CHEM-E8100
ORGANIC STRUCTURAL ANALYSIS
(5 cr)
Staff Scientist, PhD
Jari Koivisto
Room: C 308b
Phone: 040-3438574
Email: [email protected]
1
ORGANIC STRUCTURAL ANALYSIS
General Information
Lectures:
Tuesdays 13:15 – 15:00, Wednesdays 13:15 – 15:00, room Ke4 (8.9. – 14.10.2015).
Exercises and homework:
Thursdays 10:15 – 12:00, room Ke4 (10.9. – 15.10.2015). Homework (structure
determination as in the final exam).
Instrument demonstrations:
NMR and MS (probably IR) total max. 7h.
Material:
Handout.
Final exam:
Final exam (max. 100p.). Wednesday 21.10.2015. Three structure determination
problems. One A4 cheat sheet allowed.
2
ORGANIC STRUCTURAL ANALYSIS
Content and Learning Outcomes
Content:
The objective is to learn how to use mass spectrometry
(MS), infrared spectroscopy (IR) and nuclear magnetic
resonance spectroscopy (NMR) in the structural
determination and identification of organic compounds.
Learning Outcomes:
After the course the student will be able to
1. Interpret MS, IR and NMR spectra
2. Solve structures of organic molecules based on MS, IR
and NMR spectra
3. Describe the functional principles of the MS, IR and
NMR spectrometers
3
1. Introduction
1.1 MS, IR ja NMR in this course
Mass spectrometry
Fragmentation
Molecular weight
IR spectroscopy
Functional groups
(OH, C=O, NH2, ...)
Molecular formula
Double Bond
Equivalent
(DBE)
NMR spectroscopy
Conformation
(3D structure)
How the atoms are
attached to each other
(2D structure)
4
1. Introduction
1.2 2D and 3D structures of the molecules
2D structure describes how the
atoms are attached to each other
The spatial structure of the
molecule is not described
H
N
O
N
CH2CH3
Spectra give only one
(correct) solution
Usually dynamic equilibrium
involving many conformers
3D structure
describes atomic
positions in space
Determines several properties of
a substance including its reactivity,
polarity, phase of matter, color,
magnetism, and biological activity
5
1. Introduction
1.3 Electromagnetic radiation
Quantum mechanics: Discrete energy packets, i.e. photons
Oscillating magnetic
field component, M
Classical physics
http://www.chem.ufl.edu/~chm2040/Notes/Chapter_9/electromagnetic.html
Oscillating
electric field
component, E
6
1. Introduction
1.3 Electromagnetic radiation
The energy of the photon
depends on its frequency
 Wavelength
Amplitude
 Frequency
[Hz] or [s-1]
E = h hc /
where
h = Planck constant
(6,62618 x 10-34 Js)
and
c = speed of light
(2,99792 x 108 m s-1)
7
1. Introduction
1.4 Electromagnetic spectrum
rotations
vibration
electrons
http://laxmi.nuc.ucla.edu:8248/M248_98/iphysics/spectrum.html
8
1. Introduction
1.5 Spectroscopy
Spectroscopy is the study of
the interaction between matter
and electromagnetic radiation
Electromagnetic radiation, i.e.
energy is absorbed by molecules
The amount of energy
absorbed gives information
on the molecular structure
9
1. Introduction
1.5 Spectroscopy
Absorption of a photon
will occur only when the
energy of the photon
precisely matches the
energy gap between
the initial and final states
E = E2 – E1 = h
E
E2
E
E1
10
1. Introduction
1.5 Spectroscopy
The intensity of the absorption
varies as a function of frequency
(or wavelength) of the photon
Absorption spectrum
Absorption
intensity
E4  E8
E3
E2
1. excited state
E1
Ground state
Higher frequency
E2
E3 E4  E8
Shorter wavelength
Higher energy
11
1. Introduction
1.6 Double Bond Equivalent
Double Bond Equivalent (DBE)
Calculated from the
molecular formula
[(2C + 2) – H – X + N] / 2
The number of rings, double
bonds, and triple bonds
present in the compound
12
1. Introduction
1.6 Double Bond Equivalent
Molecular formula C3H6O:
DBE = [(2 x 3 + 2) – 6] / 2 = 1
DBE = 4
(three double
bonds + ring)
One double bond
or one ring
H
O
O
OH
H
H
DBE = 2
OH
O
13
2. Mass spectrometry
2.1 Introduction
The basic principle of mass spectrometry (MS) is to generate
ions from sample compounds by any suitable method
These ions are separated by their
mass-to-charge (m/z) ratio
Finally, ions are detected qualitatively and
quantitatively by their respective m/z and abundance
14
2. Mass spectrometry
2.1 Introduction
Some examples of the
applications of mass spectrometry
Determination of the
molecular weight
Sequencing of
biopolymers
Determination of the
molecular formula
Determination of the
isotopic composition
Determination of the
molecular structure
15
2. Mass spectrometry
2.2 Mass spectrometer
2.2.1 General scheme of a mass spectrometer
Sample inlet
• Directly
• Chromatograph
• Heated reservoir
Ion source
• Electron ionization (EI)
• Chemical ionization (CI)
• Field ionization/desorption (FI/FD)
• Fast atom bombardment (FAB)
• Matrix-assisted laser
desorption/ionization (MALDI)
• Electrospray ionization (ESI)
Detector
Mass analyzer
• Magnetic sector (B)
• Electron multipliers
• Linear quadrupole (Q)
• Microchannel plates (MCP)
• Quadrupole ion trap (QIT)
• Time-of-flight (TOF)
• Orbitrap
• Ion cyclotron resonance (ICR)
+
+
++
+
++
++
+ ++ + + + +
+ + + + + +
+ + + ++ + +
+ ++
+
+
Mass analyzer and detector are
always operated under high vacuum;
ion source is operated either under
vacuum or at atmospheric pressure
+
+
Mass spectrometer provides
very high sensitivity
Sample consumption
is low
16
2. Mass spectrometry
2.2 Mass spectrometer
2.2.2 Mass analysis
Ionization
Mass analyzer
Detection
Relative abundance [%]
Mass spectrum
Molecular weight [m/z]
17
2. Mass spectrometry
2.3 Sample inlet
Table 1. Examples of sample inlet methods
Sample inlet
Principle
Sample type
Typical
ionization
method
Heated reservoir
Heated reservoir
with sample vapor
Low to medium
boiling liquids
EI, CI, FI
Direct insertion probe
Sample in
heated/cooled vial
Solids, waxes, high
boiling liquids
EI, CI, FI, FAB
Gas chromatograph (GC)
Elutes directly into
ion source
Volatile components
of mixtures
EI, CI, FI
Liquid chromatograph (LC)
Elutes directly or
connected via
interface
Volatile and nonvolatile components
of mixtures
ESI (non-volatile),
EI, CI (volatile)
Direct injection
Direct injection
into ion source
using a syringe
Pure compounds in
solution
ESI
18
2. Mass spectrometry
2.4 Ionization methods
Possible to produce several
types of molecular ions
The amount of ionization
energy can be selected
CI, FAB, MALDI, and ESI produce
both positive and negative ions
Molecular ion can fragment
and thus give information
about molecular structure
The molecular weight
can be determined from
the non-fragmented
molecular ion
EI and FI/FD produce
positive ions only
19
2. Mass spectrometry
2.4 Ionization methods
Table 2. Ionization methods
Ionization method
Ionizing agent
Typical molecular
ion
Electron ionization (EI)
Electrons
M+●
Chemical ionization (CI)
Secondary ions
produced from a
reagent gas
[M+H]+, [M-H]-
Field ionization (FI)
Field desorption (FD)
High electric field
M+●, M2+, [M+H]+,
[M+A]+ (FD only)
Fast atom bombardment
(FAB)
High energy
atoms
[M+H]+, [M-H]-,
[M+A]+
Matrix-assisted laser
desorption/ionization
(MALDI)
Laser light
[M+H]+, [M-H]-,
[M+A]+
Electrospray ionization
High electric field,
heat
[M+H]+, [M+nH]n+,
[M-H]-, [M+A]+
M = intact analyte molecule, A = alkali metal (typically Na or K)
20
2. Mass spectrometry
2.4 Ionization methods
2.4.1 Electron ionization (EI)
In electron ionization (EI), energetic electrons interact
with gas phase atoms or molecules to produce
molecular ions, which are radical cations (M+●):
M + e-  M+• + 2e-
The ionization energy (IE) defines the minimum
energy required for ionization of the neutral analyte
IE’s of most molecules are in
the range of 7 – 15 eV
EI spectra are typically
acquired at 70 eV
Fragmentation depends on
the molecular structure
Molecular ion can dissociate
to produce fragment ions
(both cations and radical cations)
21
2. Mass spectrometry
2.4 Ionization methods
2.4.1 Electron ionization (EI)
EI mass spectrum of
acetophenone (70 eV)
105
77
Fragment ions
Molecular
ion
O
77
C
CH3
51
105
43
120 (M+•)
-CO
-CH3
22
2. Mass spectrometry
2.4 Ionization methods
2.4.1 Electron ionization (EI)
Table 3. Pro’s and con’s of EI
Pro’s
Consequences
Con’s
Consequences
Highly reproducible
method
Spectral libraries
Only positive ions are
produced
Limited applicability
A lot of
fragmentation
Gives information
about the molecular
structure
A lot of
fragmentation
Determination of the
molecular weight not
always possible
Highly efficient
ionization
Sensitive method
Sample must be
volatile
Limited to molecular
weight < 1000 Da
Ionization is not
selective
All volatile
components are
ionized
Ionization is not
selective
All volatile
components give
signals
Produces radical
cations
Rearrangements
complicate the
spectra
23
2. Mass spectrometry
2.4 Ionization methods
2.4.2 Chemical ionization (CI)
Chemical ionization (CI) is
less energetic method than EI
In a CI experiment, ions are produced
through the collision of the
analyte with ions of a reagent gas
Good for molecular
weight determination
Some common reagent gases
include hydrogen (H2), methane
(CH4), isobutane (i -C4H10)
and ammonia (NH3)
24
2. Mass spectrometry
2.4 Ionization methods
2.4.2 Chemical ionization (CI)
First, primary ions are formed from
the reagent gas by EI in the ion source
(the reagent gas is present in large
excess compared to the analyte)
Second, primary ions
react with excess
reagent gas to
give secondary ions
Methane as a reagent gas
Typical primary ions:
CH4+● and CH3+
Third, the secondary ions
react with the sample
Secondary ion formation:
CH4+● + CH4  CH5+ + CH3●
CH3+ + CH4  C2H7  C2H5+ + H2
The secondary ions react with the sample:
CH5+ + M  [M+H]+ + CH4
C2H5+ + M  [M+H]+ + C2H4
Typical ions: [M+H]+,
[M-H]+, [M-H]-, M+●
25
2. Mass spectrometry
2.4 Ionization methods
2.4.2 Chemical ionization (CI)
As in EI, samples must be volatile
Limited to molecular weight < 1200 Da
Comparison of isobutane and methane as reagent gases.
Analyte: 1-phenyl-1-butanone.
26
2. Mass spectrometry
2.4 Ionization methods
2.4.3 Field ionization/desorption (FI/FD)
In FI and FD, a high voltage of 8 – 12 kV is
applied between field anode and field cathode,
resulting in ionization of the analyte molecules
In FD mode, the analyte is supplied
directly on the surface of the field emitter
Typical ions: M+●, M+, [M+H]+, [M+alkali]+
In FI mode, the analyte is
introduced via external inlet system
Typical ions: M+●, [M+H]+
FI and FD are very soft methods
and have little or no fragmentation
FI and FD are mostly replaced by
modern soft ionization methods (e.g. ESI)
27
2. Mass spectrometry
2.4 Ionization methods
2.4.4 Fast atom bombardment (FAB)
In FAB, a beam of high energy atoms
(typically Xe, Ar, Ne) strikes a mixture of
sample and a matrix compound to create ions
Matrix is a non-volatile organic solvent
Common matrices include
glycerol, thioglycerol, and
3-nitrobentsyl alcohol (3-NBA)
Matrix, for example, absorbs the primary
energy and assists analyte ion formation
Some properties of the FAB method:
•
•
•
•
•
Well suited for non-volatile and thermolabile compounds
Soft ionization  good for molecular weight determination
Mass limit practically about 6000 Da
Typical ions: [M+H]+, [M+alkali]+, [M-H]Widely replaced by MALDI and ESI
28
2. Mass spectrometry
2.4 Ionization methods
2.4.4 Fast atom bombardment (FAB)
H
N
FAB spectrum of 6-aminofulvalene-2-aldimine (positive mode). Matrix 3-NBA.
m/z = 272(68) (M+), m/z = 273(100) ([M+H]+), m/z = 271(10) ([M-H]-).
C6H5
H
N
C6H5
H
http://arkat-usa.org/ark/journal/Volume1/
Part6/General/0-076A/0079.pdf
29
2. Mass spectrometry
2.4 Ionization methods
2.4.5 Matrix-assisted laser desorption/ionization (MALDI)
In MALDI, the sample is co-crystallized with
a solid matrix compound on a metal plate
The matrix serves the same
purpose as it does in FAB
The mixture is irradiated by a pulsed
laser beam, causing ionization
Common matrices include
3,5-dimethoxy-4-hydroxycinnamic acid,
2,5-dihydrobenzoic acid, and
-cyano-4-hydroxycinnamic acid
Some properties of the MALDI method:
•
•
•
•
•
Commonly used in the analysis of biomacromolecules and large organic polymers
Large mass range: about 1000 – 300 000 Da
Soft ionization  good for molecular weight determination
Background signals of the matrix in the low-mass region may cause problems
Typical ions: [M+H]+, [M+alkali]+, [M-H]-
30
2. Mass spectrometry
2.4 Ionization methods
2.4.5 Matrix-assisted laser desorption/ionization (MALDI)
MALDI spectrum of a large biomolecule.
