The dynamics of the initial thermal decomposition step of gas-phase α-HMX is investigated using the master equation method. Both the NO2 fission and HONO elimination channels were considered. The structures, energies, and Hessian information along the minimum energy paths (MEP) of these two channels were calculated at the B3LYP/cc-pVDZ level of theory. Thermal rate constants at the high-pressure limit were calculated using the canonical variational transition state theory (CVT), microcanonical variational transition state theory (μVT). The pressure-dependent multichannel rate constants and the branching ratio were calculated using the master equation method. Quantum tunneling effects in the HONO elimination are included in the dynamical 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 temperature range (500−1500 K). Both channels exhibit strong pressure dependence at high temperatures. Both reach the high-pressure limits at low temperatures. We found that the HONO elimination channel can compete with the NO2 fission, one in the low-pressure and/or high-temperature regime.

The NO 2 fission reaction of gas phase R-HMX has been studied using a direct ab initio method within the framework of microcanonical variational transition state theory (µVT). The potential energy calculations were calculated using the hybrid nonlocal B3LYP density functional theory with the cc-pVDZ basis set. The calculated results show that the potential energy of breaking the axial NO 2 groups is lower than that of breaking the equatorial NO 2 groups. No traditional transition state was found along the reaction path. Microcanonical rate constants calculation shows the variational transition state varies from 2.0 to 3.5 Å of the breaking N-N bond length as a function of the excess energy. The µVT method was used for thermal rate constants calculation over a temperature range from 250 to 2000 K. The fitted Arrhenius expression from the calculated data is k(T) ) 1.66 × 10 15 exp(-18748K/T) s -1 , which is in good agreement with the experimental data at low temperatures.