INFRARED (IR) SPECTROSCOPY (SPECTROPHOTOMETRY)
The interaction of Electromagnetic (EM) radiation with matter is one of the most useful
phenomena available to chemists for the identification and quantization of unknown substances.
You have already studied spectrophotometry involving the visible spectrum (colourimetry).
Because most organic compounds do not absorb in the visible region (most are not colored),
organic chemists commonly use non visible EM radiation to carry out spectrophotometric
analysis, e.g., IR spectroscopy uses IR radiation, UV spectroscopy uses UV radiation, and
Nuclear Magnetic Resonance (NMR) uses radio wave radiation. The Beer-Lambert relationship
between absorbance and concentration holds true in IR and UV spectrophotometry. We will be
studying these 3 methods of spectral analysis as they apply to organic compounds.
It is important that the student know the location (sequence) and wavelengths of the
major bands of the EM spectrum.
Electromagnetic Radiations:
Electromagnetic radiations are forms of radiant energy, that possess no mass or weight and are
electrically neutral. They also share 4 other common characteristics:
1) all pass through a vacuum in wavelike motion;
2) all travel at the speed of light (2.998  108 m/s in a vacuum)
3) all emit electric and magnetic fields
4) all have different energies, wavelengths, and frequencies
The spectrum of electromagnetic radiation, in order of increasing energy (decreasing
wavelength), includes radio & TV waves, microwaves, infrared radiation, visible light,
ultraviolet radiation, X-rays, and gamma rays.
No clear-cut separation exists between the bands so overlap of wavelengths, as shown below, is
reported in various literature sources.
Wavelength ‘‘ (m)
-14
10
-12
10
-10
10
Gamma rays
-8
10-6
10
10-4
Visible light
10-2
1
102
Microwaves
104
AM radio
Ultraviolet
X-rays
1022
1020
Infrared (heat)
1018
1016
1014
1012
TV and FM radio
1010
108
106
104
Frequency ‘‘(Hz)
ENERGY
Electromagnetic waves travel as photons, which are massless packets of energy. The energy of
a photon is proportional to its frequency and inversely proportional to its wavelength.
1
A photon of frequency  (or wavelength ) has an energy given by...
where h = Plank’s constant, 1.58  10-37 kcals
or
6.6210-37 kJs
hc
  h 

Multiplying by Avogadro’ number (N) gives the energy (E) of a mole of photons of frequency 
or wavelength  given in centimeters.
E  Nh  

Nhc 2.86  10 3 kcal  cm / mol 1.20  10 2 kJ  cm / mol



 (cm)
(cm)
Do problems 12.5, 12.6, and 12.7
Despite the overlap of the spectral bands, analytical chemists have assigned boundaries
(wavelength ranges) for each group and these must be memorized by the student.
Wavelength Boundaries of Important EM Radiation Bands
 (m)
 (m)
Vacuum UV
10 - 200
.01 - .20
Near UV
Visible
Near IR
Mid IR
200 - 400
400 - 800
.20 - .40
.40 - .80
.80 - 2.5
2.5 - 25
13000 - 4000
4000 - 400
25 - 1000
400 - 10
Far IR
3
Microwave
Radio & TV
 (m)
6
10 - 10
 (cm-1 )
.001 - 1
1 - 104
The frequency of IR absorption by molecules is stated not in Hz but in ‘wavenumbers’ (), often
called ‘reciprocal centimeters’ (cm-1). The wavenumber is simply the number of wavelengths
per centimeter, which equals the reciprocal of wavelength in cm.

1
 (cm)
e.g. calculate  for  = 5m
 1cm 
  0.0005 cm  
soln : 5m   4
 10 m 
or simply :  
then :  
1
1

 2000 cm-1
 (cm) .0005 cm
1
1
10 4
10 4



 2000 cm-1
 (cm)   (m)  (m) 5
 10 4 


 Calculate the wavenumbers for the boundaries of the Near, Mid, and Far IR regions for the
above table.
 Note that wavenumber is directly proportional to frequency. Mathematically, it differs from
frequency by a constant multiplier, i.e., ‘c’, the speed of light in a vacuum...
2
compare