Matrix -cyano-4-hydroxycinnamic acid.
31
2. Mass spectrometry
2.4 Ionization methods
2.4.6 Electrospray ionization (ESI)
ESI is a soft ionization method that accomplishes the transfer
of ions from solution to the gas phase at atmospheric
pressure (API = atmospheric pressure ionization)
Sample in
solution
Sample
ions
Charged
aerosol
Atmospheric pressure region
Dry N2
From LC
Vacuum
Heat
ESI needle 2.5 – 3.5 kV
H+
H+
H+
M
H+
M
[M + nH]n+
H+ H+
H+
H+
H+
H+ H+
H+
H+
Mass analyzer
M H+
H+ M
H+ H+
H+
32
2. Mass spectrometry
2.4 Ionization methods
2.4.6 Electrospray ionization (ESI)
ESI is extremely useful for the
analysis of large, non-volatile,
chargeable molecules such as
proteins and nucleic acids
Also suitable for small polar molecules
in the range of m/z 100 – 1500
Forms multiply charged ions in case
of high-mass analytes ([M+nH]n+)
Folds up the m/z scale by
the number of charges
Typical ions: [M+H]+, [M+alkali]+, [M-H]-
33
2. Mass spectrometry
2.4 Ionization methods
2.4.6 Electrospray ionization (ESI)
LeuEnk_020107, W+
LCT Premier
02-Jan-2007
10:45:21
LeuEnk_020107 53 (0.981) Cm (27:53)
TOF MS ES+
1.01e4
100
[M + H]+
%
Leucine enkephalin (acetate salt)
C28H37N5O7
ESI spectrum (positive mode)
[M+H]+ ion, calculated m/z = 556.2771
552
0
100
150
200
250
300
350
400
450
500
550
600
650
553
700
554
750
555
556
800
557
850
558
900
559
950
560
m/z
1000
561
562
563
34
2. Mass spectrometry
2.4 Ionization methods
2.4.6 Electrospray ionization (ESI)
ESI spectrum of cytochrome c (ca. 12 360 Da).
(a) Low resolution spectrum, (b) high resolution
spectrum.
Spectrum (a) shows charge states from +10 to +16
([M+10H]10+ – ([M+16H]16+). Spectrum (b) shows
the isotope distribution for [M+15H]15+.
35
2. Mass spectrometry
2.5 Mass analyzers
2.5.1 Magnetic sector
Modern magnetic sector instruments
are double-focusing having a
magnetic sector and an electric sector
Applied in all organic
MS analysis methods
Focus the ion beams both
in direction and velocity
Some properties of the magnetic sector analyzers:
•
•
•
•
•
•
•
High resolution (R > 80 000)
Accurate mass determination
High sensitivity
Very high reproducibility
Very good quantitative performance
Not well-suited for pulsed ionization methods (e.g. MALDI)
Large in size and expensive
m/z = B2r2/2V
m = ion mass
z = ion charge
B = magnetic field strength
r = magnetic field radius
V = acceleration voltage.
36
2. Mass spectrometry
2.5 Mass analyzers
2.5.2 Linear quadrupole
The linear quadrupole is composed
of two pairs of metallic rods
One set of rod is at a
positive electrical potential,
and the other set at
a negative potential
The positive rod is a
high mass filter, and the
other pair is a low mass filter
A combination of DC and RF
voltages is applied on each set
For a given amplitude of the DC and
RF voltages, only ions of a given m/z
will be able to pass the quadrupole
http://ms.mc.vanderbilt.edu/tutorials/ms/ms.htm
37
2. Mass spectrometry
2.5 Mass analyzers
2.5.2 Linear quadrupole
Linear quadrupole is commonly used in
benchtop GC/MS and LC/MS systems,
and triple quadrupole MS/MS systems
Collision gas
Triple quadrupole system
Ion source
Q1
Ion selection
Q2
Collision cell
Q3
Fragment ion
selection
Detector
Some properties of the linear quadrupole analyzers:
•
•
•
•
•
•
Good reproducibility
Good quantitative performance
Limited mass range (m/z < 4000)
Limited resolution (R < 4000)
Not well suited for pulsed ionization techniques (e.g. MALDI)
Relatively small and low-cost
38
2. Mass spectrometry
2.5 Mass analyzers
2.5.3 Quadrupole ion trap
The quadrupole ion trap store ions in a device
consisting of a ring electrode and two end cap electrodes
Commonly used in benchtop GC/MS,
LC/MS, and MS/MS systems
The ions are stabilized in the trap by
applying a RF voltage on the ring electrode
and using helium as a damping gas
By ramping the RF voltage it is possible
to destabilize the ions and eject
them progressively from the trap
Some properties of the quadrupole ion traps:
•
•
•
•
High sensitivity
Poor quantitative performance
Limited resolution (R < 5000)
Relatively small and low-cost
http://ms.mc.vanderbilt.edu/tutorials/ms/ms.htm
39
2. Mass spectrometry
2.5 Mass analyzers
2.5.4 Time-of-flight (TOF)
In a TOF instrument, ions
are accelerated to a high
velocity by an electric field
Commonly used in GC/MS,
LC/MS, and MALDI systems
The velocity reached by an ion
in an analyzer tube is inversely
proportional to its mass
In order to increase the resolution, the ion trajectory
is bent by an electronic mirror, the reflectron
Some properties of the time-of-flight analyzers:
•
•
•
•
http://pubs.acs.org/hotartcl/ac/98/jul/5176-jul01.report2-1t.html
Very large mass range (no upper theoretical limitation)
High resolution (R > 18 000)
Accurate mass determination
Well suited for pulsed ionization methods (e.g. MALDI)
40
2. Mass spectrometry
2.5 Mass analyzers
2.5.5 Orbitrap and Fourier transform ion cyclotron resonance (FTICR)
Orbitrap: Ions orbit in an electrostatic field
FTICR: Ions orbit in a magnetic field
Commonly used in GC/MS, LC/MS
MALDI, and MS/MS systems
Orbitrap
Some properties of the Orbitrap and FTICR analyzers:
•
•
•
•
Very high resolution (Orbitrap > 240 000, FTICR > 1 000 000)
Accurate mass determination
Well suited for pulsed ionization methods (e.g. MALDI)
FTICR: very expensive to purchase and maintain
(superconducting magnet), large
● Orbitrap: “reasonably” priced and sized
FTICR
The image current from
the trapped ions is detected
and converted to a mass
spectrum using the Fourier
transform of the free
induction decay (FID) signal
FT
Mass spectrum
http://upload.wikimedia.org/wikipedia/
commons/8/8a/Orbitrappe.png
http://www.chem.vt.edu/
chem-ed/ms/ftms.html
41
2. Mass spectrometry
2.6 Detection
Electron multiplier
Microchannel plate
Ion
40 V
80 V
20 V
120 V
http://hea-www.harvard.edu/HRC/mcp/fmcp.gif
60 V
160 V
100 V
140 V
106 ions
Signal is amplified and
transferred to the computer
42
2. Mass spectrometry
2.7 Mass spectrum
Mass spectra are represented
in table or graphic format
m/z
148 (M+)
120
105
77
51
27
%
8
7
100 (base peak)
66
28
20
43
2. Mass spectrometry
2.7 Mass spectrum
Continuum mode
555
556
557
558
559
560
Centroid mode
561
555
556
557
558
559
560
561
44
2. Mass spectrometry
2.8 Isotopic composition
An isotope of any given element is an
atom with the same number of protons
and electrons but a different number of
neutrons, resulting in a different overall mass
Mass number
(protons + neutrons)
Atomic number
(protons)
Mass spectrometry distinguishes elements
based on a mass-to-charge ratio, m/z
12
6
The intensity of the peak corresponds to the
relative abundance of each isotope in the sample
Isotopes give a characteristic series
of peaks with different intensities
 isotopic pattern
45
2. Mass spectrometry
2.8 Isotopic composition
Monoisotopic elements (X)
include: 19F, 23Na, 31P, and 127I
X + 2 elements include:
Cl (35Cl, 37Cl) and Br (79Br, 81Br)
X + 1 elements include: H (1H, 2H),
C (12C, 13C), and N (14N, 15N)
For practical reasons, O and S can
be regarded as X + 2 elements (O
can even be treated as an X element)
Because 2H is of low abundance,
H can be treated as monoisotopic
X - 1 elements include:
Li (6Li, 7Li) and B (10B, 11B)
The majority of elements belong
to the polyisotopic elements
46
2. Mass spectrometry
2.8 Isotopic composition
Table 4. Isotopic classifications and isotopic compositions of some elements
Element
Classif.
Isotope
Isotopic
abund.*
Isotope
Isotopic
abund.
Isotope
Isotopic
abund.
Proton
(X)**
1H
100
2H
0.0115
Fluorine
X
19F
100
Phosphorus
X
31P
100
Iodine
X
127I
100
Carbon
X+1
12C
100
13C
1.08
Nitrogen
X+1
14N
100
15N
0.369
Oxygen
(X + 2)
16O
100
17O
0.038
18O
0.205
Sulphur
(X + 2)
32S
100
33S
0.80
34S
4.52
Chlorine
X+2
35Cl
100
37Cl
31.96
Bromide
X+2
79Br
100
81Br
97.28
Isotope
Isotopic
abund.
36S
0.02
*Isotopic abundances are listed with the abundance of the most abundant isotope normalized to 100%
**Classification in parentheses: “not in the strict sense”
47
2. Mass spectrometry
Most
abundant
mass
2.8 Isotopic composition
C153H224N42O50S
Average
mass
O50S
Nominal mass = the integer mass
of the most abundant naturally
occurring stable isotope of an element
3483.6074
3482.5996
Monoisotopic
mass
Monoisotopic mass = the exact mass
of the most abundant isotope of an element
3484.6074
3481.5996
3485.6074
Relative atomic mass = atomic weight = the
weighted average of the naturally occurring isotopes
of an element (used to calculate the molecular weight)
Nominal
mass
3486.6152
3487.6152
3480
3482
3484
3486
3488
48
2. Mass spectrometry
2.8 Isotopic composition
1.
2.
3.
3.
Bromine (Br) and chlorine (Cl):
1. 1 x Cl  approx. isotope ratio
35Cl/37Cl
2. 1 x Br  approx. isotope ratio
79Br/81Br
=3:1
=1:1
3.
3. 2 x Cl, 2 x Br or Cl + Br  X + 4 peak appears
49
2. Mass spectrometry
2.9 Resolution
Mass resolution: Represents the
ability to separate two adjacent masses
Full width at half maximum (FWHM)
M
“Sharpness” of the peak
R = M/M
50%
Low resolution:
R = 100 – 1000
Medium resolution:
R = 1000 – 10 000
High resolution:
R > 10 000
(not exact definitions)
M
50
2. Mass spectrometry
2.9 Resolution
R = 1000
Molecular formulas with the same nominal mass 122:
C7H10N2
C8H10O
Formula:
C9H14
Monoisotopic: 122.1096 122.0845 122.0732
Formula:
C7H6O2
C4H10O4
C4H10S2
Monoisotopic: 122.0368 122.0579 122.0225
R = 5000
What resolution is needed to separate C9H14 and C7H10N2?
R = M/M = 122/(122.1096 – 122.0845) = 4861  5000
51
2. Mass spectrometry
2.10 Accuracy
Mass accuracy: The difference
between measured accurate and
calculated exact mass
Maccu = Mmeas – Mcalc
Maccu (ppm) =
(Mmeas – Mcalc)/Mcalc x 106
A high resolution instrument provides the
mass information with an accuracy < 5 ppm
Enough to unambiguously
determine the elemental composition
52
2. Mass spectrometry
2.10 Accuracy
Leucine enkephalin C28H37N5O7 [M+H]+:
Mmeas = 556.2777, Mcalc = 556.2771
M (ppm) = 556.2777-556.2771/556.2771 x 106 = 1.1 ppm
Mass accuracy (elements C, H, N, O):
10 ppm  24 molecular formulas
5 ppm  12 molecular formulas
1.5 ppm  4 molecular formulas
53
2. Mass spectrometry
2.11 Recognition of the molecular ion peak (EI)
The peak of highest m/z in an EI mass spectrum must
not necessarily represent the molecular ion of the analyte
The stability of the molecular ion roughly decreases in the following order:
aromatic compounds > conjugated alkenes > alkenes > alicyclic
compounds > carbonyl compounds > linear alkanes > ethers >
esters > amines > carboxylic acids > alcohols > branched alkanes
The molecular ion must be the ion
of highest m/z in the mass spectrum
(besides the corresponding isotopic peaks)
The peak at the next lowest m/z must be
explicable in terms of reasonable losses
The molecular ion has to
be an odd-electron ion, M+●
Signals at M-5 to M-14 and at
M-21 to M-25 point towards a
different origin of the presumed M+●
54
2. Mass spectrometry
2.11 Recognition of the molecular ion peak (EI)
Nitrogen rule:
If a compound contains an even number of nitrogen atoms (0, 2, 4, …),
its monoisotopic molecular ion will be detected at an even-numbered m/z
(integer value).
If a compound contains an odd number of nitrogen atoms (1, 3, 5, …),
its monoisotopic molecular ion will be detected at an odd-numbered m/z.
