Symmetry- and Anharmonicity-Controlled Intramolecular Vibrational Energy Flow in Isolated Nitromethane: Ab Initio Molecular Dynamics Simulations

Mode-specific intramolecular vibrational energy redistribution is a key elementary process in the earliest molecular response of energetic materials, yet a quantitative, mode-resolved description of its microscopic driving forces remains challenging. Here we combine density functional theory (DFT), ab initio molecular dynamics (AIMD), normal-mode projection, spectral energy density analysis (SED), and a perturbative mode-coupling model to resolve vibrational energy flow in isolated nitromethane after selective excitation of individual normal modes. The simulations reveal a nonrandom, hierarchical energy-flow network in which low- and midfrequency modes funnel energy toward a small set of accepting modes. Spectral fingerprints of acceptor-mode momenta show that most primary transfer channels are not fundamental-fundamental resonances but overtone-fundamental resonances mediated by cubic anharmonicity. This mechanism naturally explains apparently cross-symmetry transfer: the overtone of an A″ mode transforms as A' and can therefore couple to A' acceptor fundamentals. A simplified Fermi's Golden Rule model, parametrized by bilinear and third-order anharmonic force constants and finite-time Lorentzian detuning factors, predicts the dominant primary acceptors observed in AIMD. The isolated molecule exhibits coherent quantum-beating-like energy oscillations whose periods are governed by the detuning of the corresponding resonance condition. These results recast vibrational energy transfer in nitromethane as a symmetry-filtered anharmonic resonance network and provide a transferable framework for identifying early energy-flow bottlenecks in energetic molecules.