Branching Ratio and Pressure Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-Phase α-HMX:  An Ab Initio Dynamics Study

Abstract

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.

Direct dynamics study on the reaction of acetaldehyde with ozone

“…In addition, the quantum tunneling effects were included in the rate constant calculations with the SCT tunneling method [38][39][40] employed to calculate the transmission coefficient in the CVT framework, and Eckart tunneling model 29,41,42 employed in the VT framework. There are considerable literatures on the TST, CVT, and VT formalisms.…”

Ab initio calculations of the effects of H+ and NH4+ on the initial decomposition of HMX

“…The earlier calculation results have shown that the N-NO 2 bond rupture is putatively one of the most important initial reactions for the thermal decomposition of nitramine [19,[26][27][28]. Here, we first considered the N-NO 2 bond dissociation reactions for the HMX and the three complexes (PHMX1, PHMX2 and AHMX) as shown in Scheme 1.…”

Modeling the Reactions of Energetic Materials in the Condensed Phase

“…Several theoretical studies have also been reported on the energetics of gas-phase decomposition pathways for HE materials using a variety of methods. For example, we point to work in RDX and HMX where both quantum chemistry [44][45][46][47] and classical simulations of unimolecular dissociation 48,49 were used.…”