Number of
nitrogens
0
1
2
3
Compound
acetone, C3H6O
acetonitrile, C2H3N
urea, CH4N2O
triazole, C2H3N3
Cleaving off a radical (that contains no
nitrogen) from any ion changes the integer
m/z value from odd to even or vice versa
M+• [m/z]
58
41
60
69
Loss of a molecule (that contains no nitrogen) from
an ion produces even mass fragments from even
mass ions and odd mass fragments from odd mass ions
55
2. Mass spectrometry
2.12 Determination of the molecular formula
Possible combinations of elements may be
determined using computer programs or
using tables available in several textbooks
Nominal
mass
The number of molecular formula
candidates can be reduced by
combined use of MS, IR, and NMR
Monoisotopic
mass
56
2. Mass spectrometry
2.13 Symbols
Symbol
Meaning
●
unpaired electron in radicals (no charge)
+
positive even-electron ions
A: Electron and charge delocalized.
-
negative even-electron ions
B and C: Electron and charge localized.
+●
positive radical ions (odd-electron)
-●
negative radical ions (odd-electron)
+•
•
+
+
•
arrow for transfer of an electron pair
single-barred arrow for transfer of a single electron
fragmentation or reaction
A
B
C
rearrangement fragmentation
to indicate position of cleaved bond
57
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.1 Cleavage of a sigma-bond
H
C
+•
H
H
H
-e-
H
C
H
 H •+ C
H
H
Methane
M+● = 16
1
+
C
Radical
H
H
rd (rearrangement
fragmentation)
H
H
H+
H
1u
H+• C
2
H
•
H
H
H
H
The -bond cleavage
represents a simple
but widespread
type of fragmentation
H
m/z 15
• C
H
CH2+•
H2
m/z 14
2u
H
m/z 1
15 u
Carbenium
ion
58
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.1 Cleavage of a sigma-bond
The dissociation fragmentation of ions
yields a fragment ion and a neutral
Even-electron rule:
Odd-electron ions may eliminate either a radical or an even-electron neutral
species, but even-electron ions will not usually lose a radical to form an oddelectron cation, i.e. the successive loss of radicals is forbidden.
[odd]+●
[even]+ + R●
[odd]+●
[odd]+● + n
[even]+
[even]+ + n
[even]+
X
[odd]+● + R●
59
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.1 Cleavage of a sigma-bond
C4
C3
n-Hydrocarbons: The
fragmentation pattern is
characterized by clusters
of peaks separated by
14 (CH2) mass units
C5
C6
C2
C7
C8 C C
9 10 C11 C
12 C13 C14
M+•
60
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.1 Cleavage of a sigma-bond
C6
C3
Branched saturated
hydrocarbons: The smooth
curve of decreasing intensities
is broken by preferred
fragmentation at each branch
C4
C6
C2
C5
C7 C C C
8
9
10
C12
C15 M+•
61
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.1 Cleavage of a sigma-bond
Cyclic hydrocarbons:
Cleavage at the bond
connecting the ring to the
rest of the molecule is favored
83
M+•
98
Methylcyclohexane
62
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
-Cleavage: Radical-site
initiated homolytic
bond cleavage with
charge retention
Stevenson’s rule: When a fragmentation
takes place, the positive charge remains on
the fragment with the lowest ionization energy
Acetone
63
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
2-Pentanone
43
M+•
86
71
64
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
The charge-localizing heteroatom can also
be part of the aliphatic chain as it is in the
case with amines, ethers, and alcohols
The mechanism of the cleavage remains unaffected
65
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
N-Ethyl-N-methyl-1-propanamine
72
M+•
101
86
100
66
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
31
45
31
45
M+•
74
M+•
60
67
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
Aliphatic halogenated hydrocarbons do not
show abundant fragment ions due to -cleavage
1-Bromo-octane
43 57
135/137
71
29
57

43
71
91/93
29

91/93
M+•
192/194
68
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.2 Alpha-cleavage
55
The molecular ion of cyclohexanone
undergoes a double -cleavage
and an intermediate 1,5-H● shift
M+•
98
Cyclohexanone
69
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.3 Benzylic bond cleavage
91
Molecular ions possessing a benzylic bond
preferably show cleavage of that bond
as compared to the phenylic position
105
77
Propylbenzene
91
M+•
120
39 51 65 77
105
70
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.3 Benzylic bond cleavage
The EI mass spectra of phenylalkanes typically show
the ion series:
m/z 39, 51, 65, 77, 91
There are two competing reaction sequences leading
to this series:
M+●  91  65  39
M+●  77  51
(usually higher intensity)
(usually lower intensity)
71
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.4 Allylic bond cleavage
Allylic bond cleavage
There is some preference for
the cleavage of the allylic bond
in the alkene molecular ions
≡
41
3-Ethyl-2-methyl-1-pentene
83
M+•
112
72
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
McLafferty rearrangement (McL):
Fragmentation that can be described as a transfer of a -hydrogen to a doublebonded atom through a six-membered transition state with -bond cleavage.
1. The atoms A, B, and D can be carbons or heteroatoms
2. A and B must be connected by a double bond
3. At least one -hydrogen is available
4. -Hydrogen is selectively transferred to B via six-membered transition state
5. -Bond is cleaved causing alkene loss
Strictly speaking, the term McLafferty rearrangement only describes an alkene loss
from molecular ions of saturated aliphatic aldehydes, ketones, and carboxylic acids.
-bond
73
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
McLafferty rearrangement of the butanal molecular ion.
The process may either be formulated in a concerted
manner or as a stepwise process.
44
M+•
72
74
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
The mass spectra of carboxylic acids and
their derivatives are governed by both
-cleavage and McLafferty rearrangement
75
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
59
McL 60
60
87
101
73
129
113
McL 74
45
73
74
87
129
59
45
M+•
172
101
113
M+•
144
76
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
91
77
In addition to benzylic and phenylic
cleavages, phenylalkanes may undergo
alkene loss by McLafferty rearrangement
91
92
M+•
134
77
McLafferty rearrangement of the butylbenzene molecular ion
77
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
McLafferty rearrangement with
double hydrogen transfer (r2H):
Alkene loss via McL at the alkoxy group of
aliphatic and aromatic carboxylic acid esters
78
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.5 McLafferty rearrangement
Benzoyl substructures (benzaldehyde, benzoic acid
and its derivatives, acetophenone, benzophenone, etc…)
typically show the ion series:
122 McL
105
m/z 51, 77, 105
Isopropyl benzoate
123
r2H
Different from benzylic compounds, the peaks at m/z 39,
65, and 91 are almost absent.
105
77
51
123
122
M+•
164
79
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.6 Retro-Diels-Alder reaction
Retro-Diels-Alder reaction (RDA):
Molecular ions containing a cyclohexene unit
may fragment to form conjugated di-olefinic
(diene) and mono-olefinic (ene) products
Cyclohexene
54
M+•
82
80
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.7 Elimination of carbon monoxide
The most characteristic fragment ions of phenols
are caused by loss of carbon monoxide, CO,
from the molecular ion, and subsequent H● loss
M+•
94
66
65
[M-28]+● and [M-29]+ ions
Phenol
81
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.7 Elimination of carbon monoxide
Anisole
65
M+•
108
78
39
51
77
93
82
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.7 Elimination of carbon monoxide
4-Chlorophenetole
128/
130
M+•
156/
158
65
75
100/
102 111/
113
83
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.8 Loss of H2O from alcohols
Aliphatic alcohols show a strong
tendency to thermally eliminate a
water molecule prior to ionization
The mass spectra correspond
to the respective alkenes
rather than to the alcohols
The molecular ions of alcohols also
exhibit a strong tendency to H2O loss
Their second important fragmentation
route in addition to -cleavage
Loss of H2O from the molecular ion of an alcohol
84
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.8 Loss of H2O from alcohols
The EI mass spectra of 1-hexanol and 1-hexene show close similarity. However,
for example, the oxonium peak at m/z 31 is absent in the hexene spectrum.
43
56
56
41
41
oxonium
31
43
69
[M-18]+•
84
(M+•
69
M+•
84
102)
85
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.9 Ortho elimination
Ortho elimination: A hydrogen transfer
via a six-membered transition state in
ortho-disubstituted aromatic compounds
Z = hydrogen-accepting leaving group
B = hydrogen donor (e.g. hydroxyl, amino, thiol, or even alkyl)
86
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.9 Ortho elimination
2-Methylbenzoic acid
91
90
118
M+•
136
119
87
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.10 Heterocyclic compounds
57
The molecular ions of small saturated
heterocyclic compounds exhibit a strong
tendency for transannular cleavages
84
84
57
M+•
85
88
2. Mass spectrometry
2.14 Fragmentation of organic ions (EI)
2.14.10 Heterocyclic compounds
Pyridine and numerous other aromatic
N-heterocycles eliminate a molecule of
hydrogen cyanide, HCN (27 u), from
their molecular ions.
Aniline eliminates a molecule of hydrogen
isocyanide, HNC (27 u), from its molecular ion.
52
M+•
79
M+•
93
52
66
65
89
3. IR spectroscopy
3.1 IR spectrometer
Fourier transform
(FT) IR
IR source
Sample
cell
Detector
Beam splitter
Combined beam
ADC
Computer
Moving
mirror
Fixed mirror
Michelson interferometer
90
3. IR spectroscopy
3.1 IR spectrometer
Electromagnetic radiation containing all
IR wavelengths is split into two beams; one
of fixed length, other of variable length
Smooth and continuous variation
of the length of the piston adjusts
the position of the moving mirror
and varies the length of the beam
FT is done at successive points
throughout this variation
The varying distances between two
path lengths result in an interferogram
Passage of this radiation through
a sample subjects the compound
to a broad band of energies
Fourier transform (FT) converts this
interferogram from the time domain into
one spectral point of the frequency domain
The analysis of one broadbanded pass
of radiation through the sample will
give rise to a complete IR spectrum
91
3. IR spectroscopy
3.2 Sample handling
IR spectra may be obtained
for gases, liquids, or solids
Gas cells are available in lengths of a few
centimeters to tens of meters (long paths
are achieved by multiple reflection optics)
Solutions are handled in cells
of 0.1 – 1 mm thickness
Neat liquids are examined between
salt plates, usually without a spacer
A compensating cell, containing pure
solvent is placed in the reference beam
Volatile liquids are in sealed
cells with very thin spacers
The solvent must be dry and
transparent in the IR region of interest
92
3. IR spectroscopy
3.2 Sample handling
Solids are usually examined as a mull, as a
pressed disk, or as a deposited glassy film
The mull (oil suspension) is examined
as a thin film between salt plates
In the pressed disk technique, the sample is
mixed with dry, powdered KBr and then compacted
under pressure to form transparent disks
Deposited films are useful only
when the material can be deposited
from solution or cooled from a melt
as microcrystals or as a glassy film
Attenuated total reflection (ATR) technique
can also be used to examine solid samples
93
3. IR spectroscopy
3.2 Sample handling
In general, dilute solution in a nonpolar solvent
provides the best (i.e. least distorted) spectrum
Nonpolar compounds give essentially the same spectra
in the condensed phase (i.e. neat liquid, a mull, a KBr
disk, or a thin film) as they give in nonpolar solvents
Polar compounds often show hydrogen
bonding effects in the condensed phase
94
3. IR spectroscopy
3.3 Theory
Interferogram
FT
IR spectrum
IR radiation is absorbed and
converted by an organic molecule
into energy of molecular vibration
NIR – MIR – FIR
Vibrational spectra appears as bands
rather as lines because a single vibrational
energy change is accompanied by
number of rotational energy changes
4000 – 400 (200) cm-1
95
3. IR spectroscopy
3.3 Theory
X-axis: Position of the absorption band
Y-axis: Intensity of the absorption band
Wavenumber [cm-1]
Transmittance (T )
(absorbance (A))
 =  /c = 1 /
Organic chemists usually report
intensity in semiquantitative terms:
s = strong, m = medium, w = weak
96
3. IR spectroscopy
3.3 Theory
Stretching vibrations
Bending vibrations
+
-
There are two types
of molecular vibrations:
stretching and bending
Asymmetrical
stretching (as CH2)
~2926 cm-1
In-plane bending
or scissoring (s CH2)
~1465 cm-1
+
Only those vibrations that
result in a rhythmical change
in the dipole moment of the
molecule are observed in the IR
Out-of-plane bending
or twisting (CH2)
1350-1150 cm-1
+
-
Symmetrical
stretching (s CH2)
~2853 cm-1
Out-of-plane bending
or wagging (CH2)
1350-1150 cm-1
In-plane bending
or rocking (CH2)
~720 cm-1
97
3. IR spectroscopy
3.3 Theory
A molecule has as many degrees of freedom as
the total degrees of freedom of its individual atoms
Each atom has three degrees
of freedom (x, y, z)
A molecule of n atoms therefore
has 3n degrees of freedom
For nonlinear molecules,
three degrees of freedom
describe rotation and
three describe translation
The remaining 3n-6 degrees of
freedom are vibrational degrees of
freedom or fundamental vibrations
Linear molecules have
3n-5 degrees of freedom
The theoretical number of fundamental vibrations (absorption
frequencies) will rarely be observed because different phenomena
either reduce or increase the number of absorption bands
98
3. IR spectroscopy
3.3 Theory
The following will reduce the theoretical number of bands:
1. Fundamental vibrations that fall outside of the 4000 –
400 cm-1 region.
2. Fundamental vibrations that are too weak to be
observed.
3. Fundamental vibrations that are so close that they
coalesce.
4. The occurrence of a degenerate band from several
absorptions of the same frequency in highly symmetrical
molecules.
5. The failure of certain fundamental vibrations to appear
in the IR because of the lack of change in molecular
dipole.
The following will increase the theoretical number of bands:
1. Overtones having twice the frequency of the normal
vibrations will occasionally observed as weak bands.
2. Combination tones that are weak bands occasionally
appearing at frequencies that are the sum or difference of
two or more fundamental vibrations. Thus, fundamental
vibrations at x and y cm-1 may give rise to weak bands at
(x+y) or (x-y) cm-1.
