TABLE OF CONTENTS The infrared region of the electromagnetic spectrum includes radiation at wavelengths between 0.7 and 500 ?m or, in wave numbers, between 14 000 and 20 cm ?1. The relationship between wave number and wavelength scales in the infrared region is given in Table 7.1. Molecules have specific frequencies that are directly associated with their rotational and vibrational motions. Infrared absorptions result from changes in the vibrational and rotational state of a molecular bond. Coupling with electromagnetic radiation occurs if the vibrating molecule produces an oscillating dipole moment that can interact with the electric field of the radiation. Homonuclear diatomic molecules such as hydrogen, oxygen, or nitrogen, which have a zero dipole moment for any bond length, fail to interact. These changes are subtly affected by interaction with neighboring atoms or groups, as are resonating structures, hydrogen bonds, and ring strain. This imposes a stamp of individuality on each molecule s infrared absorption spectrum as portions of the incident radiation are absorbed at specific wavelengths. The multiplicity of vibrations occurring simultaneously produces a highly complex absorption spectrum that is uniquely characteristic of the functional groups that make up the molecule and of the overall configuration of the molecule as well. It is therefore possible to identify substances from their infrared absorption spectrum. [1]
