Unveiling the triggering mechanism: how intramolecular vibrational energy redistribution drives the initial thermal decomposition of o-nitrotoluene.

The initial decomposition mechanisms of energetic materials at low-moderate temperatures remain poorly understood, especially the underlying mechanism of how vibrational energy is redistributed and ultimately results in bond rupture. o-Nitrotoluene is investigated to map the flow of vibrational energy to the dominant initial reaction pathway and initial decomposition step using the combined approach of temperature-dependent Stokes and anti-Stokes Raman spectroscopy experiments, ab initio molecular dynamics (AIMD) and density functional theory (DFT). Vibrational excited-state populations were derived to reveal the intramolecular vibrational energy redistribution (IVR) characteristics and the subsequent bond-breaking positions. The results identify intramolecular proton transfer, rather than nitro-isomerization, as the dominant initial pathway. Crucially, this reaction is triggered by an IVR mechanism: thermal energy first populates the low-frequency methyl torsion (233 cm-1) and is then efficiently funneled into the high-frequency methyl C-H stretching coordinate (3079 cm-1) via strong anharmonic coupling. This IVR-enhanced effect promotes vibrational activation along the proton transfer coordinate, and the asymmetric Eckart treatment indicates an additional positive tunneling contribution, both of which synergistically promote kinetic feasibility. This work provides a new understanding of the triggering mechanism of nitro-based energetic materials and offers a detailed picture of energy flow leading to initial bond breakage in reactive systems.