The following phenomena also occur in the IR spectra:
1. Coupling: When two bond oscillators share a common
atom, they seldom behave as individual oscillators unless
the individual oscillation frequencies are widely different.
This is because there is mechanical coupling interaction
between the oscillators. The coupling phenomenon is
highly dependent on molecular geometry.
2. Fermi resonance: Interaction that occur between
fundamental vibrations and overtones or combination
tone vibrations.
99
3. IR spectroscopy
3.3 Theory
H2O molecule: 3 fundamental vibrations
CO2 molecule: 4 fundamental vibrations
Inactive in the IR
Symmetrical
stretching (s CO2)
1340 cm-1
Asymmetrical
stretching (as OH)
3756 cm-1
Asymmetrical
stretching (as CO2)
2350 cm-1
Symmetrical
stretching (s OH)
3652 cm-1
Scissoring
(s HOH)
1596 cm-1
Scissoring
(s CO2)
666 cm-1
-
+
Scissoring
(s CO2)
666 cm-1
-
100
3. IR spectroscopy
3.3 Theory
Assignments for stretching
frequencies can be approximated
by the application of Hooke’s law
Two atoms and their connecting bond are
treated as a simple harmonic oscillator
composed of two masses joined by a spring

 = the vibrational frequency [cm-1]
= velocity of light [cm s-1]
= force constant of bond [dyne cm-1]
and
= mass of atom x and atom y,
respectively [g]
The value for is approximately 5 105 dyne cm-1 for
single bonds and approximately two and three times
this value for double and triple bonds, respectively.
1 dyn = 1 g cm s-2 = 10−5 kg m s-2 = 10−5 N
101
3. IR spectroscopy
Hydrogen bonding is an interaction that
can alter the force constants of both the proton
donor (X – H) and proton acceptor (Ä) groups
X–H
Ä
3.3 Theory
The frequencies of
both stretching and
bending are altered
The X – H stretching band moves
to a lower frequency usually with
increased intensity and band widening
The X – H bending band moves
to a higher frequency (lesser
degree than the stretching band)
The Ä stretching band also
moves to a lower frequency
(lesser degree than the X – H group)
102
3. IR spectroscopy
3.3 Theory
The extent of both inter- and
intramolecular hydrogen bonding
is temperature dependent
The bands that result from
intermolecular bonding generally
disappear at low concentrations
Intramolecular hydrogen bonding
is an internal effect and persists
at very low concentrations
Ring strain, molecular geometry, and
the relative acidity and basicity of the
proton and acceptor groups affect the
strength of the hydrogen bonding
103
3. IR spectroscopy
3.4 Interpretation of the IR spectra
The two important regions for a preliminary
examination of the IR spectrum are the
regions 4000 – 1300 and 900 – 650 cm-1
The lack of strong absorption bands
in the 900 – 650 cm-1 region generally
indicates a nonaromatic structure
The high-frequency portion of the spectrum
is called the functional group region
The intermediate portion of
the spectrum, 1300 – 900 cm-1, is
referred to as fingerprint region
The characteristic stretching frequencies
for functional groups such as OH, NH, and
C=O occur in this portion of the spectrum
Absorption in this intermediate
region is probably unique
for every molecular species
104
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.1 Normal alkanes
Absorption arising from CH stretching
in the alkanes occurs in the
general region of 3000 – 2840 cm-1
Methyl group:
as (CH3) 2962 cm-1 (s)
s (CH3) 2872 cm-1 (s)
s (CH2)
Methylene group:
as (CH2) 2926 ± 10 cm-1 (s)
s (CH2) 2853 ± 10 cm-1 (s)
s (CH3)
as (CH3)
as (CH2)
105
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.1 Normal alkanes
Methyl group bending:
as (CH3) ~1450 cm-1
s (CH3) ~1375 cm-1
Methylene group bending:
s (CH2) 1465 cm-1 (scissoring)
 (CH2) and (CH2) 1350 – 1150 cm-1
(wagging and twisting)
 (CH2) 720 cm-1 (rocking, n-alkanes
of seven or more C atoms)
s (CH3)
(CH2)
as (CH3)
s (CH2)
106
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.1 Normal alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
(CH2)
s (CH3)
s (CH2)
s (CH3)
as (CH3)
s (CH2)
as (CH3)
as (CH2)
107
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.1 Normal alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
s (CH3)
s (CH3)
as (CH3)
s (CH2)
(CH2)
as (CH3)
s (CH2)
as (CH2)
108
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.2 Branched-chain alkanes
Tertiary CH stretching:
(CH) ~2890 cm-1 (w)
Isopropyl group bending:
s (CH3) 1385 – 1380 cm-1
s (CH3) 1370 – 1365 cm-1
(CH3) ~920 cm-1 (w)
s (CH3)
t-Butyl group bending:
s (CH3) 1395 – 1385 cm-1
s (CH3) ~1370 cm-1
(CH3) ~930 cm-1 (w)
as (CH3)
109
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.2 Branched-chain alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
No (CH3)
(CH)
s (CH3)
as (CH3)
s (CH3)
as (CH3)
110
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.2 Branched-chain alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
s (CH3)
s (CH2)
s (CH2)
as (CH3)
s (CH3)
as (CH3)
as (CH2)
111
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.3 Cyclic alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
as (CH2)
s (CH2)
s (CH2)
112
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.3 Cyclic alkanes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Overtone?
as (CH2)
s (CH2)
s (CH2)
several bands
113
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.4 Alkenes
The C=C stretching mode of
unconjugated alkenes usually shows moderate
to weak absorption at 1667 – 1640 cm-1
The absorption of symmetrical disubstituted
trans-alkenes or tetrasubstituted alkenes
may be extremely weak or absent
The alkene bond stretching vibrations in
conjugated dienes without a center symmetry
interact to produce two C=C stretching bands
Symmetrical dienes show
only one band resulting
from asymmetric stretching
114
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.4 Alkenes
The alkene =CH stretching mode shows
absorption bands at 3100 – 3000 cm-1
Alkene =CH bond can undergo
bending either in the same plane as
the C=C bond or perpendicular to it
The most characteristic vibrational modes of
alkenes are the strong out-of-plane =CH bending
vibrations between 1000 and 650 cm-1
115
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.4 Alkenes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Overtone?
(CH2)
 (=CH2)
<3000 cm-1
Normal
 (CH)
>3000 cm-1
(C=C)
Out-of-plane
=CH bend
116
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.4 Alkenes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
(=CH2)
(C=C)
(=CH)
Out-of-plane
=CH bend
117
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.5 Alkynes
The weak C≡C stretching
band of alkyne molecules occurs
in the region of 2260 – 2100 cm-1
Because of the symmetry,
no C≡C band is observed
in the IR for symmetrically
substituted alkynes
Monosubstituted alkynes:
 (C≡C) 2140 – 2100 cm-1
Disubstituted alkynes
(asymmetrical):
 (C≡C) 2260 – 2190 cm-1
118
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.5 Alkynes
The strong ≡CH stretching band
of monosubstituted alkynes occurs
in the region of 3333 – 3267 cm-1
The ≡CH bending vibration of alkynes or
monosubstituted alkynes leads to strong, broad
absorption in the 700 – 610 cm-1 region
The overtone of ≡CH bending appears as a weak,
broad band in the 1370 – 1220 cm-1 region
119
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.5 Alkynes
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(≡CH)
(C≡C)
Overtone
2 × ≡CH bend
≡CH bend
120
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.6 Aromatic hydrocarbons
The most prominent and most
informative bands in the IR
spectra of aromatic compounds
occur in the low-frequency range
between 900 and 675 cm-1
Out-of-plane bending
of the ring CH bonds
Aromatic CH stretching bands
occur between 3100 and 3000 cm-1
Skeletal vibrations, involving
CC stretching within the ring,
absorb in the 1600 – 1585
and 1500 – 1400 cm-1 regions
In-plane bending bands appear in
the 1300 – 1000 cm-1 region
121
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.6 Aromatic hydrocarbons
Weak combination and overtone bands
appear in the 2100 – 1650 cm-1 region
The in-phase, out-of-plane bending
of a ring hydrogen atom is strongly
coupled to adjacent hydrogen atoms
The position of absorption of the out-of-plane
bands is therefore characteristic of the number
of adjacent hydrogen atoms on the ring
Substituted benzenes show absorption
band near 710 – 675 cm-1 that is
attributed to out-of-plane ring bending
122
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.6 Aromatic hydrocarbons
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Combination and
overtone bands
 (CH3)
>3000 cm-1
Aromatic
CH stretch
>3000 cm-1
Ring CH bend
in-plane
Ring CC
stretch
Ring CH bend
out-of-plane
monosubst. 770 – 730
and 710 – 690 cm-1
Ring
CC
bend
out-ofplane
123
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.6 Aromatic hydrocarbons
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Combination and
overtone bands
Aromatic
CH stretch
>3000 cm-1
 (CH3)
>3000 cm-1
Ring CC
stretch
Ring CH bend
in-plane
Ring CH bend
out-of-plane
1,4-disubst. 840 – 810 cm-1
Ring
CC
bend
out-ofplane
124
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
The characteristic bands observed
in the spectra of alcohols
and phenols result from OH
stretching and CO stretching
The non-hydrogen bonded (free) OH
group of alcohols and phenols absorbs
strongly in the 3650 – 3584 cm-1 region
They couple with the
vibrations of adjacent groups
These vibrations are sensitive
to hydrogen bonding
Intermolecular hydrogen bonding increases
as the concentration of the solution increases
Additional bands start to appear at
lower frequencies, 3550 – 3200 cm-1,
at the expense of the free OH band
125
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
Vapor phase sample
(no hydrogen bonding)
OH) 3670 cm-1
Liquid film sample
(hydrogen bonding)
OH) 3324 cm-1
126
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
The CO stretching vibrations in
alcohols and phenols produce a strong
band in the 1260 – 1000 cm-1 region
The CO stretching mode is coupled with
the adjacent CC stretching vibration
In primary alcohols the vibration
might better be described as an
asymmetric CCO stretching vibration
Table 5. Alcoholic CO stretch absorptions
Alcohol type
Absorption range [cm-1]
Saturated tertiary
Secondary, highly symmetrical
1205 – 1124
Saturated secondary
-Unsaturated or cyclic tertiary
1124 – 1087
Secondary, -unsaturated
Secondary, alicyclic 5- or 6-membered ring
Saturated primary
1085 – 1050
Tertiary, highly -unsaturated
Secondary, di--unsaturated
Secondary, -unsaturated and -branched
Secondary, alicyclic 7- or 8-membered ring
Primary- -unsaturated and/or -branched
<1050
Further complications arise from
branching and ,-unsaturation
127
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
Mulls, pellets, or melts of
phenols absorb at 1390 – 1330
and 1260 – 1180 cm-1
The OH in-plane bending
vibration occurs in the general
region of 1420 – 1330 cm-1
These bands apparently result
from interaction between OH
bending and CO stretching
The OH out-of-plane bending
(hydrogen bonded) occurs in the
region of 769– 650 cm-1
128
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
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OH bend
in-plane
OH stretch
OH bend
out-of-plane
CO stretch
primary alcohol
1069 cm-1
129
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.7 Alcohols and phenols
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OH bend
out-ofplane
Combination and
overtone bands
OH bend
in-plane
Aromatic
CH stretch
>3000 cm-1
OH stretch
Ring CC
stretch
Ring CH
bend
out-ofplane
CO stretch
1224 cm-1
Ring
CC
bend
out-ofplane
130
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.8 Ethers
The characteristic response of ethers
in the IR is associated with the
stretching vibration of the COC group
Aliphatic ethers show a strong band
in the 1150 – 1085 cm-1 region
because of asymmetrical COC stretching
Coupled with other vibrations
within the molecule
The symmetrical band
is usually weak
Aryl alkyl ethers display an
asymmetrical COC stretching
band at 1275 – 1200 cm-1
Vinyl ethers display an
asymmetrical COC stretching
band at 1225 – 1200 cm-1
Symmetrical stretching
occurs at 1075 – 1020 cm-1
131
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.8 Ethers
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Asymmetric
COC stretch
1124 cm-1
132
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.8 Ethers
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Asymmetric
COC stretch
1245 cm-1
Symmetric
COC stretch
1049 cm-1
133
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
Ketones, aldehydes, carboxylic acids, carboxylic esters, lactones,
acid halides, anhydrides, amides, and lactams show a strong C=O
stretching absorption band in the region of 1870 – 1540 cm-1
One of the easiest bands
to recognize in IR spectra
The position of the C=O
stretching band is determined by
Physical state
Ring strain
Electronic and
mass effects of
neighboring
substituents
Conjugation
Hydrogen bonding
(intra- and intermolecular)
134
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
The absorption frequency of a neat
sample of a saturated aliphatic ketone,
1715 cm-1, is referred as normal
Changes in the environment of the
carbonyl can either lower or rise the
absorption frequency from the normal value
The absorption frequency observed for a
neat sample is increased when absorption
is observed in nonpolar solvents
Polar solvents reduce
the frequency of absorption
The overall range of solvent
effects does not exceed 25 cm-1
135
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
Replacement of an alkyl group of a
saturated aliphatic ketone by a hetero
atom (G) shifts the carbonyl absorption
G effect predominantly inductive:
G
Cl
Br
F
OH
OR
 (C=O) [cm-1]
1815 – 1785
1812
~1869
1760
1750 - 1735
The inductive effect increases
the frequency of absorption
The direction of the shift depends
on whether the inductive effect or
resonance effect predominates
G effect predominantly resonance:
G
NH2
SR
 (C=O) [cm-1]
1695 – 1650
1720 – 1690
The resonance effect reduces
the frequency of absorption
136
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
Conjugation with an alkene or
phenyl reduces the frequency of
the C=O group stretching vibration
-Diketones usually exist
as mixtures of tautomeric
keto and enol forms
Absorption occurs in the
1685 – 1666 cm-1 region
Broad C=O band appears in
the 1640 – 1580 cm-1 region
Results from intramolecular
hydrogen bonding
137
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
-Diketones show a single C=O
absorption band near 1715 cm-1
Cyclohexanone:
 (C=O) 1715 cm-1
Quinones, which have both C=O
groups in the same ring, absorb
in the 1690 – 1655 cm-1 region
Cyclopentanone:
 (C=O) 1751 cm-1
Acyclic -chloro ketones absorb at two
frequencies because of rotational isomerism:
 (C=O) ~1715 cm-1 and ~1725 cm-1
Cyclobutanone:
 (C=O) 1775 cm-1
138
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
Ketones show moderate absorption
in the 1300 – 1100 cm-1 region
as a result of CCC stretching and
bending in the C(C=O)C group
Aromatic ketones absorb in
the 1300 – 1230 cm-1 region
Aliphatic ketones absorb in
the 1230 – 1100 cm-1 region
Absorption may consist
of multiple bands
139
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
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Overtone
2 ×  (C=O)
 (C=O)
1715 cm-1
C(C=O)C
stretch
and bend
140
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.9 Ketones
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Overtone
2 ×  (C=O)
 (C=O)
1685 cm-1
141
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.10 Aldehydes
The C=O groups of aldehydes absorb
at slightly higher frequencies than
that of the corresponding methyl ketones
Aliphatic aldehydes absorb
near 1740 – 1720 cm-1
Aldehydic C=O absorption responds to structural