1

and  =
c

(by rearranging c =    )
Do problem 12.8
Principles of absorption spectroscopy:
Much information regarding the structure of a molecule can be obtained from its
absorption spectrum. The position of its absorption bands depend upon the configuration of its
atom and electrons.
To a first approximation, the internal energy E of a molecule is composed of additive
contributions from ...
 the electronic motions within the molecule (Ee)
 the vibrational motions of its atoms (Ev)
 the rotational motion of the whole molecule (Er)
E = Ee + E v + Er
The energies of these contributions are ‘quantized’, i.e., they are a set of discrete values- not
continuous.
When a molecule absorbs light, its energy is momentarily increased by an amount equal to that
of the energy of the photons of electromagnetic radiation it absorbs. The absorbed energy
changes one or more of the electronic, vibrational, or rotational states. The relative magnitudes
of electronic : vibrational : rotational excitations are ca. 1000 : 50 : 1. It is thus possible to
excite changes in rotational energy without exciting the vibrational or electronic states (Er but
not Ev or Ee). It is also possible to excite changes in rotational and vibrational states without
exciting the electronic states (Er and Ev but not Ee).
Absorption by molecules in the IR region involves changes in rotational and vibrational energies
only; absorption by molecules in the UV and Visible regions produce changes in all 3 energies.
For these reasons, molecular absorption in the UV, Visible and IR give rise to bands rather than
sharp lines which are obtained with atomic absorption, e.g. AA.
Despite the breadth of IR bands, the IR spectra of molecules are so complex that no
2 compounds with different structures have the same IR spectrum except for optical isomers and
certain high molecular weight polymers differing only slightly in molecular weight.
IR spectrometers are scanning instruments; emitting radiation from 4000 cm-1 to 650 cm-1,
(2.5 m  16 m), i.e., most of but not all of the Mid IR region. Organic compounds placed in
the radiation path absorb various frequencies of IR producing a printout of
% transmittance versus wavenumber (cm-1) and wavelength (m).
 Absorption of Far IR radiation causes molecular rotation and low energy C-C vibrations.
These are quantized absorptions and are measured using a different instrument called a
Raman Spectrometer.
3
 Absorption of Mid IR radiation induces strong molecular vibrations and is measured by IR
spectroscopy. The Mid IR region of the EM spectrum is also called the ‘fundamental’
region.
 Absorptions of Near IR radiation occur as a result of ‘overtones’ (resonance multiples) of Hstretching vibrations generated by Mid IR radiation.
VIBRATIONS
Mid IR (2.5-25 m, i.e., 4000-400 cm-1)
STRETCHING
BENDING
To understand the kinds of molecular vibrations which occur when this type of radiation is
absorbed, we need to picture a molecule and its bonds as an assembly of spheres (atoms) and
springs (bonds).
There are 2 kinds of molecular vibrations; i.e., stretching and bending.
1. Stretching vibrations involve bond length change, i.e., the distance between atoms increase
or decreases but the bond angle remains unchanged.
2. Bending vibrations involve a change in bond angle, i.e., atoms change positions while
maintaining a constant distance between atoms. Bending vibrations are also called
‘deformations’.
The frequency () of vibrations in molecules is not a continuum but is quantized and determined
by several factors... a) mass of the atoms, b) EN of atoms (dipole strength) and c) bond
strength and type (i.e., single, double or triple).
When IR radiation of the same  as the  of vibration of a bond strikes a bond, the intensity
