Molecular Dynamics Simulations of Normal Mode Vibrational Energy Transfer in Liquid Nitromethane

Abstract

Nonequilibrium molecular dynamics simulations are used to study vibrational energy relaxation (VER) in liquid nitromethane after excitation of the C−H stretching vibrations. The simulated liquid consists of a mixture of excited and unexcited molecules, where the unexcited molecules were used to monitor excitation of the bath upon vibrational relaxation of the excited molecules. A previously developed projection method (Raff, L. M. J. Chem. Phys. 1988, 89, 5680) was employed to monitor the normal-mode kinetic energies of vibrations in both excited and unexcited molecules as functions of time. Overall, the results are in qualitative agreement with experimental measurements of VER in liquid nitromethane after mid-IR excitation in the C−H stretching region (Deak, J. C., Iwaki, L. K., Dlott, D. D. J. Phys. Chem. A 1999, 103, 971). The simulation results indicate that the excited C−H stretching vibrations deposit energy predominantly into the remaining vibrations in the molecules. These vibrations relax at different rates, resulting in a multistage vibrational cooling process for nitromethane, in agreement with experimental results. The excitation of vibrations of the surrounding unexcited molecules occurs through indirect rather than direct intermolecular vibrational energy transfer processes, also in agreement with experiment.