Aminolysis of a Model
Nerve Agent: A Computational
Reaction Mechanism Study of <i>O</i>,<i>S</i>‑Dimethyl
Methylphosphonothiolate
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Abstract
The mechanism for the aminolysis of a model nerve agent, <i>O</i>,<i>S</i>-dimethyl methylphosphonothiolate, is
investigated both at density functional level using M062X method with
6-311++G(d,p) basis set and at ab initio level using the second-order
Møller–Plesset perturbation theory (MP2) with the 6-311+G(d,p)
basis set. The catalytic role of an additional NH<sub>3</sub> and
H<sub>2</sub>O molecule is also examined. The solvent effects of acetonitrile,
ethanol, and water are taken into account employing the conductor-like
screening model (COSMO) at the single-point M062X/6-311++G(d,p) level
of theory. Two possible dissociation pathways, methanethiol and methyl
alcohol dissociations, along with two different neutral mechanisms,
a concerted one and a stepwise route through two neutral intermediates,
for each pathway are investigated. Hyperconjugation stabilization
that has an effect on the stability of generated transition states
are investigated by natural bond order (NBO) approach. Additionally,
quantum theory of atoms in molecules analysis is performed to evaluate
the bond critical (BCP) properties and to quantify strength of different
types of interactions. The calculated results predict that the reaction
of <i>O</i>,<i>S</i>-dimethyl methylphosphonothiolate
with NH<sub>3</sub> gives rise to parallel P–S and P–O
bond cleavages, and in each cleavage the neutral stepwise route is
always favorable than the concerted one. The mechanism of NH<sub>3</sub> and H<sub>2</sub>O as catalyst is nearly similar, and they facilitate
the shuttle of proton to accelerate the reaction. The steps involving
the H<sub>2</sub>O-mediated proton transfer are the most suitable
ones. The first steps for the stepwise process, the formation of neutral
intermediate, are the rate-determining step. It is observed that in
the presence of catalyst the reaction in the stepwise path possesses
almost half the activation energy of the uncatalyzed one. A bond-order
analysis using Wiberg bond indexes obtained by NBO calculation predicts
that usually all individual steps of the reactions occur in a concerted
fashion showing equal progress along different reaction coordinates