Vibrational energy redistribution in crystalline nitromethane simulated by ab initio molecular dynamics

Abstract

Ab initio molecular dynamics simulations (AIMD) are systematically performed to study the Vibrational Energy Redistribution (VER) in solid nitromethane (NM) by combining normal mode decomposition and short-time Fourier transform technique. After the selective excitations of all fourteen intramolecular vibrational modes above 400 cm⁻¹, four three-dimensional (3D) excitation and detected vibrational spectra are obtained. The evolution of the kinetic energy proportion of all vibrations are also given and discussed quantitatively. These results show that, as the daughter modes, NO₂ symmetric stretches, CH₃ stretches and bends are usually excited quickly and relatively conspicuously compared with the other vibrations. Interestingly, we found that, although the stretching vibration of the CN bond which is a bridge between the methyl and nitro group can not respond immediately to the selective excitations, it always accumulates the vibrational energy slowly and steadily. Then, the underlying mechanisms are discussed based on the response of vibrational modes in both the time and frequency domain. As a result, we found that anharmonic transfers following symmetry rules which involve the couplings assisted by the overtones and rotations, as well as the transfers among the adjacent modes, play important roles in the VER of solid NM.