THE DETERMINATION OF VIBRATIONAL ANHARMONICITY IN MOLECULES FROM SPECTROSCOPIC OBSERVATIONS

This review article considers the origin of vibrational anharmonicity in molecules, and the effects that vibrational resonances have on the anharmonicity constants which may be extracted from spectroscopic observations. The importance of the effects of Darling-Dennison resonances, which increase with increasing excitation, as well as Fermi resonances, are considered. The local mode approach to X-Y stretching vibrations is dealt with, as a means of simultaneously accounting for Darling-Dennison resonances and of inter-relating normal mode stretching anharmonicity constants, thus reducing the number of parameters to be determined. The inclusion of Fermi resonances, as necessary, into the calculation is next considered, and the joint local mode-normal mode analysis explained. Applications to ethylenic and methyl group molecules are made. The success of the analyses is demonstrated through complete sets of physically representative anharmonicity constants which reproduce vibrational observations into the visible (16 500 cm-1), and which are mutually self-consistent over molecules containing the same functional groups. Extensions of the simple local mode model are considered, as means of achieving anharmonicity parameters which should describe more closely the molecular potential energy surface, and hence the concomitant physical and chemical processes which is controls.