Abstract
Theoretical and experimental results are presented for the pyrolytic decomposition of the nerve agent sarin (GB) in the gas phase. High-level quantum chemistry calculations are performed together with a semiclassical transition-state theory for describing quantum mechanical tunneling. The experimental and theoretical results for the temperature dependence of the survival times show very good agreement, as does the calculated and measured activation energy for thermal decomposition. The combined results suggest that the thermal decomposition of GB, for temperature ranging from 350 to 500 °C, goes through a pericyclic reaction mechanism with a transition state consisting of a six-membered ring structure.
Highlights
The organophosphorus (OP) chemical warfare agent (CWA) isopropyl methylphosphonofluoridate, known as sarin and GB, is a highly toxic nerve agent that acts via the inhibition of acetylcholine esterase.[1]
We found that the deep tunneling (DT) corrections proposed by Wagner[44] should be included in a 1D semiclassical transition-state theory (SCTST) calculation
We note that the DT correction requires the information of the reverse barrier height, which brings a small amount of additional computational cost to the standard SCTST calculation, but the improvement on the resulting rate constant can be significant
Summary
The organophosphorus (OP) chemical warfare agent (CWA) isopropyl methylphosphonofluoridate, known as sarin and GB, is a highly toxic nerve agent that acts via the inhibition of acetylcholine esterase.[1]. Glaude et al.[21] have reported calculated reaction mechanisms for the degradation of GB, DIMP, dimethyl methylphosphonate (DMMP), and trimethyl phosphate (TMP) under simulated incineration conditions (natural gas 94% methane, 6% ethane) of 1 atm pressure, 1500 K, and a residence time of 0.1 s. Ash et al.[22] have investigated the mechanism and kinetics of the gas-phase unimolecular isomerization and breakdown of GB and pinacolyl methylphosphonofluoridate (soman, or GD) over the temperature range of 300−1000 K using computational methods. The most favorable decomposition route identified for the decomposition of GB in this study is again via the formation of a six-membered transition state (TS) with calculated activation energy of 163.6 kJ/mol. We report combined experimental and computational study of the unimolecular gas-phase thermal degradation of GB.
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