Semiclassical molecular vibrational spectroscopy of the pre-reaction complex for the Cl - + CH 3 Cl S N 2 reaction

Semiclassical (SC) molecular dynamics theory provides a general and well-defined tool for including all quantum effects in classical mechanics.Particularly, the semiclassical initial value representation (SC-IVR) theory has been developed as a powerful and accurate tool for calculating molecular vibrational spectral densities, which exclusively relies on the classical trajectories. The pre-reaction complex Cl - ---CH 3 Cl of S N 2 reaction Cl - + CH 3 Cl is of interest to investigate the molecular vibrational power spectrum by SC-IVR methods. An analytic potential energy surface (J. Phys.Chem. 1990, 94, 2778) is used to perform molecular dynamics simulations with 10,000 ~ 500,000 trajectories. Among all semiclassical molecular dynamics trajectories, no dissociation to Cl - + CH 3 Cl or isomerization barrier crossing occurs. From the SC-IVR calculation, quantitative fundamental frequencies (vibrational quantum level 0-1 transition) are obtained with harmonic and anharmonic molecular zero-point vibrational energies (ZPE) of 24.34 and 24.12 kcal/mol respectively. The SC-IVR spectroscopy also shows qualitative overtone frequencies and vibrational modes coupling. The frequencies are further compared with second order vibrational perturbation theory (VPT2) calculations and molecular dynamics classical power spectrum. The weak coupling between vibrational modes agrees with the intrinsic non-RRKM behaviors of Cl - + CH 3 Cl S N 2 reaction.