Nonequilibrium Reaction Kinetics in Molecular Solids
MA Wood and A Strachan, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 542-552 (2016).
We explore the possibility of nonstatistical chemical reactions in condensed-phase energetic materials via reactive molecular dynamics (MD) simulations. We characterize the response of nitromethane CH3NO2, HMX cyclic (CH2-NNO2)(4), and PETN C-(CH2-ONO2)(4) to different types of insults: electric fields of various frequencies and strengths and direct heating at various rates. We find that nonequilibrium states can be created for short time scales when energy input targets specific vibrations through the electric fields and that equilibration eventually occurs even while the insults remain present. Interestingly, for strong fields these relaxation time scales are comparable to those of the initial chemical decomposition of the molecules. NM decomposes predominantly via bimolecular reactions, and while insults targeting specific modes lead to strong nonequilibrium states, they do not affect the kinetics associated with decomposition. PETN decomposes via the unimolecular formation of NO2 and, quite interestingly, exhibits faster initial decomposition and lower activation energy when excited by frequency-targeted electric fields. HMX, a larger cyclic molecule, exhibits faster internal relaxation time scales, and the degree of nonequilibrium achieved is smaller than in the other two materials. Therefore, its decomposition is rather independent of insult type and does not deviate from statistical behavior. These results provide insight into nonequilibrium or coherent initiation of chemistry in the condensed phase that would be of interest in fields ranging from catalysis to explosives.
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