2.1.4 Allowed transitions in IR spectroscopy

In order for a mode to have an allowed IR transition, the mode must involve a change in the electric dipole moment of the molecule. Stated another way, if the mode is associated with a time varying dipole, the mode can interact with infrared electromagnetic energy to produce a vibrational energy transition. A permanent dipole moment is not required. Consider carbon dioxide, a molecule with no permanent electric dipole moment. (Here, the phrase 'permanent dipole moment' refers to the dipole moment at the equilibrium geometry of an hypothetically non-vibrating molecule.

1. There are 3 atoms ($N=3$) and the molecule in its equilibrium geometry is linear. There are, therefore, 4 vibrational modes.
2. The hypothetical non-oscillating molecule has no permanent dipole moment since the two opposed individual dipole vectors cancel. {insert figure}
3. Since the dipole moment does not change during symmetric stretching, the symmetric stretch mode is inactive. {insert figure}
4. Three modes do cause the dipole moment to change: two degenerate symmetric bends (in plane and out-of-plane) {insert figure} and the asymmetric stretch. {insert figure} These modes, therefore, are infrared active.