(amplitude) of the vibration of the bond increases, i.e., energy is absorbed. If the  of the
radiation is not equal to the  of vibration of the bond, the radiation is not absorbed.
When one considers the number of different kinds of atoms, bond lengths and dipole strengths
that exist in a molecule, it turns out that there are several (perhaps many) different stretching
and bending 's so that there are several (perhaps many) different 's (and 's) of radiation
absorbed by a single molecule.
In contrast to UV and VIS spectroscopy, where only some compounds absorb, almost all known
molecules are IR-active. Exceptions include monatomics (He, Ne, Ar, Kr, etc.) and some
simple diatomics (O2, N2, H2, etc.). Consequently, IR is unexcelled as a tool for molecular
identification but is rather poor for quantitative analysis. In general IR cannot accurately detect
substances at less than about 1% wt.
Theoretical Number of Fundamental Vibrations:
Every atom has the ability to move in 3 directions, i.e., along its x, y, & z Cartesian coordinates.
A change in an atom’s position along any coordinate represents a type of motion (or ‘degree of
freedom’). Theoretically, a molecule of N atoms can have 3N different types of motion
('degrees of freedom'). Translational motion involves 3 degrees of freedom. Rotational motion
involves another 3 degrees of freedom. This leaves (3N-6) forms of vibrational motion for nonlinear molecules or (3N-5) possible vibrations for linear molecules.
For example, methane (CH4 ), which has 5 atoms, can exhibit 3(5)-6 = 9 different vibrations.
Calculate the number of possible modes of vibration of benzene and ethanol.
4
Since each type of vibration usually occurs at a different frequency, it is apparent that many
frequencies of IR may be absorbed by even simple molecules. As a result, the IR spectra of
most organic compounds shows many absorption bands.
Terminology and Types of Fundamental Vibrations:
Consider a methylene group-CH2
1. symmetrical stretching (s CH2 ~ 2853 cm-1)
(same symbol as used for frequency)
2. asymmetrical stretching (as CH2 ~ 2926 cm-1)
(Note that asymmetric stretching occurs at slightly higher frequencies
than symmetric stretching)
3. in-plane bending (scissoring) (s CH2 ~ 1465 cm-1)
(Greek symbol 'delta')
4. in-plane bending (rocking) ( CH2 ~ 720 cm-1)
(Greek symbol 'rho')
+
+
_
+
5. out-of-plane bending (wagging) ( CH2 ~ 1350-1150 cm-1)
(Greek symbol 'omega')
6. out-of-plane bending (twisting) ( CH2 ~ 1350-1150 cm-1)
(Greek symbol 'tau')
5
Requirements for Absorption of IR by Matter:
1. Radiation must be of correct energy, i.e., same  as the  of vibration of the bonds in a
molecule.
2. There must be a change in the dipole moment when the atom vibrates. If not, the radiation is
not absorbed even though the  is correct. Such a vibration is said to be "infrared-inactive".
The dipole moment () is given by
=qr
q = the product of the magnitudes of the +'ve and -'ve charges in the dipole
r = the distance between the centers of +'ve and -'ve charges
Consider CO2:
symmetric stretch
asymmetric stretch
O==C==O
O==C==O
+ O===C===O
- +
O=C====O
s
effective center of charge unchanged
q 0 but  = 0 because r = 0,
no absorption occurs (IR inactive)
as
effective center of charge changes
q 0 & r  0, so   0
absorption occurs (IR active)
The Theoretical Number of Fundamental Vibrations will Seldom be Observed:
The following phenomena will reduce the number of bands observed ...
1.  are outside the 4000 - 650 cm-1 range.
2. bands are too weak to be observed.
3. two bands are so close in  that they are not resolved by the spectrophotometer.
4. two or more bands absorbed at the same  (are 'degenerate') because sections of the
molecule are symmetrical.
5. lack of change of dipole occurs.
It occasionally happens that extra bands are observed.
