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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

Infrared Spectroscopy
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and
inorganic chemists
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The main goal of IR spectroscopic analysis is to determine the
chemical functional groups in the sample
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Hence it is used to identify chemical compounds or monitor changes occurring in the course
of a chemical reaction
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Their spectra are considerably different and tell about
important features of a molecule
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Thus, IR spectroscopy is an important and
popular tool for structural elucidation and compound identification
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IR spectra of 1-propanol (an alcohol), and propanoic acid
Infrared refers to that part of the electromagnetic spectrum between the visible and microwave regions
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These waves differ from each other in the length and frequency as shown bellow-

Figure 2
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JITENDRA SINGH, IET-DAVV, INDORE

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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

Infrared radiation spans a section of the electromagnetic spectrum having wave numbers from roughly
13,000 to 10 cm–1, or wavelengths from 0
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It is bound by the red end of the visible region
at high frequencies and the microwave region at low frequencies
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An increase in wave number corresponds to an increase in energy
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Figure 3
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This absorption is quantized but vibrational spectra appear
as bands rather than as lines because a single vibrational energy change is accompanied by a number of
rotational energy changes
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In IR spectroscopy, an organic molecule is
exposed to infrared radiation
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The frequency of absorption depends on the1
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Force constants of the bonds and
3
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The wave number, plotted on
the X-axis, is proportional to energy; therefore, the highest energy vibrations are on the left
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Transmittance, T, is the ratio of
radiant power transmitted by the sample (I) to the radiant power incident on the sample (I0)
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The percent transmittance
(%T) is plotted on the Y-axis
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Zero transmittance corresponds to 100% absorption of light at that wavelength
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Except at very, very low temperatures, all molecules are in motion in some manner
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At
temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect to
each other
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DR
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A
molecule is not a rigid assemblage of atoms
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There are two types of molecular vibrations, stretching and bending
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A bending vibration may consist of a change in bond angle between bonds with a common atom or the
movement of a group of atoms with respect to the remainder of the molecule without movement of the
atom in the group with respect to one another
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Various stretching and bending vibration of a molecule occur at certain quantized
frequencies
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The frequency of the vibration remains unchanged
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Each atom has three degrees of freedom, corresponding to motions along any of the three cartesian
coordinate axes (x, y, z)
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However, 3
degrees of freedom are required to describe translation, the motion of the entire molecule through space
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Therefore, the
remaining 3n – 6 degrees of freedom are true, fundamental vibrations for nonlinear molecules
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The net number of fundamental vibrations for nonlinear and linear molecules is therefore:

Fundamental vibrations involved no change in the center of gravity of the molecule
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Naturally, some vibrations can be both IR- and Raman-active
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) The fundamental vibrations for water, H2O, are given in Figure 4
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Figure 4
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The symmetrical
stretch of CO2 gives a strong band in the IR at 2350 cm-1
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The two scissoring or
bending vibrations are equivalent and therefore, have the same frequency and are said to be degenerate,
appearing in an IR spectrum at 666 cm–1
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JITENDRA SINGH, IET-DAVV, INDORE

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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

Figure 5
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In order to be IR active, a vibration must cause a change in the dipole
moment of the molecule
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) Of the following linear molecules,
carbon monoxide and iodine chloride absorb IR radiation, while hydrogen, nitrogen, and chlorine do not
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Only two IR bands (2350 and 666 cm –1) are seen for carbon dioxide, instead of four corresponding to the
four fundamental vibrations
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In the case of CO2, two bands are degenerate, and one
vibration does not cause a change in dipole moment
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(The 3n–6 rule does not apply since the -CH2- group represents only a portion of a molecule
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Figure 6
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JITENDRA SINGH, IET-DAVV, INDORE

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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

The theoretical number of fundamental vibrations will seldom be observed because of the following
reasons –
1
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Overtones (multiple of a given frequency) and
b
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The following will reduces the theoretical number of bands
a
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b
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c
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d
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e
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Stretching Vibrations
The stretching frequency of a bond can be approximated by Hooke’s Law
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7
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Thus, the
energy or frequency is dependent on how far one stretches or compresses the spring, which can be any
value
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However, vibrational motion is quantized: it must follow the rules of quantum mechanics, and the only
transitions which are allowed fit the following formulaE = (n +1/2) hν
Where
ν is the frequency of the vibration and n is the vibrational quantum number (0, 1, 2, 3,
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JITENDRA SINGH, IET-DAVV, INDORE

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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

The lowest energy level is E0 = 1/2 hν, the next highest is E1 = 3/2 hν
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This rule is not inflexible, and occasionally transitions
of 2hν, 3hν, or higher are observed
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They
are of lower intensity than the fundamental vibration bands
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A bond can come apart, and it cannot be
compressed beyond a certain point
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As the interatomic
distance increases, the energy reaches a maximum, as seen in Figure 8
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The
allowed transitions, hν, become smaller in energy
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The following formula has been derived from Hooke’s law
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As the force constant increases, the vibrational frequency
(wavenumber) also increases
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Using the following mass values
C, carbon 12/6
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02 x 1023 ν for a C–H bond is calculated to be 3032 cm–1
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The region of an IR
spectrum where bond stretching vibrations are seen depends primarily on whether the bonds are single,
double, or triple or bonds to hydrogen
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Bond
C–C, C–O, C–N
C=C, C=O, C=N, N=O
C≡C, C≡N
C–H, N–H, O–H

