CLASSICAL AND SEMICLASSICAL DYNAMICS OF A CH-BOND BELONGING TO AN ALMOST FREELY ROTATING METHYL GROUP

Abstract

In the course of the years $Cavagnat et al^{(1)}$ have recorded the intrared and Raman spectra of molecules containing $CHD_{2}$-groups in the CH-stretching region and, most recently, they have recorded spectra also in the nearintrared. In the $3000 cm^{-1}$ region they found two features, which they interpreted, in an adiabatic-tpe approximation, as due to the CH-stretching in a freely rotating frame and to the CH-stretching in an oscillating frame respectively. The ratio of intensities is consequentely ascribed to the ratio of the densities of the two different types of states. The frequency gap of the two features is found to be moderately independent of the hindering potential. We have performed classical trajectory studies for the following two degrees of freedom system: a CH-stretching under the action of a Morse potential and a torsion under the action of a potential with a simple cosine form. The coupling was assumed to be in the kinetic energy term. At low energies the Poincare surfaces of sections show that the regular CH-stretching+rotation motion and the regular CH-stretching+torsion motion are separated by resonance and instabilty zones. These regions increase with increasing energy, exhibiting several overlapping resonances. Their role will be discussed. Power spectra have also been calculated, to be compared to the experimental absorption spectra.