Infrared Spectroscopy

The structure of new compounds that are
isolated from natural sources or prepared
in the lab must be determined (and/or
verified).
Chemical analysis
 Spectroscopy


Spectroscopic techniques are nondestructive and generally require small
amounts of sample
Infrared Spectroscopy

Four common spectroscopic techniques
used to determine structure:
Infrared Spectroscopy (IR)
 Mass Spectrometry (MS or Mass Spec)
 Nuclear Magnetic Resonance Spectroscopy
(NMR)
 Ultraviolet Spectroscopy

Infrared Spectroscopy

Infrared spectroscopy:


Used to determine the functional groups
present in a molecule
Mass spectrometry
Breaks molecule into fragments
 Analysis of the masses of the fragments
gives MW and clues to the structure of
the molecule

Infrared Spectroscopy

Nuclear Magnetic Resonance Spectroscopy
Observes the chemical environment of the
hydrogen (or carbon) atoms in the molecule
 Helps provide evidence for the structure
of alkyl groups


Ultraviolet Spectroscopy


Observes electronic transitions
Provides information on the electronic
bonding in a molecule
Infrared Spectroscopy

Infrared spectroscopy:

Used to determine the functional groups
present (or absent) in a molecule
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology,
1/5/11)
Infrared Spectroscopy

Infrared spectroscopy is a type of absorption
spectroscopy:

Any technique that measures the amount of light
absorbed by (or transmitted through) a
compound as a function of the wavelength of
light
 Sample
is irradiated by a light source
 Amount
of light transmitted (or absorbed) at
various wavelengths is measured by a detector
A
spectrum is obtained.
 Graph of light transmitted (or absorbed) as a
function of wavelength
Infrared Spectroscopy

Infrared Spectrometer
Infrared Spectroscopy

Fourier Transform Infrared Spectrometer
(FTIR)
Uses an interferometer
 Interferogram simultaneously contains
all frequencies
 Has better sensitivity.
 Less energy is needed from source.
 Completes a scan in 1-2 seconds.
 Takes several scans and averages them.
 Has a laser beam that keeps the
instrument accurately calibrated.

Infrared Spectroscopy
 Infrared spectroscopy uses
light from the infrared region
of the electromagnetic
spectrum.
 The absorption of IR radiation
leads to absorption bands in the
IR spectrum.
The position of a band is
reported in wavenumbers ( u )
 the number of wavelengths
per cm
 u = 10,000 where l = mm
l
 Directly proportional to energy

Infrared Spectroscopy

The atoms in a molecule are in constant
motion.

The covalent bond between two atoms acts
like a spring, allowing the atoms to vibrate
(stretch and bend) relative to each other.
Infrared Spectroscopy

The absorption of IR radiation increases the
amplitude of the various types of bond
vibrations.
 Stretching
 Symmetric
 Asymmetric
 Bending
Infrared Spectroscopy

Since energy is quantized, covalent bonds can
vibrate/stretch only at certain allowed
frequencies.

The position of an absorption band
correlates with the type of chemical bond.
Infrared Spectroscopy

The frequency of an absorption band in
an IR spectrum depends primarily on:

Type of vibration
 Stretching vibrations: higher frequency
 Bending vibrations: lower frequency

Masses of the atoms in a bond
AW

Freq
Strength of the bond or bond order
BO
Freq
Infrared Spectroscopy

The polarity of a bond has a significant impact
on the intensity of an IR absorption band.

Vibrations that cause a significant change in
the dipole moment of a chemical bond lead
to strong absorption bands.

Vibrations that result in no change/very
little change in dipole moment lead to very
weak or no absorption band.
 Symmetrical
bonds often exhibit very
weak or no absorption band.
Infrared Spectroscopy

Each molecule has a unique IR spectrum.
 The IR spectrum is a “fingerprint” for the
molecule.

