QUASI-CLASSICAL DYNAMICS OF BENZENE OVERTONE RELAXATION ON AN AB-INITIO FORCE-FIELD - I - ENERGY-FLOW AND SURVIVAL PROBABILITIES IN PLANAR BENZENE FOR CH(V=2,3)

Computational studies based upon ensembles of quasiclassical trajector ies are presented for 21-mode (planar) benzene, with which we examined relaxation of the first and second (v = 2,3) CH stretch overtones. Th e most complete ab initio force field available for this molecule was used for short time (t<240 fs) and long time (t=2.4 ps) dynamical calc ulations. Local mode analyses indicate that energy flow from the initi ally excited CH stretch is fast (t<240 fs) and irreversible. For both overtones examined, energy from the CH chromophore activates the follo wing local modes in sequence: ipso in-plane CH wag, adjacent CC stretc h, terminal CC stretch, ortho CH in-plane wag, meta CH in-plane wag, i ntermediate CC stretch, meta CH stretch, para CH in-plane wag, and par a CH stretch. Long time calculations indicate that the in-plane CH wag and CH stretching local modes are at steady-state quantities by 2.4 Q s, while the CC stretches and CCC bends continue to take on energy. Th e ortho and meta CH stretches were of minor importance in intramolecul ar vibrational energy redistribution in benzene, gaining little or no energy over the time scales we studied. Survival probabilities indicat e that the half life of the excited CH stretch decreases by almost two thirds as the overtone energy is raised. The results of our study are compared to other classical and quantum dynamical investigations of b enzene and are found to be in good qualitative agreement. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)70343-5].