Hydrolysis of a VX-like Organophosphorus Compound through Dissociative Chemisorption on the MgO(001) Surface

Organophosporous VX agent O-ethyl S-(2-dlisopropylethylamino)ethyl methylphosphonothioate is one of the main nerve agents. For this reason, the search for ways to deactivate it is very important. In this work, hydrolysis and adsorption reactions of a VX-like compound (O,S-dimethyl methylphosphonothioate, DMPT) on the MgO(001) surface were studied by density-functional theory (OFT) using periodic boundary conditions. A degradation reaction mechanism was proposed and theoretically investigated on two types of MgO(001) surfaces: the terrace and the Al-doped. Conformations, free-energy differences, transition states, reaction barriers, and minimum-energy paths were computed. We found that the P-S bond, related to the agent toxicity, breaks via hydrolysis occurring spontaneously throughout the analyzed temperature range, 100-600 K. In the dissociative chemisorption of the DMPT molecule, the formation of the MgO: [PO(CH3)(OCH3))+[SCH3](-) intermediate is catalytically favored from a temperature of about 335 K for the Al-doped surface, a value considerable smaller than the 500 K value for the same process on the terrace. At 335 K, the dissociation fragments on the Al-doped surface are less stable in comparison to the hydrolysis products. The possible reconstitution of the P-S bond on both surfaces does not occur according to kinetic analysis; however, the electronic energy barrier for the direct dissociation reaction on the Al-doped sites is about 49.0 kJ/mol lower than the value for the terrace. After recombination with the OH- and H+ ions, the HOPO(CH3)(OCH3) and HSCH3 products do not accumulate on either surface because these molecules desorb below the DMPT dissociation temperatures. The Al-doped sites of MgO(001) are thus more active in the catalytic hydrolysis process of the VX-like organophosphorus compound than is the nondoped surface.