IR spectrum results from a combination of all
possible stretching and/or bending vibrations
of the individual bonds and the whole
molecule.
 Simple stretching: ~1600-4000 cm-1.
 Complex vibrations: 600-1400 cm-1,
called the “fingerprint region.”
Infrared Spectroscopy

IR Spectrum of n-octane
Infrared Spectroscopy

An IR spectrum is used to identify functional
groups that are present (or absent).


Cannot conclusively identify a structure by IR
alone unless an IR spectrum of an authentic
(known) sample of the compound is available.
Absorptions from specific functional groups
are found in certain regions of the IR
spectrum.
Carbon-Carbon Bonds

Increasing bond order leads to higher frequencies:
 C-C
1200 cm-1
(fingerprint region)
 C=C
1600 - 1680 cm-1
 CC
2200 cm-1
(weak or absent if internal)

Conjugation lowers the frequency:
 isolated C=C
1640-1680 cm-1
 conjugated C=C
1620-1640 cm-1
 aromatic C=C
approx. 1600 cm-1

C=C peaks are generally weak to moderate in
intensity.
Carbon-Hydrogen Bonds

Bonds with more s character absorb at a
higher frequency.

sp3 (alkane) C-H
 just below 3000 cm-1 (to the right)

sp2 (alkene or aromatic hydrocarbon) C-H
 just above 3000 cm-1 (to the left)

sp (alkyne) C-H
 at 3300 cm-1
O-H and N-H Bonds

Both O-H and N-H stretches appear
around 3300 cm-1, but they look
different.
Alcohol O-H
 broad with rounded tip when hydrogen
bonding is present (sharp in the absence
of hydrogen bonding)
 Secondary amine (R2NH)
 Broad (usually) with one sharp spike
 Primary amine (RNH2)
 Broad (usually) with two sharp spikes.
 No signal for a tertiary amine (R3N)

NH Bend

A broad, round peak may be observed around 1600
cm-1 for the N – H bend, especially with primary
amines.
NH2
stretch
N-H bend has
a different
shape than an
aromatic ring
or C=C
N-H
bend
Carbonyls

Carbonyl stretches are generally strong:





Aldehyde
Ketone
Carboxylic acid
Ester
Amide
~1710 cm-1
~1710 cm-1
~1710 cm-1
~1730 - 1740 cm-1
~1640-1680 cm-1

Conjugation shifts all carbonyls to lower
frequencies.

Ring strain shifts carbonyls to higher
frequencies.
H3C
-1
1745 cm
O
Aldehydes
Carboxylic Acids
Ketones
Esters

C=O stretch at ~ 1730-1740 cm-1

C-O stretch at 1000-1300 cm-1 (broad)
and
strong
(Note: other functional groups may have peaks in the
1000-1300 cm-1 region too!)
O
O
1743
1245
Amides

C=O stretch at 1640-1680 cm-1

N-H stretch (if 1o or 2o) around 3300 cm-1
double peak)
(sometimes a
Nitriles

C  N absorbs just above 2200

The alkyne C  C signal is much weaker and is
just below 2200 cm-1
cm-1 (med – strong)
IR Spectroscopy
Example: Interpret the following IR spectrum
by assigning each of the major peaks.
Identify what functional group(s) are present.
3403
cm-1
1604
cm-1
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology,
12/30/09)
IR Spectroscopy
Example: Interpret the following IR spectrum
by assigning each of the major peaks.
Identify what functional group(s) are present.
2814
cm-1
2733
cm-1
1691
cm-1
1642
cm-1
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology,
12/30/09)
Infrared Spectroscopy
Example: Which one of the following
compounds is the most reasonable structure for
the IR spectrum shown below?
OCH3
O
O
O
OH
O
1721
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology,
12/30/09)
Infrared Spectroscopy
Example: Which of the following compounds is
the most reasonable structure for the IR
spectrum shown below?
O
NH2
OH
OH
O
OH
O
O
OH
1603
1689
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology,
12/30/09)