changes in the same manner as ketones
Electronegative substitution
on the a carbon increases the
frequency of C=O absorption
Internal hydrogen bonding shifts
the absorption to lower frequencies
Conjugated aldehydes absorb
near 1710 – 1685 cm-1
142
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.10 Aldehydes
The majority of aldehydes show
aldehydic CH stretching absorption
in 2830 – 2696 cm-1 region
Two moderately intense
bands are frequently observed
This is attributes to Fermi resonance between the
fundamental aldehydic CH stretch and the first overtone
of the aldehydic CH bending vibration at ~1390 cm-1
Only one CH stretching band is observed
if the CH bending band has been
shifted appreciably from ~1390 cm-1
143
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.10 Aldehydes
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Aldehydic
CH stretch
2818 cm-1
2716 cm-1
Aldehydic
CH bend
1390 cm-1
 (C=O)
1729 cm-1
144
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.10 Aldehydes
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Aldehydic
CH stretch
2820 cm-1
2738 cm-1
 (C=O)
1703 cm-1
Aldehydic
CH bend
1391 cm-1
145
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
In the liquid or solid state, and in CCl4 solution at
concentrations much over 0.01 M, carboxylic acids
exist as dimers due to strong hydrogen bonding
Hydrogen bonding is strengthened by the large
contribution of the ionic resonance structure
Carboxylic acid dimers display
very broad, intense OH
stretching absorption in the
region of 3300 – 2500 cm-1
(usually centers near 3000 cm-1)
Free carboxylic acid
OH stretching
occurs near 3520 cm-1
146
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
Vapor phase sample
(no hydrogen bonding)
OH) 3578 cm-1
Liquid film sample
(hydrogen bonding)
OH) ~3000 cm-1
147
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
The C=O stretching bands of
acids are considerably more intense
than ketonic C=O stretching bands
The monomers of saturated aliphatic
acids (free acids) absorb near 1760 cm-1
The carboxylic acid dimer has a center
of symmetry; only the asymmetrical
C=O stretching mode absorbs in the IR
The C=O group in dimerized
saturated aliphatic acids absorbs in
the region of 1720 – 1706 cm-1
Hydrogen bonding and resonance weaken
the C=O bond, resulting in absorption
at a lower frequency than the monomer
148
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
Internal hydrogen bonding reduces
the frequency of the C=O stretching
absorption to a greater degree than
does intermolecular hydrogen bonding
Salicylic acid
(internal)
 (C=O) 1665 cm-1
p-Hydroxybenzoic acid
(intermolecular)
 (C=O) 1680 cm-1
Unsaturation in conjugation with
the carboxylic C=O group decreases
the frequency of absorption of both the
monomer and dimer forms only slightly
,-Unsaturated and aryl
conjugated acids show
absorption for the dimer in the
1710 – 1680 cm-1 region
Substitution in the -position with
electronegative groups (e.g.
halogens) increases the C=O absorption
frequency about 10 – 20 cm-1
149
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
Two bands arising from CO stretching
and OH bending appear near 1320 – 1210
and 1440 – 1395 cm-1, respectively
The out-of-plane
bending of the
OH group of dimers
appears near 920 cm-1
Both of these bands involve some interaction
between CO stretching and in-plane COH bending
The more intense band,
near 1315 – 1280 cm-1 for
dimers, is generally referred
to as the CO stretching band
The COH bending band
near 1440 – 1395 cm-1
is of moderate intensity
150
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
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OH stretch
dimer
3300 – 2500 cm-1
Normal
 (CH)
<3000 cm-1
 (C=O)
dimer
1712 cm-1
OH bend
out-of-plane
937 cm-1
COH bend
in-plane
1415 cm-1
CO stretch
dimer
1285 cm-1
151
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.11 Carboxylic acids
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OH bend
out-of-plane
939 cm-1
OH stretch
dimer
3300 – 2500 cm-1
 (C=O)
dimer
1696 cm-1
COH bend
in-plane
1417 cm-1
CO stretch
dimer
1288 cm-1
152
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.12 Carboxylate anion
CH3CONH4
Asymmetric
carboxylate anion
C(C=O)2- stretch
1568 cm-1
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Symmetric
carboxylate anion
C(C=O)2- stretch
1402 cm-1
153
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
The intense C=O stretching occurs
at higher frequencies than that of
normal ketones due to the inductive
effect of the adjacent oxygen atom
The C=O absorption band of saturated
aliphatic esters (except formates)
is in the 1750 – 1735 cm-1 region
The C=O absorption bands of
vinyl or phenyl esters, with
unsaturation adjacent to the CO
group, are observed at considerably
higher frequencies (~1770 cm-1)
The C=O absorption bands of formates,
,-unsaturated, and benzoate esters
are in the region of 1730 – 1715 cm-1
Occurs along with a lowering
of the CO stretching frequency
The C=O absorption band of
-halogen substituted esters
is observed near 1770 cm-1
154
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
The C=O absorption bands of
oxalates and -keto esters are
in the region of 1755 – 1740 cm-1
The C=O absorption
band of -keto esters is
observed near 1650 cm-1
Little or no interaction
between the two C=O groups
Hydrogen bonding between the
ester C=O and the enolic OH
The C=O absorption of saturated
-lactones (six-membered ring)
occurs in the same region as
straight-chain, unconjugated esters
Unsaturation  to the C=O
reduces its absorption frequency
Unsaturation  to the -Oincreases its absorption frequency
155
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
The CO stretching vibration of esters
actually consists of two asymmetrical
coupled vibrations: CC(=O)O (more
important) and OCC (less important)
Occur in the region of
1300 – 1000 cm-1
The corresponding symmetric
vibrations are of little importance
The CC(=O)O band of saturated
esters, except for acetates, shows
strongly in the 1210 – 1163 cm-1 region
Acetates of saturated alcohols
display this band at ~1240 cm-1
Vinyl and phenyl acetates absorb
in the 1190 – 1140 cm-1 region
156
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
The CC(=O)O stretch of esters
of ,-unsaturated acids
result in multiple bands in
the 1300 – 1160 cm-1 region
The OCC band of esters of
primary alcohols occurs at
about 1064 – 1031 cm-1
Esters of aromatic acids absorb
in the 1310 – 1250 cm-1 region
Esters of secondary alcohols
absorb near 1100 cm-1
The analogous type of stretch
in lactones is observed in
the 1250 – 1111 cm-1 region
Aromatic esters of primary
alcohols show this
absorption near 1111 cm-1
157
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
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Asymmetrical
OCC stretch
1042 cm-1
 (C=O)
1743 cm-1
Acetate
CC(=O)O
stretch
1242 cm-1
158
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.13 Esters and lactones
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
 (C=O)
1765 cm-1
Acetate
CC(=O)O
stretch
1215 cm-1
Asymmetrical
OCC stretch
1193 cm-1
159
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.14 Acid halides
Unconjugated acid chlorides
show C=O stretching absorption
in the 1815 – 1785 cm-1 region
Conjugated acid halides
absorb at a slightly lower
frequency due to resonance
Aromatic acid chlorides
absorb at 1800 – 1770 cm-1
A weaker band near 1750 – 1735 cm-1
appearing in the spectra of aroyl chlorides
probably results from Fermi resonance
between the C=O band and the overtone
of a lower wavenumber band near 875 cm-1
160
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.14 Acid halides
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
 (C=O)
1806 cm-1
161
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.14 Acid halides
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Fermi
resonance
band
1730 cm-1
 (C=O)
1774 cm-1
162
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.15 Carboxylic acid anhydrides
Anhydrides display two
C=O stretching bands
in the carbonyl region
Asymmetrical and
symmetrical stretching
Saturated acyclic anhydrides
absorb near 1818 and 1750 cm-1
Unconjugated straight-chain
anhydrides show C(C=O)O(C=O)C
stretching vibrations near 1047 cm-1
Conjugated acyclic anhydrides
absorb near 1775 and 1720 cm-1
Acetic anhydrides show this
stretching band near 1125 cm-1
163
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.15 Carboxylic acid anhydrides
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C(C=O)O(C=O)C
stretch 1125 cm-1
Asymmetric
 (C=O)
1827 cm-1
Symmetric
 (C=O)
1766 cm-1
164
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
In dilute solution in non-polar solvents
primary amides show two moderately
intense NH stretching frequencies near
3520 and 3400 cm-1
In the spectra of solid samples, these
bands are observed near 3350 and
3180 cm-1 because of hydrogen bonding
Asymmetrical and
symmetrical stretching
165
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
In dilute solutions, secondary
amides show the free NH stretching
vibration near 3500 – 3400 cm-1
In more concentrated solution and in solid
samples, the free NH band is replaced by
multiple bands in the 3330 – 3060 cm-1 region
Multiple bands are observed because
the amide group can bond to produce
dimers with an s-cis conformation and
polymers with an s-trans conformation
Solid lactams show a strong NH
stretching absorption near 3200 cm-1
166
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
The C=O absorption of amides (amide I band)
occurs at lower frequencies than normal
carbonyl absorptions due to the resonance effect
Primary amides have a strong C=O
band in the region of 1650 cm-1
when examined in the solid phase
The position of absorption depends
on the same environmental factors as
the C=O absorption of other compounds
Acetamide absorbs
at 1694 cm-1
In dilute solution the absorption
band is observed near 1690 cm-1
167
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
Open-chain secondary amides show the
C=O absorption band near 1640 cm-1
when examined in the solid state
In dilute solution the absorption
band is observed near 1680 cm-1
or even near 1700 cm-1 (anilides)
The C=O absorption of tertiary amides
occurs in the range of 1680 – 1630 cm-1
The C=O absorption of
lactams with six-membered
rings or larger is near 1650 cm-1
Five-membered ring lactams absorb
in the 1750 – 1700 cm-1 region
168
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
All primary amides show a sharp absorption band in dilute
solution (amide II band) resulting from NH2 bending
at a somewhat lower frequency than the C=O band
In mulls and pellets the
band occurs near 1655 –
1620 cm-1 (may be under the
envelope of the amide I band)
In dilute solutions the band
appears at 1620 – 1590 cm-1
(separated from the amide I band)
Secondary acyclic amides in solid
state display the NH2 bending band
in the region of 1570 – 1515 cm-1
In dilute solutions, the band occurs
in the 1550 – 1510 cm-1 region
169
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
The CN stretching band of primary
amides occur near 1400 cm-1
A broad, medium band in the 800 – 666 cm-1
region in the spectra of primary and secondary
amides results from out-of-plane NH bending
The NH out-of-plane bending in lactams causes
broad absorption in the 800 – 700 cm-1 region
170
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
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Primary amide
s (NH)
3189 cm-1
Primary amide
as (NH)
Amide I band
3363 cm-1
 (C=O)
1662 cm-1
 (CN)
1426 cm-1
Out-of-plane
NH bend
~640 cm-1
Amide II band
NH bend
1634 cm-1
171
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
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Amide I band
 (C=O)
Secondary
1655 cm-1
amide
 (NH)
Out-of-plane
NH bend
~720 cm-1
Amide II band
NH bend
1567 cm-1
172
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.16 Amides and lactams
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
 (C=O)
1640 cm-1
173
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
Primary amines display two
weak absorption bands
Free (dilute solution):
as (NH) ~3500 cm-1
s (NH) ~3400 cm-1
Hydrogen-bonded:
as (NH) 3400 – 3300 cm-1
s (NH) 3330 – 3250 cm-1
In the spectra of liquid primary and secondary
amines, a shoulder usually appears on the lowfrequency side of the NH stretching band
Secondary amines show
a single band in the
3350 – 3310 cm-1 region
Overtone of the NH bending band
intensified by Fermi resonance
174
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
Primary amine
NH stretch
Secondary amine
NH stretch
175
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
The NH bending (scissoring) vibration
of primary amines is observed
in the 1650 – 1580 cm-1 region
Liquid samples of primary
and secondary amines display
medium to strong broad absorption
in the 909 – 666 cm-1 region arising
from out-of-plane NH bending
The NH bending is seldom detectable in
the spectra of aliphatic secondary amines
Secondary aromatic amines show the
NH bending band near 1515 cm-1
176
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
Absorption bands for the unconjugated CN bond
in primary, secondary, and tertiary aliphatic amines
appear in the region of 1250 – 1020 cm-1
Aromatic amines display CN stretching
absorption in the 1342 – 1266 cm-1 region
177
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Primary
amine
as (NH)
3369 cm-1
Primary
amine
s (NH)
3369 cm-1
s (NH)
1617 cm-1
 (CN)
1070 cm-1
Out-of-plane
NH bend
~810 cm-1
178
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.17 Amines
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Secondary
amine
 (NH)
3416 cm-1
 (CN)
1320 cm-1
s (NH)
1510 cm-1
179
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.18 Nitriles
The IR spectra of nitriles are characterized
by absorption band in the triplebond stretching region of the spectrum
Aliphatic nitriles absorb
near 2260 – 2240 cm-1
Conjugation, such as occurs in aromatic nitriles, reduces
the frequency of absorption to 2240 – 2222 cm-1
180
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.18 Nitriles
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 (CN)
2248 cm-1
181
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.19 Nitro compounds
In the nitroalkanes, the two NO stretching
bands occur near 1550 and 1372 cm-1
Asymmetrical and
symmetrical stretching
of the NO2 group
Conjugation lowers the frequency of
both bands, resulting in absorption near
1550 – 1500 and 1360 – 1290 cm-1
Nitroaromatic compounds show a CN
stretching vibration near 870 cm-1
182
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.19 Nitro compounds
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 (CN)
859 cm-1
as (N=O)2
1524 cm-1
s (N=O)2
1347 cm-1
183
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.20 Thiols
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
 (SH)
2554 cm-1
 (CS)
710 cm-1
184
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.21 Organic halogen compounds
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
 (CCl)
711 cm-1
185
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.22 Heteroaromatic compounds
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Aromatic
CH stretch
>3000 cm-1
 (C=C)
 (C=N)
 (NH)
3401 cm-1
Ring CH bend
in-plane
Ring
CH bend
out-of-plane
186
3. IR spectroscopy
3.5 Characteristic group absorptions of organic molecules
3.5.22 Heteroaromatic compounds
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
Aromatic
CH stretch
>3000 cm-1
 (C=C)
 (C=N)
1600 – 1430 cm-1
Ring CH bend
in-plane
Ring CH bend
out-of-plane
784 cm-1 and 704 cm-1
187
4. NMR spectroscopy
4.x Theory and instrumentation
Due to time limitations, NMR theory and instrumentation are not presented in the English material.
