Decomposition of O,S-dimethyl methylphosphonothiolate by ammonia on magnesium oxide: a theoretical study of catalytic detoxification of a chemical warfare agent.

The adsorption of a model nerve agent, O,S-dimethyl methylphosphonothiolate (DMPT), on the hydroxylated and unhydroxylated nano-crystalline magnesium oxide surface followed by the nucleophilic attack of ammonia (NH3) is investigated at the M06-2X/6-311++G(d,p) level of theory using the representative cluster models. The geometries of DMPT and NH3 are fully optimized, while the geometry of the oxide fragment is kept frozen. The main insight of this study is the incorporation of the Eley-Rideal mechanism for the first time in the detoxification process, where one of the reactant molecules (DMPT) is adsorbed and the other one (NH3) reacts with it directly impinging from the gas phase. There are two possible pathways of nucleophilic detoxification, either concerted or stepwise. The nature of the first transition state of nucleophilic attack in both pathways is the vital step for degradation. Our calculated results predict that the reaction of DMPT with NH3 gives rise to both P-S and P-O bond cleavage completely. Also, the P-S cleavage is found to be the favorable one over P-O bond breaking. The exploration of the overall reaction mechanism has established the catalytic activity of nano-crystalline MgO in nucleophilic DMPT degradation, as in all cases the activation barriers have reduced compared to the previously reported aminolysis of DMPT in the gas phase. Interestingly, the hydroxylated model has better catalytic performance than the unhydroxylated one.