Page 261 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 251

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Wavenumber 13,000–4,000 cm 4,000–200 cm 200–10 cm
Wavelength 0.78–2.5 µm 2.5–50 µm 50–1,000 µm

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This chapter focuses on the most frequently used mid IR region, between 4000 and 400 cm (2.5
to 25 µm). The far IR requires the use of specialized optical materials and sources. It is used for analysis
of organic, inorganic, and organometallic compounds involving heavy atoms (mass number over 19).
It provides useful information to structural studies such as conformation and lattice dynamics of sam-
ples. Near IR spectroscopy needs minimal or no sample preparation. It offers high-speed quantitative
analysis without consumption or destruction of the sample. Its instruments can often be combined with
UV-visible spectrometer and coupled with fiberoptic devices for remote analysis. Near IR spectroscopy
has gained increased interest, especially in process control applications.

Theory of Infrared Absorption

At temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect
to each other. When the frequency of a specific vibration is equal to the frequency of the IR radiation
directed on the molecule, the molecule absorbs the radiation.
Each atom has three degrees of freedom, corresponding to motions along any of the three Carte-
sian coordinate axes (x, y, z). A polyatomic molecule of n atoms has 3n total degrees of freedom.
However, 3 degrees of freedom are required to describe translation, the motion of the entire molecule
through space. Additionally, 3 degrees of freedom correspond to rotation of the entire molecule.
Therefore, the remaining 3n – 6 degrees of freedom are true, fundamental vibrations for nonlinear
molecules. Linear molecules possess 3n – 5 fundamental vibrational modes because only 2 degrees
of freedom are sufficient to describe rotation. Among the 3n – 6 or 3n – 5 fundamental vibrations (also
known as normal modes of vibration), those that produce a net change in the dipole moment may re-
sult in an IR activity and those that give polarizability changes may give rise to Raman activity. Nat-
urally, some vibrations can be both IR- and Raman-active.
The total number of observed absorption bands is generally different from the total number of fun-
damental vibrations. It is reduced because some modes are not IR active and a single frequency can
cause more than one mode of motion to occur. Conversely, additional bands are generated by the ap-
pearance of overtones (integral multiples of the fundamental absorption frequencies), combinations of
fundamental frequencies, differences of fundamental frequencies, coupling interactions of two funda-
mental absorption frequencies, and coupling interactions between fundamental vibrations and over-
tones or combination bands (Fermi resonance). The intensities of overtone, combination, and difference
bands are less than those of the fundamental bands. The combination and blending of all the factors thus
create a unique IR spectrum for each compound.
The major types of molecular vibrations are stretching and bending. The various types of vibrations
are illustrated in Fig. 15.2. Infrared radiation is absorbed and the associated energy is converted into
these type of motions. The absorption involves discrete, quantized energy levels. However, the individ-
ual vibrational motion is usually accompanied by other rotational motions. These combinations lead to
the absorption bands, not the discrete lines, commonly observed in the mid IR region.