Infrared spectroscopy analyzes the vibrational and rotational absorption bands of molecules within the infrared region of the electromagnetic spectrum. Different functional groups absorb infrared radiation at characteristic wavelengths that can be used for compound identification. Molecular vibrations occur when bonds stretch, bend, scissor, rock, wag, or twist, causing a change in dipole moment that allows absorption of infrared radiation.

1. ‘Theory of IR Spectroscopy and Modes of Molecular Vibrations’
Presented by: Facilitated to:
Mr. L. Sanathoiba Singha
M. Pharm 1st Semester
Pharmaceutical Analysis.
Mr. T. Sreenivas Rao
Assistant Professor
Pharmaceutical Chemistry.
Karnataka College of Pharmacy
Bengaluru-64, Karnataka.
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2. Region Wavelength (λ) range in µ Wave number (ῡ) range in cm-1
Near IR 0.78 - 2.5 12800 - 4000
Middle IR 2.5 – 50 4000 - 200
Far IR 16 - 200 625 - 10
THEORY OF IR SPECTROSCOPY:
• Almost any compound having covalent bonds, whether organic or inorganic, absorbs various frequencies of EMR radiation in
the infrared region of the EMR.
• The infrared region of EMR may be divided into the following sections:
• At wavelengths below 25µm the radiation has sufficient energy
to cause changes in the vibrational energy levels of the
molecule, and these are accompanied by changes in the
rotational energy levels.
• The main region of interest for analytical purposes is from
wavelengths 2.5 to 25µm i.e. wavenumbers 4000 to 400cm-1.
Table 1. Subdivision of IR.
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3. Fig. 1. A portion of EMR spectrum showing relationship of vibrational infrared to other types of radiation.
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4. • Molecules contain bonds of specific spatial orientation and energy.
• These bonds are seldom completely rigid, and when energy is supplied, they may bend, distort or stretch.
• A very approximate model compares the vibration to that of a harmonic oscillator, such as an ideal spring.
• In the spring-ball system depicted above, if the spring connecting the two balls is struck with a force, vibrations are produced
which can be described by Hooke’s law of simple harmonic motion.
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5. • If the spring has a force constant, k and masses m1 and m2 at the ends, then the theoretical vibration frequency, ν is given
by:
ν =
1
2π𝑐
√(
𝑘
μ
)
Where, μ =
m1.m2
m1 + m2
and c is the velocity of light in vacuum.
• For example, C─O bond in methanol (CH3OH) has:
 k = 5 x 102 N m-1
 μ = 6.85 x 1.660 x 10-27 kg
Using these values in the above formula, ν can be calculated and found to be 1113cm-1.
• Practically, the observed C─O band for methanol is at 1034cm-1, which is approximately close to the calculated value above.
• Therefore, if force constant is known, approximate frequency of band can be calculated.
• The force constant for a bond varies slightly from one compound to another.
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6. • For absorption of energy to occur from the incident infrared radiation and vibrational transitions to occur, it is essential that a
change in dipole moment occurs during the vibration.
• The electric field associated with the infrared radiation will interact with the molecule to change its electrical properties.
• Vibration between two atoms in diatomic molecules, will not result in a change of electrical symmetry or dipole moment of the
molecule, and such molecules will not absorb in the infrared region.
• Some molecules have a dipole moment due to charge separation and will interact with the field. E.g. HCl
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7. • Others may acquire a dipole moment when they vibrate. E.g. methane, CH4, has no dipole, but when one of the C─H bonds
stretches, the molecule will develop a temporary dipole.
• Since every type of bond has a different natural frequency of vibration, and since two of the same type of bonds in two different
compounds are in two slightly different environments, no two structure have exactly the same infrared absorption pattern.
• The vibrational modes or transitions are characteristic of the groups in a molecule and are useful in the identification of a
compound, particularly in establishing the structure of an unknown compound.
• Thus, the infrared spectrum can be used for molecules much as a fingerprint can be used for humans.
• For identification of a pure compound, the spectrum of the unknown substance is compared with the spectra of a limited number
of possible substances suggested by other properties. When a match between spectra is obtained, identification is complete.
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8. Group Wavenumber (cm-1)
C─H (aliphatic) 2700-3000
C─H (aromatic) 3000-3100
O─H (phenolic) 3700
O─H (phenolic, hydrogen bonding) 3300-3700
C═O (aldehyde) 1720-1740
C═O (ketone) 1705-1725
C═O (acid) 1650
C═O (ester) 1700-1750
Table 2. Approximate positions of some IR absorption bands.
Fig. 2. IR spectrum of toluene.
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9. MODES OF MOLECULAR VIBRATIONS:
Modes Types
(i) Stretching vibrations
(ii)
Bending
vibrations
In plane bending
Out of plane
bending
Symmetric Asymmetric
Scissoring Rocking
Wagging Twisting
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10. REFERENCES:
 Pavia D. L, Lampman G. M and Kriz G. S. Introduction to Spectroscopy. Thomson Learning Inc. USA. 2001 (3rd Edition). 13:15.
 Kasture A. V, Mahadik K. R, Wadodkar S. G and More H. N. A Textbook of Pharmaceutical Analysis Instrumental Methods
Volume II. Nirali Prakashan, Pune. 2007 (17th Edition). 208.
 Jeffery G. H, Bassett J. Mendham J and Denny R. C. Vogel’s Textbook of Quantitative Chemical Analysis. Longman Scientific and
Technical, England. 1989 (5th Edition). 741-744.
 Wikipedia contributors. (2018, June 30). Infrared spectroscopy. In Wikipedia, The Free Encyclopedia. Retrieved 14:24, October 8,
2018, from https://en.wikipedia.org/w/index.php?title=Infrared_spectroscopy&oldid=848160003
 Kealey D and Haines P. J. Instant Notes Analytical Chemistry. BIOS Scientific Publishers Limited, UK. 2005. 233-234.
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