Ab initio molecular dynamics simulation of vibrational energy redistribution of selective excitation of C-H stretching vibrations for solid nitromethane

Vibrational Energy Transfer in Energetic Ionic Liquid 4-Amino-1H-1,2,4-triazolium Nitrate: Ab Initio Molecular Dynamics Simulations

Energetic ionic liquids (EILs) represent a distinctive class of energetic materials with substantial research significance and promising energetic applications. In this work, we delved into the vibrational energy transfer mechanism within the EILs, specifically focusing on 4-amino-1H-1,2,4-triazolium nitrate (ATN), utilizing ab initio molecular dynamics simulations. Our work illustrates distinct energy transfer patterns for different vibrational modes. Upon exciting the stretching vibration of the NH group in the cationic group, vibrational energy preferentially migrates to the neighboring CH bond within the aromatic ring on the femtosecond to picosecond time scales and notably in an in-phase coherent energy transfer fashion. In contrast, exciting the stretching vibration of the N9H11 bond triggers the transfer of vibrational energy to its neighboring N9H10 bond in an out-of-phase coherent fashion. Conversely, exciting the stretching vibration of the N9H10 bond leads to energy transfer predominantly through intermolecular pathways due to the hydrogen-bonding interaction between this bond and the anion. The vibrational energy of the excited N9H10 stretch is shown to dissipate very rapidly, displaying a fast component (with a time constant as short as ca. 7 fs) and a slow component (ca. 230 fs).

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

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.

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.