A Combined Theoretical and Experimental Study of Sarin (GB) Decomposition at High Temperatures

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.

Calculations on the unimolecular decomposition of the nerve agent VX

“…In this study, we use one-dimensional semiclassical transition state theory (1D SCTST) 36,54 to calculate the canonical rate constants, k(T), at high pressure limit. We showed in a previous study 21 that our method is also applicable to the calculation of the microcanonical rate constant, k(E). Recent developments of SCTST, including the 1D approximation can be found in ref.…”

“…Alternatively, a relatively more cost efficient method proposed by Truhlar and co-workers, [43][44][45][46]53 the multi-structure all-structure (MS-T) method, can be easily adapted into the SCTST rate constant calculation. 21,22 The predicted full two-step mechanism of VX thermal decomposition through pericyclic H transfer reaction has been shown in ref. 22.…”

“…Numerous researches have been devoted to investigate different methods to degrade V-series agents including, among others, surface sorption, [4][5][6][7] hydrolysis, [8][9][10][11][12] perhydrolysis, 13 novel catalysts, [14][15][16][17][18] binding to metal ions, 19 and incineration. In recent studies, the thermally activated pericyclic H-transfer reaction in gas-phase via a 6-memberedring transition state (TS) has been investigated for the G-series OP nerve agent sarin (GB), 20,21 and for VX simulants. 22 Note that this mechanism is only possible for gas-phase reactions; in the solution state this is not a feasible degradation pathway.…”

“…In order to overcome these challenges, we first employ density functional theory (DFT) for the quantum chemistry calculations. In our previous studies, 21,22 we tested the performance of various combinations of DFT functionals and basis sets on calculating the quantum tunneling contribution to reaction rate constants of H-transfer reactions of VX simulants. In the current study, we use the functional with the best performance, M06-2X, 24 and the basis set jul-cc-pVTZ 25 in the calculation.…”

“…In practice, the numerical computations of these anharmonic constants can introduce unnecessary error in a FD calculation. In the current study, we employ the one-dimensional (1D) SCTST, 21,36,[39][40][41] which is a much less expensive and yet sufficiently accurate approximation to the FD SCTST. We have used the 1D SCTST in a combined experimental and theoretical study of a pericyclic H-transfer reaction of sarin, 21 similar to the title reactions, and found excellent agreement between the two.…”