fundamental vibrations but are ‘overtones’.
Bands may occur which are not
Overtones are integral (whole number) multiples of a fundamental vibration, , e.g., IR
absorption at 600 cm-1 may be accompanied by weak overtone absorptions at 1200 cm-1 (2),
1800 cm-1 (3), etc. Overtone absorptions are less intense than the fundamental bands by a
factor of 10 to 100 times. Most absorption bands in the Near IR (0.75 - 2.5 m) are overtones of
H-stretching vibrations. Overtones are commonly seen in the 5-6 m (2000-1650 cm-1) range
of the Mid IR.
6
Phenomena which alter the position () and shape of IR absorption bands are ‘vibrational
coupling’ of peaks, ‘Fermi resonance’ and ‘hydrogen bonding’.
 Vibrational Coupling may result when 2 vibrating bonds share a common atom. For
example, in a C-C-O group, both C-C stretch and C-O stretch can interact (combine) to
produce a single IR absorption band whose position is different than the positions of either
of individual stretches.
 Fermi Resonance is a special case of vibrational coupling in which the frequency of a
fundamental vibration and an overtone of another vibration happen to coincide. For
example, in cyclopentanone, the C=O stretch at 1750 cm-1 coincides with the overtone of an
-methylene (-CH2) bend. With good resolution, the apparently single absorption band is
shown to be a ‘doublet’, i.e., it’s absorption band has a split end.
 Hydrogen Bonding occurs most frequently in alcohols (-OH groups), amines (-NH groups)
and amides (CONH groups) all of which contain electropositive H and electronegative O, or
N atoms. Hydrogen bonding alters the strength of these bonds and results in increased band
intensity and band widening. H bonding is increased when samples are run in polar solvents
and is reduced when run in nonpolar solvents. Other proton acceptor groups, beside O and N
are the halogens (X), and unsaturated groups (C=C).
7
Intermolecular Hydrogen Bonding:
Molecules with polar hydrogen groups
usually associate readily and their spectra are
very dependent on the state of the sample.
O
H
Consider cyclohexanol: Cyclohexanol normally
exists as a polymeric aggregate held together by
hydrogen bonds.
O
H
If the concentration is decreased, this tends to
break up H-bonding into monomers.
O
H
Trimers  Dimers  Monomers
The following spectrum of cyclohexanol
illustrates hydrogen bonding dependence. The
spectra were measured in CCl4 solution.
0.01 M
3700
3300
0.03 M
3700
3300
0.1 M
3700
0.2 M
3700
3300
1.01 M
3300
3700
3300
-1
wavenumber (cm )
Intramolecular Hydrogen Bonding:
Intramolecular hydrogen bonds are formed when the
proton donor and acceptor are present in the same
molecule under conditions that would allow formation
of a 5- or 6-membered ring.
For example, o-hydroxyacetophenone exhibits strong
intramolecular H-bonding. A broad, medium-to-strong
band is observed near 3100 cm-1 even as a dilute
solution in a non polar solvent.
CH3
C
O
H
O
8
The Relative Position of Stretching Frequencies of 2-atom Vibrations can be easily
estimated since the main participants in most vibrations are just 2 bonded atoms. The  of their
vibrations depend upon the 2 masses and the bond strength between them, i.e.,
1.  is proportional to () the bond strength ('force constant', 'f') which  EN & # of bonds
2.  is inversely  the masses of the atoms
For example,
 strong CC bonds absorb at higher wavenumbers than C=C which in turn is higher
than C-C bonds.
 C-H absorbs at a higher wavenumber than C-D because the CH group has lower mass.
 The highly polar O-H bond absorbs at a higher wavenumber than N-H which is higher
than C-H, even though the mass of these groups is decreasing.
The Exact Position of Stretching Frequencies of 2-atom Vibrations can be calculated
(approximately) using Hooke's Law.


