Nuclear Magnetic Resonance
Proton NMR
Theory
 An atomic nucleus can be viewed as a
spinning, positively charged sphere.
 If the nucleus has an odd mass number or
atomic number, it generates a magnetic
field just like a spinning bar magnet
would.
Theory
 When placed in the magnetic field generated
by a larger bar magnet, a small bar magnet
can either align with the field (lower energy)
or against the field (higher energy).
Theory
 In the absence of a magnetic field,
the spins of a group of nuclei are
randomly oriented.
 When a magnetic field is applied, the
nuclei align either:
 with the field
 a spin state
 lower energy
 against the field
 b spin state
 higher energy
Theory
 The difference in energy (DE)
between the spin states
depends on both the strength
of the magnetic field and the
identity of the nuclei.
 Absorption of a photon with
the correct energy causes the
nuclei to flip between the a
and b spin states
Theory
 When the nucleus is subjected to the right
combination of both magnetic field and the
right frequency of electromagnetic radiation,
the spin of the nucleus changes.
 a state  b state
 “in resonance”
 NMR spectrometer detects the absorption
(or emission) of energy at a specific
magnetic field.
Simple NMR Spectrometer
RF is held constant,
most common u = 60,
100 or 300 MHz
Strength of magnetic
field is varied.
Theory
 The nuclei present in an organic compound
are not “naked”; they are surrounded by
electrons.
 The electrons generate a small induced
magnetic field that opposes the external
field and partially “shields” the nuclei from
the applied magnetic field.
 Partially “shielded”
 The magnetic field experienced by the
nucleus is weaker than the applied
magnetic field.
Theory
 In order for “resonance” to occur (i.e. in
order for the nucleus to flip from one spin
state to the other), the strength of the
applied magnetic field must be increased in
order for the shielded proton to “flip” at
the same frequency.
Theory
 Protons in an organic molecule are shielded
by different amounts due to differences in
their chemical environment.
More shielded
Absorbs at
higher field
H
H C O
H
H
Less shielded due to high
electronegativity of O;
Absorbs at lower field
Theory
 Proton NMR Spectrum of Methanol
UPFIELD
More Shielded
Lower chemical shift
DOWNFIELD
Less shielded
Higher chemical shift
Information from NMR Spectrum
 Number of signals
 Number of different kinds of protons
 Location of signals
 Chemical environment of protons
 Degree of shielding or deshielding
 Intensity of signal
 Number of protons of a particular type
 Splitting
 The number of protons on adjacent carbon
atoms
Information from NMR Spectrum
 Number of signals = number of different
types of protons
 Chemically equivalent protons
 same chemical environment
 exact same chemical shift
 How many groups of chemically equivalent
protons are present in each of the following?
CO2H
CH3
O
CH3CH2CO2CH2CH3
O
CH3
Information from NMR Spectrum
 The location of each signal is described by
its chemical shift
 the ratio of how far a signal is shifted
downfield relative to a standard (TMS) to
the total spectrometer frequency
 d =
shift downfield from TMS (Hz)
instrument frequency in MHz
 Since MHz = 1,000,000 Hz, the
chemical shift is reported as ppm
Chemical Shifts
 “Substituents” with electronegative atoms
(O, halogens, N) or p systems (like aromatic
rings and vinyl groups) result in deshielding
of protons.
 Downfield shift
 Downfield shift increases as the
electronegativity of a substituent increases.
d
(CH3)4C
(CH3)3N
0.9 ppm
2.2 ppm
3.2 ppm
4.3 ppm
3.04
3.44
3.98
e.n. 2.50
(CH3)2O
CH3F
Chemical Shifts
 The amount of deshielding decreases as the
distance between the electronegative atom
increases.
CH3Br
CH3CH2Br
CH3CH2CH2Br
CH3CH2CH2CH2Br
2.69 ppm
1.66 ppm
1.06 ppm
0.93 ppm
 Shielding and deshielding effects are
additive.
