INTRODUCTION TO INFRARED (IR) SPECTROSCOPY
Virtually all organic compounds absorb IR radiation. The frequency absorbed varies with the
functional groups present, e.g., OH, NH, C=O, C=C, etc. Thus IR spectroscopy is a powerful
method of classifying unknown organic compounds by identifying the functional groups present
in the compounds.
The IR portion of the electromagnetic spectrum lies between visible light and microwaves. It is
divided into three regions; the near IR, mid IR and far IR as shown
Wavelength Boundaries of Important EM Radiation Bands
 (m)
 (m)
Vacuum UV
10 - 200
.01 - .20
Near UV
200 - 400
.20 - .40
Visible
Near IR
Mid IR
400 - 800
.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 last column labeled  (cm-1) is a frequency unit called ‘wavenumbers’ or ‘reciprocal
centimeters’. It is calculated as the mathematical inverse of the wavelength in cm. Thus
wavenumber (  ) has units of cm-1 and is directly proportional to the frequency of the radiation.
An example of the conversion between wavenumbers and wavelength is shown below.

1
 (cm)
e.g. calculate  for  = 5m
 1cm 
1
1
  0.0005 cm  
soln : 5m   4
then :  

 2000 cm -1
 (cm) .0005 cm
 10 m 
1
1
10 4
10 4
or simply :  



