File:Chapter-17 Infrared spectroscopy (Vibrational Modes) pp 151-155.pdf

The simplest types or modes of vibrational motion in a molecule that are infrared (IR) active are the stretching and bending modes. In general, asymmetric stretching vibrations occur at higher frequencies than symmetric stretching vibrations. If any group or moiety contains three (03) or more atoms in which two atoms are identical, then this group exhibits two modes of stretching vibrations namely symmetric and asymmetric stretching vibrations. Only stretching vibration occurs for linear molecules having atomicity 02. Molecules with more than 2 atoms can vibrate in different ways (stretching and also bending vibrations). Linear molecules with N number of atoms, have (3N – 5) degrees of vibrational modes, whereas nonlinear molecules have (3N – 6) degrees of vibrational modes (also called vibrational degrees of freedom). IR active molecules are polar. Polarity should be measured w.r.t. electronegativity difference END ≠ 0 of each bond but not w.r.t. their permanent dipole moment (µ = 0 D).More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.