188
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
NMR parameters for the
structural analysis of
organic molecules in solution
Signal integrals
Nuclear Overhauser
effect (NOE)
Chemical shift ()
Indirect spinspin coupling (J)
189
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.1 Signal integrals
The area under the signal curve is referred to
as the intensity or the integral of the signal
Important parameter
in 1H NMR
Comparing these integrals in a
spectrum directly gives the ratios
of the protons in the molecule
Integrals cannot be
measured in routine
13C NMR spectra
Integral curve
Integral label
190
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
The chemical shift is the resonance
frequency of a nucleus relative to a
standard in an external magnetic field
It is caused by the magnetic
shielding of the nuclei by their
surroundings, mainly by the electrons
Shielding of nuclei from an external
magnetic field by electrons
Electron(s)
External
magnetic
field, B0
Nucleus
Gives information about the local
structure around the nucleus of interest
The expression for the resonance frequency
in terms of shielding is given by:
0
0
0 = resonance frequency [Hz]
= gyromagnetic ratio [107 rad T-1 s-1]
= shielding constant [dimensionless]
0 = external magnetic field [T]
Small induced magnetic
field opposing the external field
and thus shielding the nucleus
191
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
The exact resonance frequency of the nucleus
depends on the external magnetic field
There is no absolute
chemical shift scale
The compound used as a reference
is usually tetramethylsilane, TMS
A relative scale is used in which the frequency
difference, , between resonance signals of
sample and the reference compound is measured



With the simplification:
The chemical shift, , in parts
per million (ppm) of a given
nucleus in the sample is defined in
terms of the resonance frequency
∆ The -value for the reference compound
TMS is, by definition, zero, since in this
case  = 0; thus:
(TMS) = 0
192
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
-High frequency
-Large chemical shift
-Deshielded
-Low field (down field)
-Low frequency
-Small chemical shift
-Shielded
-High field (up field)
TMS
193
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
Factors affecting the
NMR chemical shift
1H
Inductive effects
Magnetic anisotropy effects
In 13C NMR, intra- and intermolecular
effects, expressed in ppm, are similar
in magnitude to those in 1H NMR
Hydrogen bonding
and solvent effects
Mesomeric (resonance) effects
When 13C NMR chemical shifts are
considered in relation to the total chemical
shift range, these effects are less important
194
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
The electron density around the
nucleus, and therefore also the shielding,
are greatly influenced by substituents
Electronegative substituents reduce
the shielding owing to their electronwithdrawing inductive effect (-I)
Electropositive substituents
cause an increase in shielding
The -value increases
The -value decreases
195
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
Chemical bonds are in general
magnetically anisotropic
Shielding of a nucleus depends
on its geometrical position in
relation to the rest of the molecule
They have different susceptibilities
along the three directions in space
The magnetic moments
induced by an external
magnetic field B0 are not
equal for different directions
It is possible to define a double cone around the
magnetically anisotropic group separating regions
of positive and negative shielding contributions
196
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
+
+
+
H C C H
C=C bond
Ethene protons are deshielded
 = 5.28 ppm
-
-
C C
C≡C bond
Acetylene protons are shielded
 = 2.88 ppm
+
+
-
C O
-
+
C=O bond
Aldehydic protons are deshielded
  9 – 10 ppm
+
The axial proton
is more shielded
than the equatorial
(  0.1 – 0.7 ppm)
-
C
C
H
H
-
+
197
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
An induced ring current effect is observed
when an aromatic molecule with its delocalized
-electrons is placed in a magnetic field
B0
H
The ring current generates an additional magnetic field,
whose lines of force at the center of the aromatic ring are
in the opposite direction to the external magnetic field, B0
+
Leads to regions of increased and reduced
shielding in the vicinity of the aromatic molecule
-
H
H
-
The 1H NMR signal of the protons in benzene
is found at  = 7.27 ppm (ethylene  = 5.28)
In 13C NMR the ring current effect is less important; the large
chemical shift  = 128.5 ppm is explained by a different mechanism
+
198
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
Mesomeric effects (resonance
effects) are a property of substituents
or functional groups in a molecule
Examples of -M substituents
Describe the electron donating or
withdrawing properties of substituents
based on resonance structures
Negative when the substituent
is an electron-withdrawing
group  deshielding (-M)
Positive when the substituent
is an electron-donating
group  shielding (+M)
Examples of +M substituents
199
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.2 Chemical shift
Hydrogen bonding
Protons attached to oxygen by hydrogen
bonds have particularly low shielding values
Solvent effects
If the dissolved substance interacts
with the solvent this shows up as a
shift in the positions of the signals
The -values can be greater
than 10 ppm in many cases
The exact -values cannot be specified,
as the signal positions depend on the
temperature and the concentration
Effects are clearly seen when the spectrum
is measured in turn in a non-polar solvent
(CCl4), in polar solvent (DMSO), and in a
magnetically anisotropic solvent (benzene)
200
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.3 1H NMR chemical shifts
1H
phenol -OH
acid -COOH
H
H
NMR chemical shifts
alcohol -OH
thioalcohol -SH
amine -NH2
H
aldehyde -CHO
N
N
arene -H
alkene -CH=
alkene =CH2
-CH-O-CH2-OH3C-Oalkyne -C≡CH
H3C-NH3C-SPh-CH3
-(C=O)-CH2-(C=O)-CH3
-CH2H3C-C=
halogen-C-CH3
cyclopropyl -CH2H3C-M (Si, Li, Al, Ge,...)
12
11
10
9
8
7
6
 [ppm]
5
4
3
2
1
0
201
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.4 1H NMR chemical shifts of alkanes and cycloalkanes
The main influence on the chemical shift of
protons in alkanes is that due to substituents
Inductive effects decrease with the distance
between the substituents and the observed nucleus
CH3Cl
: 3.05
CH3CH2Cl
1.42
CH3CH2CH2Cl
1.04
For multiple substitution the influence of
each additional substituent is slightly less
CH4
CH3Cl
CH2Cl2
CHCl3
: 0.23
3.05
5.33
7.26
:
2.82
2.28
1.93
For a given substituent, X, CH3 protons
are usually more strongly shielded than
CH2 protons, and these in turn are
more strongly shielded than CH protons
(CH3)2CHCl
:
4.13
CH3CH2Cl
3.51
CH3CH2CH2Cl
3.05
202
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.4 1H NMR chemical shifts of alkanes and cycloalkanes
Table 6. 1H NMR chemical shifts of
CH3 protons for different substituents X
Table 7. 1H NMR chemical
shifts of cycloalkanes
X
(X-CH3)
E X*
Compound

Li
-1
1.0
Cyclopropane
0.22
R3Si
0
1.8 (Si)
Cyclobutane
1.94
H
0.4
2.2
Cyclopentane
1.51
CH3
0.8
2.5 (C)
Cyclohexane
1.44
NH2
2.36
3.0 (N)
Cycloheptane
1.54
OH
3.38
3.5 (O)
Cyclooctane
1.54
I
2.16
2.5
Br
2.70
2.8
Cl
3.05
3.0
F
4.25
4.0
COOH
2.08
NO2
4.33
*EX: electronegativities according to
Pauling
203
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.4 1H NMR chemical shifts of alkanes and cycloalkanes
c
a
b
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204
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.5 1H NMR chemical shifts of alkenes
In the case alkenes, the effects
of substituents can be
inductive, mesomeric or steric
Table 8. 1H NMR chemical shifts of
monosubstituted ethenes
X
(H1)
gem
(H2)
trans
(H3)
cis
H
5.28
5.28
5.28
CH3
5.73
4.88
4.97
C6H5
6.72
5.20
5.72
F
6.17
4.03
4.37
Cl
6.26
5.39
5.48
Br
6.44
5.97
5.84
I
6.53
6.23
6.57
OCH3
6.44
3.88
4.03
OCOCH3
7.28
4.56
4.88
CHO
6.37
6.52
6.35
NO2
7.12
5.87
6.55
205
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.5 1H NMR chemical shifts of alkenes
-M effect
+M effect
206
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.5 1H NMR chemical shifts of alkenes
a
b
c
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
207
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.6 1H NMR chemical shifts of alkynes
The region where the chemical
shifts of acetylenic protons are
found (  2 – 3 ppm) overlaps
with those of many types of protons
The chemical shifts of acetylenic protons
depend on the substituent electronegativities,
on the conjugation, and on the solvent
208
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.6 1H NMR chemical shifts of alkynes
a
b
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209
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.7 1H NMR chemical shifts of arenes
In aromatic compounds the shielding is determined
mainly by mesomeric effects of the substituents
For benzene
 = 7.27 ppm
In nitrobenzene, all
protons are less shielded
than those in benzene.
The protons in ortho and
para positions are less
shielded than the meta
(-M effect).
In aniline, all protons are
more strongly shielded
than those in benzene.
The protons in ortho and
para positions are more
shielded than the meta
(+M effect).
210
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.7 1H NMR chemical shifts of arenes
c
a
b
d
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211
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.8 1H NMR chemical shifts of aldehydes
The signals of aldehydic protons RCHO
can be recognized from their characteristic
position in the region  = 9 – 11 ppm
Typically the substituent
effects are quite small
212
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.8 1H NMR chemical shifts of aldehydes
c
a
b
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
213
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.9 1H NMR chemical shifts of protons in OH, NH, and SH groups
The protons in OH, NH, and SH groups
form are subject to considerable variation
Table 9. Chemical shift range
for OH, NH, and SH protons
Compound type
 (H)
OH: alcohols
phenols
acids
enols
1–5
4 – 10
9 – 13
10 - 17
NH: amines
amides
amido groups
in peptides
1–5
5 – 6.5
7 – 10
SH: thiols
aliphatic
aromatic
1 – 2.5
3–4
They form hydrogen bonds, undergo exchange,
and have varying degrees of acidic character
Hydrogen exchange can cause
broadening of the signals
Chemical shifts are further influenced
by concentration, temperature,
solvent, and impurities such as water
The measured -values are only reproducible
under well-defined experimental conditions
214
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.9 1H NMR chemical shifts of protons in OH, NH, and SH groups
c
b
a
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215
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.9 1H NMR chemical shifts of protons in OH, NH, and SH groups
c
b
a
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216
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.10
13C
NMR chemical shifts
ketone -C=O
nitrile -C≡N
aldehyde -CHO
13C
alkyne -C≡C-
acid -COOH
alkene -C=Camide, ester -CON, -COOheteroaromatic
N
thioketone S=C-
NMR chemical shifts
arene
azomethine -N=C-
-C-O-
quaternary -C-
-N-C
cyclopropyl -C-C-Shalogen-Ctertiary -CH-O-CH-HC-N-S-CHhalogen-CHsecondary -CH2-O-CH2-N-CH2-H2C-Shalogen-CH2primary -CH3
-O-CH3
-N-CH3
-S-CH3
halogen-CH3
230
210
190
170
150
130
110
 [ppm]
90
70
50
30
10
217
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.11
13C
NMR chemical shifts of alkanes and cycloalkanes
In alkanes the chemical shift of a particular
13C nucleus depends on the number of
neighboring carbon atoms at the - and positions and on the degree of branching
Iodine is an exception
 heavy atom effect
The -effect causes the 13C nuclei to be
more strongly shielded  steric interactions
Substituents have considerable influence on
the chemical shifts  -, -, and -effects
The -effect increases with increasing
electronegativity of the substituent
The -effect is much smaller,
and is always a deshielding effect
218
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.11
13C
NMR chemical shifts of alkanes and cycloalkanes
Table 10. 13C NMR chemical
shifts of alkanes
Table 11. 13C NMR chemical shifts of
propane derivatives: XCH2-CH2-CH3
Table 12. 13C NMR chemical
shifts of cycloalkanes
X
(C)
(C)
(C)
Compound

-2.3
H
16.1
16.3
16.1
Cyclopropane
-2.8
H3C-CH3
6.5
CH3
24.9
24.9
13.1
Cyclobutane
22.4
CH2(CH3)2
16.1
16.3
NH2
44.6
27.4
11.5
Cyclopentane
25.8
(H3C-CH2)2
13.1
24.9
OH
64.9
26.9
11.8
Cyclohexane
27.0
CH(CH3)3
24.6
23.3
NO2
77.4
21.2
10.8
Cycloheptane
28.7
C(CH3)4
27.4
31.4
F
85.2
23.6
9.2
Cl
46.7
26.0
11.5
Br
35.4
26.1
12.7
I
9.0
26.8
15.2
Compound
(C1)
CH4
(C2)
219
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.11
13C
NMR chemical shifts of alkanes and cycloalkanes
3
2
1
4
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220
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.12
13C
NMR chemical shifts of alkenes
The 13C resonances of alkene
carbons are found in a broad
range of   100 – 150 ppm
Table 13.