 2000 cm -1
 (cm)   ( m)   ( m) 5
 10 4 


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...
compare

1

and  =
c

(by rearranging c =    )
Most organic functional group absorptions occur in the mid IR range, between 4000 and 400
cm-1 (25 and 1000 m, respectively).
IR Spectroscopy Introduction
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When IR radiation strikes a molecule it is absorbed and causing the molecule to vibrate.
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’.
VIBRATIONS
STRETCHING
BENDING
Terminology and Types of Fundamental Vibrations:
Consider a methylene group (CH2)
1. symmetrical stretching (s CH2 @ 2850 cm-1)
(same symbol as used for frequency)
2. asymmetrical stretching (as CH2 @ 2930 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')
IR Spectroscopy Introduction
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The mid IR region is subdivided as shown below and the approximate frequency (cm -1) at
which various functional groups absorb is shown.
congested region
fingerprint region
group frequency region
N-H
O-H
4000
C-H
3000
CC
C=C
C=N
C=O
CN
2000
1400
1250
1000
800
 (cm-1)
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 sometimes ignored.
In general, more polar bonds absorb more intensely than less polar (and non polar) bonds.
IR analysis can be carried out on gas, liquid, or solid samples.
Obtaining and Interpreting and IR Spectrum
IR spectrometers are scanning instruments. A sample of the substance to be tested is irradiated
with the entire range of mid IR radiation; various functional groups absorbing at different
frequencies. An IR spectrum is produced. It is a plot of transmittance vs. frequency (cm-1) and
wavelength (m). Peaks (absorption bands) appear as dips in the baseline; the baseline running
at the top of the spectrum, near 100% transmittance.
The theory of IR spectroscopy will be covered in greater detail in CHM 526, Organic Chemistry 3.
The purpose of this introductory unit is to give the student a head start at training your eye to
recognize the main functional groups of organic compounds.
Credits:
The following IR scans are from SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ (May, 2004).
This is an excellent Spectroscopy database web site that contains IR, MS, NMR and CNMR
scans for many organic compounds.
IR Spectroscopy Introduction
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Alkanes:
  CH3 @ 2960 & 2850 cm-1 and @ 1460 & 1380 cm-1
  CH2 @ 2930 & 2850 cm-1 and @ 1460 & 720 cm-1
 multiple  C-C bands appear over a broad range of 1200 to 800 cm-1
 Note: all CH bands are below 3000 cm-1
CH3
 CH2 & CH3
< 3000 cm-1
CH2
4
CH3
hexane
CH2 & CH3
1460 cm-1
 CH3
‘umbrella’
1380 cm-1
 CH2
720 cm-1
Alkenes: have all the same absorptions as alkanes plus:
 terminal  =CH2 @ 3080 or internal  =CHR @ 3020 cm-1 (both above 3000 cm-1)
  C=C @ 1650 cm-1 and  =CH2 between 1000 to 720 cm-1
 =CH2
3080 cm-1
CH2 CH(CH2)3CH3
1-hexene
 C=C
1650 cm-1
 =CH2
910 & 990 cm-1
Alkynes: have all the same absorptions as alkanes plus:
 terminal  C-H @ 3300 cm-1 and  C-H between 700 to 600 cm-1
  CC @ 2120 cm-1
CH C(CH2)3CH3
1-hexyne
 C-H
3300 cm-1
IR Spectroscopy Introduction
 CC
2120 cm-1
 C-H
2120 cm-1
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Nitriles: have the same absorptions as alkanes plus
  CC @ 2240 cm-1 is stronger than an alkyne otherwise it looks like a terminal alkyne
CH3
CH2
4
C
N
hexanenitrile
e
 CC
2240 cm-1
Aromatics: have some unique absorption bands
 several  =C-H bands @ 3030 cm-1, ring stretches @ 1500 &/or 1600 cm-1
 group of 2 to 6 small CH bands (‘overtones’) between 2000 & 1700 cm-1
 characteristic overtones and oop patterns are appended
 group of 1 to 3 large  CH ‘out of plane bends’(‘oop’) between 900 & 700 cm-1
Monosubstituted aromatics: have  oop bands @ ca. 700 & 750 cm-1
CH3
toluene
overtones
2000 – 1700 cm-1
 =C-H
3030 cm-1
phenyl ring 
1600 & 1500 cm-1
mono oop 
700 & 750 cm-1
Meta disubstituted aromatics: have  oop bands @ ca. 700, 770 & 870 cm-1
H3C
CH3
m-xylene
 =C-H
3030 cm-1
overtones
2000 – 1700 cm-1
IR Spectroscopy Introduction
phenyl ring 
1600 & 1500 cm-1
meta oop 
700, 770 & 870 cm-1
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Ortho disubstituted aromatics: have a single  oop band below 800 cm-1
CH3
CH3
overtones
2000 – 1700 cm-1
o-xylene
phenyl ring 
1600 & 1500 cm-1
 =C-H
3030 cm-1
ortho oop 
750 cm-1
Para disubstituted aromatics: have a single  oop band at or above 800 cm-1
H3C
CH3
p-xylene
overtones
2000 – 1700 cm-1
phenyl ring 
1600 & 1500 cm-1
 =C-H
3030 cm-1
para oop 
800 cm-1
Ethers: have the same absorptions as alkanes plus
aliphatic ethers: one broad  C-O-C band between 1150 & 1085 cm-1
CH3CH2 O
CH2CH3
diethyl ether
 C-O-C
1125 cm-1
IR Spectroscopy Introduction
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Ethers (Continued):
aryl-alkyl, diaryl, and vinyl ethers:
two broad  C-O-C bands between 1275 & 1200 cm-1 and between 1075 & 1020
O
CH3
methoxybenzene
(anisole)
 C-O-C
1250 & 1030 cm-1
Also note the aromatic bands in methoxybenzene
Alcohols: are easily identified. Characteristic bands are:
 strong, broad  O-H band @ ca. 3300 cm-1
 broad  C-O band between 1260 & 1000 cm-1 (like the ether C-O-C stretch)
 broad, weak  O-H band between 770 & 650 cm-1
CH3CH2CH2CH2 OH
1-butanol
 C-O
1050 cm-1
 O-H
3300 cm-1
 O-H
650 cm-1
Phenols: show the OH and CO stretches as well as the aromatic bands
OH
phenol
 O-H
3300 cm-1
IR Spectroscopy Introduction
 C-O
1240 cm-1
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Carbonyl Compounds: all contain the C=O group which absorbs strongly between
1870 & 1540 cm-1. Electron withdrawing groups (-NO2, -Cl, etc.) increase its frequency while
