Hidrólise de um composto organofosforado tipo-vx pela quimissorção dissociativa na superfície de MgO(001), por cálculos ab initio

Abstract

The VX agent, O-ethyl S-(2-diisopropylethylamino)ethyl methylphosphonothioate, is one of the main neurotoxic agents, thus the search for ways to degrate it is considerably important. In this work, the hydrolysis of a VX-like organophosphorus compound (O,S-dimethyl methylphosphonothioate, DMPT) by the dissociative chemisorption on the MgO(001) surface was studied by density-functional theory using periodic boundary conditions. A degradation mechanism involving the reactions of the DMPT and water molecules was proposed and investigated on two types of MgO(001) surfaces: terrace and Al-doped. Conformations, free energy differences, transition states e reaction barriers were calculated. Firstly, it was verified that only the P-S neurotoxic bond breaks in the hydrolysis of the DMPT compound, which can occur spontaneously throughout the analyzed temperature range (100-600 K). In the dissociative chemisorption of the DMPT molecule, the formation of intermediate MgO:[PO(CH3) (OCH3)]+[SCH3]− is thermodynamically fewer stable than the hydrolysis products from the temperature of about 335 K for the Al-doped surface, which is less than the same process calculated on the surface without defects (from 500 K). According to a reactional barrier analysis, the possible reconstitution of the P-S bond does not occur on both cases of analyzed MgO(001) surface models. However, the electronic energy barrier for the dissociation reaction on the Al-doped sites is about 49.0 kJ/mol less than the one on the terrace. At the same time, the process of the formation of H+ and OH− ions on the MgO(001) terrace is relevant as the hydroxylation initial step of this surface and it is part of the catalyzed hydrolysis mechanism of the DMPT compound. The adsorption of one, two and three water molecules were only obtained on the MgO(001) terrace because it is known that water molecules are dissociated spontaneously on point defects. The variation of the Gibbs free energy for the adsorption and dissociation processes was calculated in the 100-600 K temperature range. The thermodinamic results showed that the adsorption of a single water molecule does not lead to dissociation. For the dimer and trimer of water molecules, one molecule dissociates while the others co-adsorbed stabilize the H+ and HO− ionic species on the surface. In the two cases, the dissociation products on the surface converged for the formation of hydrogen bonds among the formed hydroxyl and water molecules. As a consequence of these interactions, the protonated surface coexists with the adsorbed hydroxyl ions. The electronic energy barriers are not large enough to forbid the partial dissociation of two (23.2 kJ/mol) and three (24.9 kJ/mol) water molecules because they would be easily surmounted. Therefore, the initial step for the hydrolysis on the MgO terrace starts from two water molecules, but the dissociated product is more stable when there are three water molecules III chemisorbed. Regarding the migration of the H+ and HO− ions after the dissociation, the calculated electronic energy barriers showed that this process on the MgO(001) surface is unfavorable. Thus, the dissociation processes of the DMPT and H2O molecules on the MgO(001) surface should happen in close regions to facilitate the next step of the proposed reaction mechanism, which is the ionic recombination of [PO(CH3)(OCH3)]+, [SCH3]-, HO- and H+ for the subsequent formation of the P1 [HOPO(CH3)(OCH3)] and P2 [HSCH3] products. The products P1 and P2 did not accumulate on the Al-doped surface because these molecules are desorbed from 197 K. Therefore, if compared to the hydrolysis reaction, 335 K is an ideal temperature to avoid the accumulation of the products on the analyzed point defects, with the consequent spontaneous desorption of P1 and P2 and the MgO reconstitution in the final step of the catalytic process. However, the sites of the terrace can also participate of the DMPT catalyzed hydrolysis mechanism from 500 K. In this work, the MgO(001) surface works as a catalyst for the degradation of VX agent, but with higher selectivity of the Al-doped sites than that of the terrace sites. In the same way, these results have an important variety of applications, as well as reference for further studies of the VX compound reaction on the MgO(001) surface with other kinds of defects or other surfaces. Thus, these results scientifically contribute to the area of catalysis and oxide surfaces in the chemical deactivation of neurotoxic agents, especially the V-type agents. Furthermore, this work will enable the development of new technologies for national defense in order to enable the chemical degradation of these types of compounds without affecting the environment.