1
f

c 2c mr
where mr = reduced mass =
Mx  My
Mx + My
c = 3.00  1010 cm/s
Mx and My are masses of atoms (g)
f = force constant (bond strength) in
dynes/cm
Values of force constants for these calculations are tabulated below.
Bond
'f' (dyn/cm)
Bond
'f' (dyn/cm)
H-F
H-Cl
H-Br
H-I
H-O
H-S
H-N
H-C
9.7  105
4.8  105
4.1  105
3.2  105
7.8  105
4.3  105
6.5  105
5  105
Cl-C
C-C
C=C
CC
N-N
N=N
NN
C-O
C=O
3.4  105
5  105
10  105
15  105
4.5  105
13  105
23  105
5.4  105
12.5  105
Example:
Calculate the absorption wavenumber for OH, CH, and CD bond stretching.
9
Generalizations re: IR Absorption Band Positions:
 fundamental stretching range = 4000-800 cm-1 (2.5-12.5 m)
 fundamental bending range = 1700-400 cm-1 (6-25 m)
The MID IR region is divided into 2 regions:
1. The 'group frequency' region = 4000-1250 cm-1 (2.5-8 m)
2. The 'fingerprint' region = 1250-400 cm-1 (8-25 m)
In the group frequency region, absorption bands are characteristic of specific functional groups
(OH, NH2, C=O, C-H, etc.). These appear at fairly constant positions, rather independent of the
rest of the molecule.
In the fingerprint region, vibrational frequencies are greatly affected by the whole molecular
structure and spectra are considered specific for a particular molecule. Some functional group
absorption can be identified in the fingerprint region, especially below 1000 cm-1.
The region between 1400-1000 cm-1 is often congested with deformation (bending) bands and is
difficult to interpret and many (but not all) bands in this region are often ignored.
Recall the relationship:   f  1/mass and note the general locations of functional group
stretches in the functional group region of the IR spectrum.
congested region
fingerprint region
group frequency region
N-H
O-H
4000
C-H
3000
CC
CN
2000
C=C
C=N
C=O
1400
1250
1000
800
 (cm-1)
Intensity of Absorption Bands:
Intensity  change in dipole moment. Bond with weak dipoles absorb weakly, e.g., ...
C=C, extinction coefficient () ~ 5. Similarly, C=N, and C-C give weak stretching bands.
O-H, C=O, C-Cl, C-F and Si-O are intense,  = 100-1000.
 Note that the intensity of even 'intense' IR bands are 2-3 orders of magnitude less than those
of UV and visible absorption bands. The low energy of IR radiation causes instrumentation
problems. For example, the signals measured are often of the same order of magnitude as
electronic noise in a detecting circuit. In addition, warm components of the spectrometer
radiate IR and this false radiation must be distinguished from the signal.
 The low energies of IR require wide slits in IR spectrometers; often the same width as the
absorption band, i.e., several m. As a result, the resolution of IR spectrometers is much
poorer than Visible spectrometers where slit widths are around 1 m.
10
 Quantitation is poor by IR. About  5% accuracy is optimum. Absorption band intensities
are reported simply as very strong (vs), strong (s), medium (m), weak (w),
very weak (vw), or weak to medium (wm).
Physical State of the Sample Affects Group Frequencies and Band Shapes:
IR analysis can be carried out on gas, liquid, or solid samples. The variation of  of absorption
bands in various phases is small (only 1-2%) unless specific interactions like hydrogen bonding
or solvent effects occur. However, the appearance and complexity of bands of spectra are
strongly affected by the physical state of the sample.
 Gas phase IR scans are the simplest (smoothest) since few intermolecular interactions occur.
Absorption band tend to broader than in the liquid or solid state.
 Liquid phase IR scans show more squiggles and extra small peaks due to restricted rotation
caused by interaction between molecules in the liquid phase. Absorption bands are often
symmetrical, similar to the ‘normal’ curve.
 Solid phase IR scans are most sharp and most complex. Many bands are split.
 Dilute solutions of solute in nonpolar solvents give the most reproducible band frequencies
and shapes. Polar solvents tend to shift bands to lower  (longer ), e.g., OH from 3620 
3300 cm-1. Solvent concentration affects intermolecular hydrogen bonding but not
intramolecular.
 Temperature has little effect on IR spectral band position or shapes.
11
IR Instrumentation:
mirror
reference
cell
detector
monochromator
IR source
sample
cell
mirror
chopper
%T
chart recorder
DISPERSIVE IR SPECTROMETER
The IR beam is split; ½ is directed through the sample cell and ½ through the reference cell (e.g.
solvent only or air). The beams are chopped by a rotating mirror (chopper) that alternately directs
the sample and reference beams to the monochromator.
The monochromator separates the beams into its various wavelengths that strike the detector. A
chart recorder prints a plot of %T vs. wavenumber (cm-1) and wavelength (m). A full scan