electron withdrawing groups (-NH2, -OH, phenyl, etc.) lower its frequency.
Ketones: without donating/withdrawing groups show a strong  C=O band @ 1715 cm-1
O
CH3 C
CH2CH2CH2CH3
2-hexanone
 C=O
1715 cm-1
The aryl group in acetophenone lowers the  C=O frequency to 1690 cm-1 and shows
the characteristic aromatic bands
O
C
CH3
acetophenone
 C=O
1690 cm-1
Aldehydes: The normal (aliphatic) aldehyde  C=O band occurs at 1740 to 1720 cm-1.
 Aldehydes have a unique  C-H stretch @ 2820 – 2695 cm-1 (otherwise they are
indistinguishable from ketones)
 C-H
2700 cm-1
O
C
 C=O
1725 cm-1
IR Spectroscopy Introduction
H
cyclohexanecarbaldehyde
8
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In Aromatic Aldehydes the frequency of the  C=O stretch is lowered to 1705 cm-1 and
the characteristic aromatic absorption bands are evident
O
C
H
 C-H
2700 cm-1
benzaldehyde
 C=O
1700 cm-1
Carboxylic Acids:
 The  C=O band occurs at 1710 cm-1 in aliphatic carboxylic acids.
 A very broad  O-H band occurs from 3300 all the way to 2500 cm-1
  C-O and in-plane  O-H occur in the 1400-1200 cm-1 region
 broad  O-H oop occurs near 920 cm-1-
O
CH3CH2CH2CH2CH2 C
OH
hexanoic acid
 C-O
1260 cm-1
 O-H
3300 - 2500 cm-1
 C=O
1710 cm-1
 O-H
1420 & 940 cm-1
In Benzoic Acid the  C=O frequency is lower (1690 cm-1) due to resonance with the
phenyl ring and the monosubstituted aromatic bands are evident
O
C
OH
benzoic acid
 O-H
3300 - 2500 cm-1
 C=O
1690 cm-1
IR Spectroscopy Introduction
 O-H
1430 & 940 cm-1
 C-O
1300 cm-1
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Acid Anhydrides: have two C=O groups joined by an O atom. Their IR spectra are unique as
their carbonyl peak is split into two or more peaks.
  C=O peak stretch occurs between 1860 to 1720 cm-1
 a broad  C-O-C stretch occurs between 1300 to 1050 cm-1
 C-O-C
1100 cm-1
O
O
H3C C O C CH3
acetic anhydride
split  C=O
1800 cm-1
In Phthalic Anhydride, the  C=O band is split into a multiplet.
O
C
O
C
O
phthalic anhydride
split  C=O
1800 cm-1
Acid Chlorides: have an IR spectra much like ketones except that the  C=O band occurs at a
higher frequency (1800 to 1700 cm-1)
 C-Cl
590 cm-1
O
H3C
C
Cl
acetyl chloride
 C=O
1800 cm-1
IR Spectroscopy Introduction
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Esters: show characteristic absorption bands for  C=O and  C-O stretches. Esters might be
mistaken for ketones except for the broad  C-O ester bands
  C=O at 1750 to 1735 cm-1 for aliphatic esters and at 1730 to 1715 cm-1 for aromatic
esters
 all esters have two broad  C-O bands at ca. 1300 and at ca. 1050 cm-1
O
H3C
C
O
CH3
methyl acetate
 C=O
1740 cm-1
 C-O
1250 & 1050 cm-1
Phenyl Benzoate: has a  C=O band @ 1725 cm-1,  C-O bands at 1050 and 1275 cm-1
as well as characteristic absorption bands of a monosubstituted aromatic.
O
C
O
phenyl benzoate
 C=O
1725 cm-1
 C-O
1270 & 1050 cm-1
Halogenated Alkanes: The C-H stretch may be raised above 3000 cm-1 with several
electronegative halogens. Halogens are difficult to identify by IR. Mass Spec is much better.
  CH2-X occurs between 1300 and 1150 cm-1
  C-Cl occurs between 850 and 550 cm-1
  C-Br occurs between 690 and 515 cm-1
Cl
chlorocyclohexane
 C=O
1725 cm-1
methyl acetate
 C-Cl
850 - 550 cm-1
Amides:
IR Spectroscopy Introduction
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Amides:
 slightly below 1700 cm-1, all amides show a  C=O band (‘amide 1’ band) (reduced by
resonance with unpaired electrons on N)
 between 1650 and 1515 cm-1, 1° and 2° amides and some lactams show and  N-H
bend (‘amide 2’ band). This band is usually partially or completely overlapped by the
amide 1 band and may not be distinguishable. 3° have no N-H bonds.
 between 800 and 630 cm-1, 1° and 2° amides (and amines) show medium to weak
 N-H oop bands (similar to the O-H bend of alcohols)
 between 3500 and 3300 cm-1, 1° amides show two  N-H bands while 1° amides show
one  N-H bands. 3° amides have none. They are narrower and less intense than
 O-H bands of alcohols and phenols.
 between 1400 and 1050 cm-1, all amides show a medium intensity  C-N band
O
H
C
N
H
benzamide
(1° amide)
 N-H
3200 & 3370 cm-1
amide 1
1670 cm-1
O
C
amide 2
1580 cm-1
 C-N
1400 cm-1
 N-H
630 cm-1
CH3
N
H
N-methylbenzamide
(2° amide)
 N-H
3300 cm-1
amide 1
1640 cm-1
O
C
amide 2
1550 cm-1
 C-N
1320 cm-1
 N-H
650 cm-1
CH3
N
CH3
N,N-dimethylbenzamide
(3° amide)
has no
 N-H or  N-H
IR Spectroscopy Introduction
amide 1
1630 cm-1
 C-N &  CH3
1380 cm-1
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Amines: The  and  N-H bands and  C-N bands in amines are basically the same as in
amides.
-1
 in the 3500-3300 cm region, 1º amines (R-NH2) show two  N-H bands, 2º amines
(R2-NH) show one  N-H band and 3º amines (R3-N) have no N-H bonds
-1
 between 1650-1500 cm , in-plane  N-H band (scissoring) occurs. (equivalent to the
Amide 2 band of 1 and 2 amides). The band is strongest in 1º amines, small and
difficult to locate in 2º amines, and does not occur in 3º amines
-1
 between 910-670 cm , out-of-plane  N-H occurs. The band is strongest in
1º amines, and does not occur in 3º amines
-1
 between 1250-1020 cm , medium to weak  C-N occurs in aliphatic amines. Oddly, the
frequencies are increased to 1340-1265 cm-1 in aromatic amines
CH3
CH2
4
CH2 NH2
hexanamine
(1° amine)
 N-H
3300 & 3450 cm-1
‘ amide 2’
 N-H
1600 cm-1
CH3CH2 NH
 N-H
800 cm-1
CH2CH3
diethylamine
(2° amine)
 N-H
3300 cm-1
 N-H,  CH2 & CH3
1465 -1480 cm-1
 C-N
1130 cm-1
 N-H
730 cm-1
N(CH2CH3)3
triethylamine
(3° amine)
 CH2 &  CH3
1480 cm-1
has no
 N-H or  N-H
IR Spectroscopy Introduction
 C-N
1100 cm-1
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IR Spectroscopy Introduction
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