CH4
0.23 ppm
CH3Br
2.69 ppm
CH2Br2
4.94 ppm
CHBr3
6.82 ppm
Information from NMR Spectrum
Alkane C – H Chemical Shifts
CH3
CH3 ~0.9 ppm
CH2
CH2 ~ 1.2 ppm
CH ~ 1.5 ppm
CH
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Technology, 10/16/09)
http://riodb01.ibase.aist
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Institute of Advanced
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Technology, 10/17/09)
Carbonyl Compounds
O
 CHn with an a Carbonyl
a
c
CH3CH2CO2CH3
b
CHnC
c
Typical range:
a
b
1.9 – 3.3 ppm
with 2.0-2.5
being quite
common
Carbonyl Compounds
 CHn with an a Carbonyl
O
CH3CCH3
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O
CHnC
Typical range:
1.9 – 3.3 ppm
with 2.0-2.5
being quite
common
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Aromatic Compounds
b
H
b
H
H3C
a
CH3
a
H
b
Aromatic CH:
6.5 -8.5 ppm
b
H
b
a
Benzylic CHn:
2.2 – 3.0 ppm
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Alkyl Halides
a
c
CH3CH2CH2Br
b
CHn with a halogen:
2.1 – 4.5 ppm
(3 – 4 ppm most
common)
c
a
b
Downfield shift
decreases as distance
from halogen
increases.
Downfield shift
increases as e.n. of
halogen increases.
Alcohols
OH:
0.5 – 5.0 ppm
b
CH3CH2OH
a
c
Carbinol CHn:
3.2 – 5.2 ppm
with 3 – 4 ppm
most common
a
b
c
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Advanced Industrial Science and Technology, 10/17/09)
Downfield shift for
protons decreases as
distance from
hydroxyl group
increases.
Ethers
a
OCH3
b H
H b
CHnOR:
Similar to alcohol’s
carbinol CH
Hc
c H
3.2 – 5.2 ppm
H
d
c
11
a
10
9
8
b & d
7
6
(3 -4 ppm most
common)
5
4
3
2
1
0
Esters
a
a
c
CH3CO2CH2CH3
b
-CHnCO2R
1.9 – 3.3 ppm
c
O-CHn
b
(2-2.5 most
common)
3.5 – 5 ppm
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Amines
N – H :
Variable position
1.5 – 4.0 ppm
b
H
N
a
CHnN:
~ 2 – 4 ppm
10
8
6
4
2
0
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Alkenes
a
b
c
H
H
H
C
C
c
H
H
a
C C
H
CH2CH2
b
d d
Vinyl protons:
a
b
Allylic protons:
d
1.6 – 2.5 ppm
c
4.5 – 7.3 ppm
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Technology, 10/17/09)
Aldehydes
a
H3C
Aldehyde proton:
9.5 – 10.5 ppm
c
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a
CH3
C
a
C Hc
H O
b
b
Carboxylic Acids
R-COOH
CH3CO2H b
a
9.7 - 12.5 ppm
b
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a
Integration
 The area under each NMR signal is
proportional to the number of protons that
give rise to that signal.
 The relative area is measured by an
integrator and is depicted as an integration
curve above the spectrum.
 The height of the integration curve is
proportional to the area
Integration
 The height of the integration curve is
proportional to the relative number of
protons.
Splitting
 The magnetic fields generated by nonequivalent protons on adjacent carbon atoms
will either align with or against the external
magnetic field.
 Magnetic coupling causes the proton to
absorb at a slightly different chemical shift:
 Slightly downfield when external field is
reinforced
 Slightly upfield when external field is
opposed
Splitting
 The protons (Hb) on carbon
2 of 1,1,2-tribromoethane
appear as a doublet.
 If Ha has spin aligned
with b, then b is
reinforced and peak
shifts downfield.
 If Ha has spin opposed
to b, then the peak
shifts upfield.
Splitting
 Since all possibilities occur, the signal is
split into multiple peaks when non-equivalent
protons are present on an adjacent carbon
atom.
b
CH3CH2OH
a
c
Notice that the hydroxyl
proton is not split by
the protons on the
adjacent methylene and
vice versa!
Splitting
 A signal that is split by N equivalent protons
will be split into N + 1 peaks.
Splitting
 Equivalent protons do not split each other.
 Protons attached to the same carbon will
not split each other unless they are nonequivalent.
 Protons separated by four or more bonds do
not couple (split each other).
 Protons attached to O or N will not split the
protons attached to an adjacent atom (and
vice versa).
“Classic” Ethyl Group Pattern
“Classic” Isopropyl Pattern
Example
 Predict the splitting pattern and approximate
chemical shifts for the following compounds:
O
OH
O
O
2,5-hexanedione
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Isopropyl alcohol
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Technology, 10/18/09)
Propiophenone
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Example
 Assign each peak in the NMR spectrum to
the appropriate protons.
a
d
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/ (National Institute of Advanced
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Technology, 10/19/09)
b
c
O
Example
 Draw the structure for the following
compound if its molecular formula is C4H10O.
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Example
 Draw the structure of the following
compound.
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Example
A compound with the formula C6H10O3 has strong
peaks at 1746 and 1719 cm-1 in its IR spectrum.
Identify its structure using the following proton
NMR.
3
3
2
2
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