Adiabatic approach to the coupling between local- and normal-like modes

Abstract

An adiabatic method of incorporating harmonic bending and stretching modes into the local mode model is developed. This is achieved by defining local normal mode coordinates which enable one to treat some of the stretches as local modes and the other vibrational degrees of freedom as normal modes in a perfectly systematic fashion. An energy level expression for a system of local modes coupled to local normal modes (harmonic oscillators) is derived. The expression has the same form as the standard normal mode formula. A simple formula for the anharmonicity constant xbs is obtained. The formulas are shown to reproduce well the experimental results for H2O, H2S, and H2Se and are tested against previous calculations.