13C
The chemical shifts are affected by alkyl
substituents an by substituents with widely
differing inductive and mesomeric properties
NMR chemical shifts of alkenes
Compound
(C1)
H2C1=C2H2
123.5
H3C3C1H=C2H2
133.4
H3CCH=CHCH3 (cis)
124.2
11.4
H3CCH=CHCH3 (trans)
125.4
16.8
(H3C)2C=CH2
141.8
Cyclohex-1-ene
127.4
(C2)
(C3)
115.9
19.9
111.3
24.2
For alkyl-substituted ethenes the double
bond carbons carrying the substituents are
less shielded (  120 – 140 ppm) than those
at the terminal position (  105 – 120 ppm)
25.4
(C4: 23.0)
221
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.12
13C
NMR chemical shifts of alkenes
Table 14. 13C NMR chemical
shifts of monosubstituted ethenes
With exceptions of Br, CN and I, the substituent
effects reduce the C1 shielding and increase the C2
shielding compared with ethene  inductive effects
CH3O, CH3COO, and CHO as substituents show
very large -effects  mesomeric effects
X
(C1)
(C2)
H
123.5
123.5
CH3
133.4
115.9
CH=CH2
137.2
116.6
C6H5
137.0
113.2
F
148.2
89.0
Cl
125.9
117.2
Br
115.6
122.1
I
85.2
130.3
OCH3
153.2
84.1
OCOCH3
141.7
96.4
NO2
145.6
122.4
CN
108.2
137.5
CHO
138.5
138.0
222
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.12
13C
NMR chemical shifts of alkenes
-M effect
+M effect
223
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.12
13C
NMR chemical shifts of alkenes
2
1
3
4
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
224
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.13
13C
NMR chemical shifts of alkynes
3
Table 15. 13C NMR chemical
shifts of monosubstituted
acetylenes: HC1≡C2X
X
(C1)
H
71.9
Alkyl
68.6
84
HC≡C
64.7
68.8
C6H5
77.2
83.6
CH3CH2O
23.4
89.6
(C2)
2
1
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
225
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.14
13C
NMR chemical shifts of arenes
The 13C signals of benzene, alkyl-substituted
benzenes, fused ring arenes, and annulenes are
found within the narrow range of  = 120 – 140 ppm
Signals of substituted arenes
can be expected anywhere within
the range  = 100 – 150 ppm
Table 16. 13C NMR chemical shifts
of monosubstituted benzenes
X
(C1)
H
128.5
Li
(C2)
(C3)
(C4)
186.6
143.7
124.7
133.9
CH3
137.7
129.2
128.4
125.4
COOH
130.6
130.1
128.4
133.7
F
163.3
115.5
131.1
124.1
OH
155.4
115.7
129.9
121.1
NH2
146.7
115.1
129.3
118.5
NO2
148.4
123.6
129.4
134.6
I
94.4
137.4
131.1
127.4
The chemical shifts depend mostly
on the inductive and mesomeric
properties of the substituents
226
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.14
13C
NMR chemical shifts of arenes
In aromatic heterocycles the shielding
of the ring carbon nuclei are essentially
determined by the heteroatom
227
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.14
13C
NMR chemical shifts of arenes
2
3
4
1
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
228
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.15
13C
NMR chemical shifts of carbonyl and carboxy compounds
Table 17. 13C NMR chemical shifts of aldehydes
and ketones
The 13C resonances of C=O carbons in aldehydes
and ketones are in the range of   190 – 220 ppm
With increasing alkyl substitution the
carbonyl shieldings are further reduced
Conjugation with an unsaturated
moiety increases the shielding
Compound
(C1)
(C2)
H3C2-C1HO
200.5
31.2
H3CCH2-CHO
202.7
36.7
5.2
(CH3)2CH-CHO
204.6
41.1
15.5
(CH3)3C-CHO
205.6
42.4
23.4
H2C=CH-CHO
193.3
136.0
136.4
C6H5-CHO
191.0
CH3C2OC1H3
30.7
206.7
C4H3C3H2C2OC1H3
27.5
206.3
35.2
7.0
(CH3)2CHCOCH3
27.5
212.5
41.6
18.2
(CH3)3CCOCH3
24.5
212.8
44.3
26.5
(CH3)3CC3OC2(CH3)3
28.6
45.6
218.0
Ph-CO-Ph
195.2
Cl3C1C2OCCl3
90.2
175.5
H2C1=C2HC3OCH3
128.0
137.1
(C3)
197.5
(C4)
25.7
229
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.15
13C
NMR chemical shifts of carbonyl and carboxy compounds
2
4
5
3
1
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
230
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.15
13C
NMR chemical shifts of carbonyl and carboxy compounds
The 13C resonances of C=O carbons in monocarboxylic acids
and their derivatives are in the range of   160 – 180 ppm
Table 18. 13C NMR chemical shifts of acetic acid
derivatives
The carboxylate ion formation in alkaline
solutions causes reduction in shielding
of the C=O carbons, and also in those of
the 13C nuclei at the -, -, and -positions
The shielding in amides, acyl
halides, esters, and anhydrides is
greater than in the parent acids
Compound
(C1)
(C2)
C2H3C1OOH
176.9
20.8
(pD 1.5)*
CH3COO-
182.6
24.5
(pD 8)*
CH3CON(CH3)2
170.4
21.5
CH3: 35.0 and 38.0
CH3COCl
170.4
33.6
CH3COOCH3
171.3
20.6
OCH3: 51.5
CH3COOCH=CH2
167.9
20.5
=CH: 141.5
=CH2: 97.5
(CH3CO)2O
167.4
21.8
CH3COSH
194.5
32.6
*Solvent: D2O
231
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.15
13C
NMR chemical shifts of carbonyl and carboxy compounds
If the hydrogen atoms of the CH3 group in
acetic acid are replaced by methyl groups, the
C=O carbon signal appears at a higher -value
The chemical shifts of amino
acids are strongly pH-dependent
Carboxylic acids form dimers by hydrogen
bonding  chemical shift changes
of several ppm are generally found
Where the C=O group undergoes
conjugation with an unsaturated group, the
-value is smaller than that for acetic acid
Table 19. 13C NMR chemical shifts of
-substituted acetic acids
Compound
(C1)
(C2)
H-C2H2C1OOH
175.7
20.3
C3H3C2H2C1OOH
179.8
27.6
9.0
(CH3)2CHCOOH
184.1
34.1
18.1
(CH3)3CCOOH
185.9
38.7
27.1
H2NCH2COOH (D2O)
pD 0.45
pD 12.05
171.2
182.7
41.5
46.0
HOCH2COOH (D2O)
177.2
60.4
ClCH2COOH
173.7
40.7
Cl3CCOOH
167.0
88.9
H2C3=C2HC1OOH
168.9
129.2
C6H5COOH
168.0
(C3)
130.8
232
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.1.15
13C
NMR chemical shifts of carbonyl and carboxy compounds
2
3
4
1
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
233
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Neighboring magnetic dipoles in a molecule interact with each
other through chemical bonds  indirect spin-spin coupling
Affects the magnetic field at the
position of the nuclei being observed
The effective field is stronger or weaker than
it would be in the absence of the coupling
Alters the resonance frequencies 
causes splitting of the NMR signals
234
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Coupling constants are independent of
the external magnetic field strength, B0
1J
= coupling between the nuclei of atoms
directly bonded to each other (one bond)
2J = geminal coupling (two bonds)
3J = vicinal coupling (three bonds)
3+nJ = long-range coupling (more than three bonds)
Coupling constants between protons
are denoted by J(H,H), those between
13C nuclei by J(C,C), and between
protons and 13C nuclei by J(C,H)
The number of bonds between
the coupled nuclei is indicated by
a superscript preceding the J
The unit of J is Hz
Couplings between equivalent
nuclei cannot be observed
In practice the most important types of
couplings are those between protons
235
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
A spectrum which contains only singlets
is said to be a zero-order spectrum
If the frequency interval  between the coupled nuclei is large compared
with the coupling constants J, i.e.  >> J, the spectrum is first-order
If the first-order condition is not
satisfied a spectrum is described as
being of second-order (higher-order)
In second-order spectra the couplings
between equivalent nuclei also become evident
236
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
The values of coupling constants vary over quite wide ranges
 the internuclear distances alone cannot account for the values
The majority of spectra
are analyzed by
first-order methods
The factors influencing coupling constants include:
•
•
•
•
•
•
The hybridization of the atoms involved in the coupling
Bond angles and torsional angles
Bond lengths
The presence of neighboring -bonds
Effects of neighboring electron lone-pairs
Substituent effects
Gives the absolute
magnitudes of the
coupling constants
Table 20. General summary of the orders of magnitude and signs of
H,H, C,H, and C,C coupling constants
J(H,H) [Hz]
Sign
J(C,H) [Hz]
Sign
J(C,H) [Hz]
Sign**
1J
276*
+
125 – 250
+
30 – 80
+
2J
0 – 30
-
-10 – +20
+/-
< 20
+/-
3J
0 – 18
+
1 – 10
+
0–5
+
3+nJ
0–7
+/-
<1
+/-
<1
+/-
*For H2
**Determined in only a few cases
Sufficient for most problems
237
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Several nuclei coupled together
constitutes a spin system
If the molecule contains several spin systems that are
not coupled to each other, the spectrum can be divided
into partial spectra which are mutually independent
Chemically non-equivalent nuclei
are identified by different letters
of the alphabet, beginning with A
Depending on the type of spectrum the labeling follows
the order of their signals from left to right in the spectrum
Chemically equivalent nuclei
are labelled with the same letter
and the number of equivalent
nuclei is added as a subscript
In cases where  between the coupled nuclei
is much greater than J, they are represented
by letters that are well separated in the alphabet
238
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Chemical equivalence
Magnetic equivalence
Two nuclei i and k are chemically
equivalent if they have the same
resonance frequency, i.e. i = k
Two nuclei i and k are magnetically equivalent if:
• They are chemically equivalent (i = k)
• For all couplings of the nuclei i and k to other
nuclei such as l in the molecule, the relationship
Jil = Jkl is satisfied
2
In 4-nitrophenol the protons H2 and H6 are
chemically equivalent but not magnetically
equivalent, since the coupling constants
3J(H2,H3) and 5J(H6,H3) are unequal, as
are 3J(H6,H5) and 5J(H2,H5)  the correct
description of the spectral type is AA’XX’.
When two or more nuclei are chemically
equivalent but not magnetically equivalent,
the same letter is repeated with primes (’)
239
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Doublet
Coupling to one neighboring
nucleus (AX spin system)
Doublet
HA
HX
3J(H
HA
A,HX)
= 3.66 Hz
HX
9,2700
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
9,2600
9,2500
5,8200
5,8100
5,8000
5,7900
240
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Doublet
HX
Coupling to two equivalent neighboring
nuclei (AX2 spin system)
Triplet
HA
3J(H
A,HX)
HX
3J(H
A,HX)
= 1.70 Hz
HA
9,6350
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
9,6300
9,6250
9,6200
9,6150
4,0900
4,0850
4,0800
4,0750
4,0700
241
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Doublet
HX
Coupling to three or more equivalent
neighboring nuclei (AXn spin system)
Quartet
HA
3J(H
A,HX)
HX
3J(H
A,HX)
= 2.90 Hz
HA
9,8100
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
9,8000
9,7900
9,7800
9,7700
2,2200
2,2100
2,2000
2,1900
242
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
The number of lines in a multiplet,
which is called the multiplicity M,
can be calculated from the equation:
For couplings to nuclei with I = ½ the signal
intensities within each multiplet correspond
to the coefficients of the binomial series, which
can be obtained from Pascal’s Triangle.
