Medical brain notes 14

  INTRODUCTION The  infrared spectrum  provides the largest number of characteristic properties of a compound. It also serves as a powerful ‘ analytical tool ’ for the extensive and intensive study of molecular structure.   In fact,  infrared absorption spectra  are due to changes in vibrational energy accompanied by changes in rotational energy. Broadly speaking, the range in the electromagnetic spectrum that extends from 0.8 to 200  μ  is referred to as the infrared region. In usual practice, however, either the wavelength ( μ ) or the wave number (cm –1 ) is employed to measure the position of a given infrared absorption. More precisely, the infra-red regions may be categorized into three distinct zones based on their respective wave numbers and wave-lengths as stated below : Another school of thought advocates that there are two general regions in the infrared spectrum, namely : ( a )  Group frequency region :  having a wavelength ra...

THEORY The underlying principle of infrared spectroscopy is based upon the molecular vibrations which is further composed of the stretching and the bending vibrations of a molecule.   Therefore, it would be necessary to have a clear concept of various modes of vibrations often encoun-tered in different molecules having a variety of functional moieties, laws governing them and the mathemati-cal derivations related to them.   1. MOLECULAR VIBRATIONS   A molecule may not be looked upon as a rigid assemblage of atoms. Rather it may be regarded as a sort of flexible system comprising of balls of varying masses representing the atoms of a molecule and springs of varying strengths representing the chemical bonds of a molecule.   The vibrations for molecules are of  two  types, namely :   ( a ) Stretching, and   (b) Bending (or deformation).   1.1.  Stretching   Vibration causes stretching whereby the distance between the two atoms increase...