STRETCH-BEND COUPLING EFFECT ON THE INTRAMOLECULAR VIBRATIONAL-RELAXATION IN A 2-MODE MODEL OF CH OVERTONE EXCITED FLUOROFORM

A classical dynamic study of the effect of stretch-bend coupling and CH anharmonicity on the occurrence of Fermi resonance and intramolecular vibrational relaxation (IVR) has been carried out for a two-mode Hamiltonian proposed by Halonen et al. for CH overtone excited fluoroform. The analysis is made in terms of variations of the phase space structure and overtone relaxation rate constants, with respect to CH stretch anharmonicity and various approximations to the kinetic and potential couplings. CH bond anharmonicity is confirmed as the most important factor in the dynamics of overtone relaxation and it controls the occurrence and intensity of the 2:1 Fermi resonance between the CH stretch and the adjacent bends. The kinetic coupling is much more important than the potential, and, contrary to the effect of this, it tends to decrease the stability of the CH periodic orbit. The overtone relaxation rate constants, estimated by linear stability analysis of the CH periodic orbit, are in good agreement with the experimental results. © 1990.