typically requires ~ 5 min.
Note that the sample is placed before the monochromator, so that the monochromator can dispose
of extraneous radiation.
In general, lenses are avoided and focusing is accomplished with variously shaped mirrors usually made of glass or fused quartz with aluminum deposited on the front face. Such mirrors
reflect up to 96% of IR.
With more expensive instruments, called Fourier Transform IR spectrometers, the entire IR
spectrum is passed through the sample several times per second and the data is averaged. Higher
resolution and much faster scan times are achieved.
Radiation Sources:
1. The Nernst Glower or Nernst filament, which is fabricated from a binder and oxides of Zr, Th,
and Ce or Y, is shaped into a hollow rod. When electrically connected, the material heats to ~
1500 C and emits IR (maximum emission at 7100 cm-1).
2. The Glowbar is a small rod of SiC, which when heated to ~1500 C by electrical conduction,
emits IR with maximum emission at 5500-5000 cm-1.
Detectors:
1. Thermocouples convert heat (from radiant energy) into emf.
2. 'Bolometers' use a Wheatstone bridge as a detector. The electrical resistance of a filament
changes with its temperature, which varies with the intensity of IR radiation transmitted
through a sample.
12
Sample Handling:
Gases are viewed in evacuated cells (up to10 cm. wide).
Liquids are scanned in either pure ('neat') form or in solution with a solvent. Non volatile neat
liquids are examined between flat salt (NaCl) plates without a spacer. Capillary thickness is
0.01mm or less. Volatile liquids (b.p. < 70 C) must be examined in very thin sealed cells which
have thin spacers between salt plates.
Solutions are handled in cells of 0.1-1 mm path length at concentrations ranging from 0.05 to
10%. All solvents absorb at some part of the MID IR range, so if the entire spectrum is to be
studied, several solvents must be used in sequence.
For example: CCl4 is absorbance free above 1333 cm-1
CS2 is absorbance free below 1333 cm-1
Solvent-solute interactions must be avoided, e.g., CS2 cannot be used with 1 or 2 amines and
amino alcohols react slowly with CCl4 and CS2.
Solids are examined as a 'mull', pressed disk (KBr pellet) or a cast film
Mulls are prepared by thoroughly grinding 2-5 mg of a solid sample in a small agate mortar, after
which 1-2 drops of a mulling oil is added and grinding is continued. Particle size should be < 2
m (the shortest Mid IR wavelength) to avoid scattering radiation. Mulling oils include 'Nujol'
and 'Fluorolube'.
 Nujol is a high-boiling HC petroleum oil (paraffin or mineral oil) with C-H stretching at
2800-3000 cm-1 (3.3-3.5 m) and C-H bending at 1450 and 1360 cm-1 (6.81and 7.25
m).
 Fluorolube is a completely halogenated polymer containing F or Cl, e.g.,
hexachlorobutadiene and hexafluorobutadiene. It is absorption free above ~ 1400 cm-1
(below ~ 7.7 m).
Pressed Disk: Dry powdered KBr and other metal halides can be compacted under pressure in
vacuum to form IR-transparent disks. 1.5 mg of solid sample is intimately ground with
~ 200 mg of IR-grade KBr (~ 1% mixture). The mixture is transferred to a die and evacuated for
~ 10 minutes and then pressed (still under vacuum) at 8-10 tons for ~ 5 minutes more. Although
KBr is completely transparent from 10,000-400 cm-1, it is hygroscopic and readily absorbs
moisture so that bands may appear at 3400 and 1640 cm-1 if not properly dried and evacuated. IR
grade KBr should be dried at 110 ºC overnight and then desiccated before use.
Deposited films (Cast films) can be prepared by evaporating a solution of the solid on a NaCl
disk.
13
Transmission Properties of Various IR Cell Materials
IR transparent up to ...
 (m)
IR transparent down to ...
 (cm-1)
glass
2
5000
quartz
4
2500
LiF
6
1667
CaF2
9
1111
NaCl
15.5
645
KBr
25
400
CsBr
40
250
CsI
50
200
Other References:
 McMurry, Sec. 12.5 - 12.9, p.451-466
 Silverstein, Bassler & Morrill, Spectrophotometric Identification of Organic Compounds,
(copy in Chemical stockroom and College library).
 Infrared Spectral Interpretation work book and audio cassette tape available on loan from
instructor. Sign out for 3 days.
 Spectra of Organic Compounds on the computers in Rm. 402A. Shows IR, UV, NMR spectra
of 100 organic compounds. See Instructor or stock room attendant to access.
 McMurry Organic Chem. Toolbox on computers in Rm. 402A. Supplement to text. See
Instructor or stock room attendant to access.
 Aldrich Chemical Handbook of IR Scans in the College library has actual IR scans for
thousands of organic chemicals.
Homework:
Do problems 12.9, 10, 11, 12, 13, 29, 30, 31, 34, 35, 36, 37, 39, 40, 41 in McMurry.
Complete the assignment, i.e., INTERPRETATION OF IR SCANS-HOMEWORK ASSIGNMENT,
located at the end of the notes on interpretation of IR scans.
14