Absorption region, (cm–1)
800–1300
1500–1900
2000–2300
2700–3800
DR
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In a molecule, two oscillating bonds can share a
common atom
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As one bond contracts,
the other bond can either contract or expand, as in asymmetrical and symmetrical stretching
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In the case of the –CH2– group in Figure 6, you note there are two bands in the region for
C—H bonds: 2926 cm–1 and 2853 cm–1
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JITENDRA SINGH, IET-DAVV, INDORE

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CHEMISTRY - BE-I Year (July’ 2014), Sem -A

Examples
Alkanes
The spectra of simple alkanes are characterized by absorptions due to C–H stretching and bending (the C–C
stretching and bending bands are either too weak or of too low a frequency to be detected in IR spectroscopy)
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-1
• C–H stretch from 3000–2850 cm
-1
• C–H bend or scissoring from 1470-1450 cm
-1
• C–H rock, methyl from 1370-1350 cm
-1
• C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm
The IR spectrum of octane is shown below
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The C-H scissoring (1470), methyl rock (1383), and long-chain methyl rock (728) are noted on this spectrum
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The region from about 1300-900 cm-1 is called the fingerprint region
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Usually, this region is quite complex and
often difficult to interpret; however, each organic compound has its own unique absorption pattern (or fingerprint)
in this region and thus an IR spectrum be used to identify a compound by matching it with a sample of a known
compound
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The stretching vibration of the C=C bond
usually gives rise to a moderate band in the region 1680-1640 cm-1
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The strongest bands in the
spectra of alkenes are those attributed to the carbon-hydrogen bending vibrations of the =C–H group
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-1
• C=C stretch from 1680-1640 cm
-1
• =C–H stretch from 3100-3000 cm
-1
• =C–H bend from 1000-650 cm
The IR spectrum of 1-octene is shown below
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The C=C stretch band is at 1644 cm-1
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DR
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Compounds that do not have a C=C bond show C-H stretches
only below 3000 cm-1
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The –C≡C– stretch appears as a weak
band from 2260-2100 cm-1
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A terminal alkyne (but not an internal alkyne) will show a C–H stretch as a strong, narrow
band in the range 3330-3270 cm-1
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A terminal alkyne will show a
C–H bending vibration in the region 700-610 cm-1
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Note the C–H stretch of the C–H bond adjacent to
the carbon-carbon triple bond (3324), the carbon-carbon triple bond stretch (2126), and the C–H bend of the C-H
bond adjacent to the carbon-carbon triple bond (636)
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DR
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Conjugation of the
carbonyl group with carbon-carbon double bonds or phenyl groups, as in alpha, beta-unsaturated aldehydes and
benzaldehyde, shifts this band to lower wavenumbers, 1685-1666 cm-1
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This is a saturated ketone, and the C=O band appears at 1715
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It's usually not necessary to mark any of the bands in the
fingerprint region (less than 1500 cm-1)
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When
run as a thin liquid film, or "neat", the O–H stretch of alcohols appears in the region 3500-3200 cm-1 and is a very
intense, broad band
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-1
• O–H stretch, hydrogen bonded 3500-3200 cm
-1
• C–O stretch 1260-1050 cm (s)
The spectrum of ethanol is shown below
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DR
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Note that this is at slightly higher frequency than is the
–C–H stretch in alkanes
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Bands in the region 1250-1000 cm-1 are due to C–H
in-plane bending, although these bands are too weak to be observed in most aromatic compounds
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The pattern of overtone bands in the region 2000-1665 cm-1 reflect
the substitution pattern on the ring
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Details of the correlation between IR patterns in these two
regions and ring substitution are available in the literature references linked in the left frame (especially the books
by Shriner and Fuson, Silverstein et
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, and the Aldrich Library of IR Spectra)
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If you are presented with two spectra and told that one is aromatic and one is not, a
quick glance at the sheer multitude of bands in one of the spectra can tell you that it is the aromatic compound
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Note the =C–H stretches of aromatics (3099, 3068, 3032) and the –C–H
stretches of the alkyl (methyl) group (2925 is the only one marked)
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Also note the carbon-carbon stretches in the aromatic ring (1614, 1506, 1465), the in-plane C–H bending
(1086, 1035), and the C–H oop (738)
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As in ketones, if the
carbons adjacent to the aldehyde group are unsaturated, this vibration is shifted to lower wave numbers, 1710-1685
cm-1
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This band generally appears as one or
two bands of moderate intensity in the region 2830-2695 cm-1
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DR
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Note that the O=C stretch of the alpha, betaunsaturated compound -- benzaldehyde -- is at a lower wavenumber than that of the saturated butyraldehyde
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•

General Uses of IR spectroscopy:
• Identification of all types of organic and many types of inorganic compounds
• Determination of functional groups in organic materials
• Determination of the molecular composition of surfaces
• Identification of chromatographic effluents
• Quantitative determination of compounds in mixtures
• Nondestructive method
• Determination of molecular conformation (structural isomers) and stereochemistry (geometrical isomers)
• Determination of molecular orientation (polymers and solutions)
Common Applications:
• Identification of compounds by matching spectrum of unknown compound with reference spectrum
(fingerprinting)
• Identification of functional groups in unknown substances

DR

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