Nonequilibrium Reaction Kinetics in Molecular Solids

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.