Elucidating the Coupling Mechanisms of Rapid Intramolecular Vibrational Energy Redistribution in Nitromethane: Ab Initio Molecular Dynamics Simulation

Abstract

Ab initio molecular dynamics simulations are presented to investigate the intramolecular vibrational energy redistribution (IVR) of an isolated nitromethane molecule. A number of IVR processes are simulated by monitoring the kinetic energy of vibrational modes under selective low-lying vibrational excitations from their ground states (Δν = 1 or 2). Evolution of the normal-mode kinetic energy gives the ultrafast energy transfer processes from parent modes to daughter modes intuitively. From the ultrafast vibrational transfer made by Fourier transformation of the time-dependent normal-mode kinetic energy, we can capture that the symmetry of the normal modes plays an important role in the anharmonic coupling between the vibrational modes. The results show three symmetry-dependent coupling mechanisms: direct symmetric coupling, overtone-assisted coupling, and rotation-assisted coupling. Furthermore, the calculated efficiencies of IVR also coincide with these mechanisms.

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

“…According to our previous work, 26,27 anharmonic couplings in nitromethane, which dominate these VER processes, are strongly associated with the vibrational symmetry. Hence the coupling mechanisms especially the role of symmetry-dependent couplings in the VER processes were mainly discussed as below.…”

“…The previous research of IVR in a single NM molecule 27 suggests that a mode with B 1 symmetry could couple with modes with B 2 symmetry with the assistance of rotations (including CH 3 torsion), and couple with those with A 1 symmetry through its overtone which also has A 1 symmetry. And these mechanisms can be applied to solid NM.…”

“…It's important to note that our calculation here ignores the nuclear quantum effect (NQE) which would not have a signicant inuence on the coupling mechanism and transfer process according to our simulations at 300 K (400 K) for solid (gaseous) NM. 26,27 For the purpose of visualization and comparison, fourteen vibrational modes were divided into four groups in terms of their frequencies: M1-M3 (2950-3200 cm À1 ), M4-M8 (1250-1600 cm À1 ), M9-M11 (850-1150 cm À1 ), and M12-M14 (400-700 cm À1 ). In one selective excitation, the four-fold degenerate lattice modes corresponding to one molecular vibration were excited simultaneously.…”

“…And three symmetry-dependent coupling mechanisms are concluded: direct symmetric coupling, overtoneassisted coupling and rotation-assisted coupling. 27 To date, most of the existing VER studies of NM focus on the response of vibrations aer selectively exciting high-frequency CH stretching vibrations. Further studies are required to better understand the vibrational dynamics and vibrational energy transfer efficiency aer the selective excitation of lowand middle-frequency vibrational modes.…”

“…And three symmetry-dependent coupling mechanisms are concluded: direct symmetric coupling, overtone-assisted coupling and rotation-assisted coupling. 27 …”