M = 2nI + 1
n = the number of equivalent nuclei
I = spin
For nuclei with I = ½ (e.g. 1H and
equation simplifies to:
M=n+1
Number of peaks (n + 1):
1
2
3
4




singlet (s)
doublet (d)
triplet (t)
quartet (q)
multiplet (m)
13C),
n
n
n
n
n
n
n
=
=
=
=
=
=
=
0
1
2
3
4
5
6
1
1
1
3
1
2
1
3
1
1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
243
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
HA ( = 6.692)
J AM
Coupling between three
non-equivalent nuclei
(AMX spin system)
3J(H
Doublet of
doublets (dd)
J AX
A,HM)
= 17.6 Hz
3J(H ,H ) = 10.9 Hz
A
X
2J(H ,H ) = 1.0 Hz
M
X
Hbenzene
HM ( = 5.737)
dd
HX ( = 5.226)
dd
HM
HA
HX
SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
J AM
J MX
J AX
J MX
244
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
JAM = 10 Hz
JAX = 5 Hz
JMX = 2 Hz
Triplet of
quartets (tq)
AMX spin system in which:
HA = one 1H
HM = two equivalent 1H
HX = three equivalent 1H
Doublet of
triplets (dt)
Doublet of
quartets (dq)
HA ( = 6)
HM ( = 4)
JAM
HX ( = 2)
JAX
JAM
JMX
JMX
JAX
245
4. NMR spectroscopy
4.1 NMR parameters for the structural analysis
4.2 Indirect spin-spin coupling
Doublet of doublet
of doublets (ddd)
JAK = 12 Hz
JAM = 7 Hz
JAX = 2 Hz
HA ( = 6)
AKMX spin system in which:
HA – Hx = one 1H
(only HA and HK are shown)
ddd
HK ( = 4)
JAK = 12 Hz
JKM = 9 Hz
JKX = 5 Hz
JAK
JAK
JAM
JAX
JKM
JKX
246
4. NMR spectroscopy
4.2 Relationship between the spectrum and the structure
4.2.1 Equivalence, symmetry and chirality
Equivalent nuclei have the
same resonance frequency
Couplings between equivalent nuclei
cannot be observed in first-order spectra
Equivalence can result not only from molecular symmetry, but also
from conformational mobilities such as rotations or inversions
The greater the extent of equivalence
between the nuclei in a molecule, the simpler
will be its spectrum and fewer lines in it
Three hydrogen nuclei
in methyl group are
always equivalent
In benzene, all the 1H
nuclei and all the 13C
nuclei are equivalent  only
one signal in each spectrum
NMR spectra of two
enantiomers are
exactly the same
247
4. NMR spectroscopy
4.2 Relationship between the spectrum and the structure
4.2.1 Equivalence, symmetry and chirality
13C
1,2-dichlorobenzene  three signals
1,3-dichlorobenzene  four signals
1,4-dichlorobenzene  two signals
1H
1,2-  symmetrical spectrum (AA’BB’)
1,3-  three chemically different protons (AB2C)
1,4-  one signal
13C
1,1-, cis- and trans-dichlorocyclopropane
 two signals
1H
1,1-  one signal
cis-  a plane of symmetry  three sorts of protons
trans-  C2 axis of symmetry  two sorts of protons
248
4. NMR spectroscopy
4.2 Relationship between the spectrum and the structure
4.2.2 Homotopic, enantiotopic and diastereotopic groups
Homotopic CH2 protons are equivalent, and
therefore give one signal in the spectrum
The two protons in methylene chloride,
CH2Cl2, are homotopic because the
molecule has a two-fold (C2) axis of symmetry
C2
In some CH2 groups the protons only
appear to be equivalent, even though
they would exchange positions if reflected
in a plane (mirror symmetry plane)
The substituents in bromochloromethane
are arranged in the pro-R configuration if
looking from H1 towards the carbon, whereas
viewed from H2 the order of the substituents
is reversed being in the pro-S configuration
The CH2 protons in most ethyl groups
are equivalent as a result of rapid rotation
If one of the hydrogen atoms is
replaced by deuterium, two
enantiomers would be formed 
enantiotopic CH2 protons 
indistinguishable in the 1H NMR
spectrum  prochiral compound
249
4. NMR spectroscopy
4.2 Relationship between the spectrum and the structure
4.2.2 Homotopic, enantiotopic and diastereotopic groups
If the two hydrogen atoms of a CH2 group cannot
be imagined to exchange positions either by rotation
about an axis of symmetry or by reflection in a plane
of symmetry, they are said to be diastereotopic
If proton HA or HB in the CH2
group in 1,2-propandiol is replaced
by a substituent R, two different
diastereomers would be obtained
HA and HB are attached to a prochiral
center  they are not equivalent
HA and HB always give separate signals,
expect in cases where the resonance
frequencies are accidentally equal (isochronous)
Even rapid rotation around the C1-C2
bond does not make them equivalent
250
4. NMR spectroscopy
4.2 Relationship between the spectrum and the structure
4.2.2 Homotopic, enantiotopic and diastereotopic groups
The nearest and second nearest neighbors for HA:
OH , CH3 / H , H (1)
OH , OH / H , CH3 (2)
OH , H / H , OH (3)
The nearest and second nearest neighbors for HB:
H , H / OH , OH (4)
H , CH3 / OH , H (5)
H , OH / OH , CH3 (6)
All six environments are different. If the rotation
around the central C-C bond were frozen out the
chemical shifts observed for HA would be 1 in
rotamer I, 2 in rotamer II, and 3 in rotamer III,
while for HB the corresponding -values would be 4,
5, and 6. However, as very fast rotation occurs at
room temperature, these -values become averaged:
A = I1 + II2 + III3
B = I4 + II5 + III6
I, II and III are the weightings (i.e. mole fractions).
Even in the case when all three rotamers are present
in equal amounts (I + II + III = 1/3),A andB
are not equal unless they coincide accidentally.
CH2 groups or C(CH3)2 groups are
always non equivalent when the
molecule contains an asymmetrical atom,
i.e. when the molecule is chiral
251
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.1 Geminal couplings 2J(H,H)
See pages 291 – 293 in the Finnish material.
252
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
Compound
3J(H,H)
Range*
[Hz]
Typical
value*
Cyclopropane
cis
trans
6 – 10
3–6
8
5
Cyclobutane
cis
trans
6 – 10
5–9
-
Cyclohexane
a,a
a,e
e,e
6 – 14
3–5
0–5
9
3
3
Benzene
ortho
6 – 10
9
Pyridine
2,3
3,4
5–6
7–9
5
8
H-C-C-H
0 – 12
7
=CH-CH=
9 – 13
10
5 – 14
11 – 19
10
16
>CH-CHO
1–3
3
=CH-CHO
5–8
6
CH-NH**
4–8
5
CH-OH**
4 – 10
5
CH-SH**
6–8
7
-CH=CH2
cis
trans
Table 21. Ranges and typical values
of vicinal H,H coupling constants
The factors influencing 3J(H,H) are:
• The torsional or dihedral angle
• The substituents
• The distance between the two carbon
atoms concerned
• The H-C-C bond angle
*All values are positive
**Not exchanging
253
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
The Karplus equation, named
after Martin Karplus, describes the
correlation between 3J-coupling
constants and dihedral torsion angles
= A cos2 + B cos + C, where 
is the dihedral angle, and A, B, and C are
empirically derived parameters whose values
depend on the atoms and substituents involved
Graph of the Karplus relation
The coupling constants are largest for
 = 0 or 180, and smallest for  = 90
3J()
Used for determining the conformations
and configurations of ethane derivatives
and saturated six-membered rings
Applied also when the coupling is
transmitted through N, O, or S atoms,
provided that no exchange of
protons attached to heteroatoms occurs
https://de.wikipedia.org/
wiki/Karplus-Beziehung
254
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
The 3J(H,H) coupling constants
for ethane derivatives, e.g. in
ethyl groups, are usually about 7 Hz
Corresponds to an averaged coupling,
since at room temperature there is rapid
exchange between the different rotamers
In rotamers I and III there is a gauche coupling (3Jg) with  = 60, while in rotamer II there is a trans coupling (3Jt)
with  = 180  3Jg  3 – 5 Hz and 3Jt  10 – 16 Hz. If all rotamers are involved in the equilibrium in equal amounts,
fast rotation results in a vicinal coupling constant which is the arithmetic average:
3J
= 1/3(2 × 3Jg + 3Jt)  7 Hz
However, if the three rotamers have different energies the position of the equilibrium is determined by the energy
difference between them:
3J
= I Jg + II Jt + III Jg
where  represents the mole fraction of each rotamer. If the spectrum is recorded at different temperatures the
equilibrium ratios are shifted, and in such cases the observed coupling constant 3J is temperature-dependent.
60
180
60
255
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
The preferred conformation of
the six-membered rings, e.g.
cyclohexane, is the chair form
A distinction can be made between
the axial (a) and equatorial (e)
positions of the hydrogen atoms
Depending on the relative positions
of the coupled hydrogen nuclei three
different vicinal couplings are thus
possible: 3Jaa , 3Jae and 3Jee
 = 180
3J
aa  7 – 9 Hz
 = 60
3J
ae  2 – 5 Hz
 = 60
3J
ee  2 – 5 Hz
3J
In cyclopropane and its
derivatives, the 3J values
for cis proton pairs are usually
6 – 10 Hz, while those for
trans proton pairs 3 – 6 Hz
cis
 6 – 10 Hz
3J
trans
 3 – 6 Hz
256
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
Electronegative substituents reduce the 3J
coupling constant, but the effect is not very large
The relationship between coupling constants and
the differences between the electronegativities
EX of the substituents and the value EH for
hydrogen can be described by a simple equation
3J(H,H)
= 8.0 - 0.8(EX - EH)
Table 22. Vicinal H,H coupling constants
in monosubstituted ethanes: X-CH2-CH3
X
3J(H,H)
[Hz]
E X*
Li
8.4
1.0
H
8.0
2.2
CH3
7.3
2.5
Cl
7.2
3.0
OR
7.0
3.5
*EX: electronegativities according to
Pauling
Usually only an averaged vicinal coupling is
observed owing to rotation about the CC bond
257
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
In ethylene and ethylene derivatives
the coupling between cis proton pairs is
smaller than that between trans proton pairs
Both these types of couplings are greatly affected
by substituents, and become smaller as the
electronegativities of the substituents increase
Table 23. Vicinal H,H coupling constants
in monosubstituted ethylenes
X
3J
cis [Hz]
3J
trans [Hz]
Li
19.3
23.9
1.0
H
11.6
19.1
2.2
Cl
7.3
14.6
3.0
OCH3
7.1
15.2
3.5
F
4.7
12.8
4.0
*EX: electronegativities according to
Pauling
3J
E X*
3J
cis
trans
3J
= 6 – 14 Hz
(usually 10 Hz)
cis
3J
= 14 – 20 Hz
(usually 16 Hz)
trans
The dependence of the cis and
trans coupling constants on
substituent electronegativity is
empirically described by equations
3J
cis = 11.7 3J
trans = 19.0
4.7(EX - EH)
- 3.3(EX - EH)
258
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.2 Vicinal couplings 3J(H,H)
In small strained cycloalkene rings the
cis vicinal couplings remain significantly
smaller than the normally expected value
The vicinal coupling to an aldehyde
proton is relatively small  for
acetaldehyde 3J = 2.9 Hz and for
propionaldehyde 1.4 Hz
In seven-membered rings the coupling
increase to the expected 10 Hz  the
H-C-C bond angle has an important effect
Table 24. Vicinal H,H cis coupling
constants in cycloalkenes
2.92 Hz
Compound
3J
Cyclopropene
1.3
Cyclobutene
3.0
Cyclopentene
5.0
Cyclohexene
9.0
Cycloheptene
10.0
1.4 Hz
cis [Hz]
259
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.3 H,H couplings in aromatic compounds
In benzene and its derivatives the ortho (3J),
meta (4J), and para (5J) couplings are different
It is possible to determine the
arrangement of the substituents
Table 25. H,H coupling constants in
benzene derivatives
J [Hz]
Benzene
Derivatives
J(ortho)
7.5
7–9
large
J(meta)
1.4
1–3
small
J(para)
0.7
<1
small
For naphthalene two different
ortho coupling constants are
found: 3J(1,2) = 8.3 Hz and
3J(2,3) = 6.9 Hz (J(1,3) = 1.2 Hz
and J(1,4) = 0.7 Hz)
The difference between
the ortho couplings is
probably due mainly to
the different bond lengths
260
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.3 H,H couplings in aromatic compounds
In heteroaromatics the coupling
constants depend on the electronegativity
of the heteroatom, the bond lengths, and
the charge distribution in the molecule
In pyridine and its derivatives the ortho
coupling is, as in benzene, considerably
larger than the meta and para couplings
It is usually possible from the couplings
and the chemical shifts to completely
determine the pattern of substitution
Table 26. H,H coupling constants in pyridine
and pyridine derivatives
J [Hz]
Pyridine
Derivatives
J(ortho)
2,3
3,4
4.9
7.7
5–6
7–9
large
J(meta)
2,4
3,5
2,6
1.2
1.4
-0.1
1–2
1–2
0–1
small
J(para)
2,5
1.0
0–1
small
In five-membered aromatic heterocycles
such as furan, thiophene, and pyrrole the
differences between the ortho, meta, and
para couplings are much smaller  not
always lead to structural assignments
261
4. NMR spectroscopy
4.3 H,H coupling constants and chemical structure
4.3.4 Long-range couplings
See pages 302 – 303 in the Finnish material.
262
4. NMR spectroscopy
4.4 1D NMR
4.4.1 DEPT
See pages 341 – 343 in the Finnish material.
263
4. NMR spectroscopy
4.5 2D NMR
4.5.1 COSY
See pages 348 – 350 in the Finnish material.
264
4. NMR spectroscopy
4.5 2D NMR
4.5.2 HMQC and HSQC
See pages 357 – 359 in the Finnish material.
265
4. NMR spectroscopy
4.5 2D NMR
4.5.3 HMBC and CIGAR-HMBC
See pages 360 – 362 in the Finnish material.
266