Branching ratio and pressure dependent rate constants of multichannel unimolecular decomposition of gas-phase alpha-HMX: An ab initio dynamics study

The dynamics of the initial thermal decomposition step of gas-phase alpha - HMX is investigated using the master equation method. Both the NO? fission and HONO elimination channels were considered. The structures, energies, an d Hessian information along the minimum energy paths (MEP) of these two cha nnels were calculated at the B3LYP/cc-pVDZ level of theory. Thermal rate co nstants at the high-pressure limit were calculated using the canonical vari ational transition state theory (CVT), microcanonical variational transitio n state theory (mu VT). The pressure-dependent multichannel rate constants and the branching ratio were calculated using the master equation method. Q uantum tunneling effects in the HONO elimination are included in the dynami cal calculations and found to be important at low temperatures. At the high -pressure limit, the NO2 fission channel is found to be dominant in the tem perature range (500-1500 K). Both channels exhibit strong pressure dependen ce at high temperatures. Both reach the high-pressure Limits at low tempera tures. We found that the HONO elimination channel can compete with the NO2 fission, one in the low-pressure and/or high temperature regime.