Estudo da desativação do agente VX usando o MgO por cálculos ab initio

Abstract

The organophosphates are used in industry, in the fields of dyes, varnishes, artificial leather, electrical insulation, waterproofing, plastics, oil additives and solvents, are also used in medicine to treat diseases such as glaucoma, in the household and in agriculture are applied as insecticides and pesticides. Besides these various civilian facilities, the organophosphates are used as well as chemical weapons of mass destruction, and have similar structure to those of compounds used as insecticides and pesticides. In the case of VX, an organophosphate used as a chemical weapon, the fall in P­S binding is essential in its chemical degradation. Although many chemical reactions can be used to decompose the chemical agents of war, only some can be used in practice in a breakthrough, because these reactions need to be simple and the reagents used should be stable, inexpensive and low molecular weight. However, many of the reactions that may be useful for the neutralization of the neurotoxic VX agent still follow under discussion in recent polls, but most of them are confined to some successes catalyzed hydrolysis. In this dissertation we studied processes of the VX agent catalyzed hydrolysis by MgO (001) by means of ab initio calculations. We used the program PWscf ­ Plane Waves Self­Consistent Field. The PWscf using the Density Functional Theory from a set of basic autofunction given by plane waves and pseudopotentials. Among other attributes, this code is able to calculate the energy of the ground state of the Kohn­Sham orbital for an electron, and atomic force under different conditions of stress, structural optimization and transition state. The VX molecule was replaced by a smaller molecule, called a p­VX, which replaced some of the radicals VX for methyl groups. This was done to reduce the size of the molecule, which reduces the computational cost, but not substantially affect the chemistry of the problem, the study of breaking the link P­S. The proposed mechanism of hydrolysis is related to the breaking of the link heterolytic P­S, with the consequent formation of intermediate ion R­P+ and R'­S­, they, in turn, stabilized by quimissorption the MgO(0010 surface. Together with this reaction, is the dissociation of water molecules to the formation of ions H+ and HO­, whose ions are stabilized on the MgO(001) surface. The final step is the recombination of these ions, to generate products of hydrolysis and R­POH and R'­SH, followed by the process of desorption of molecules from the surface of the catalyst. For the overall reaction of hydrolysis of the molecule p­VX, the variation of internal energy was calculated to be ­5.66 kcal/mol. Were determined the structures of ion R­P+ and R'­S­ stabilized on the MgO(001) surface, with an formation energy calculated at ­0.20 kcal/mol, indicating that the middlemen have good stability on the surface is compared with the p­VX molecule original. In dissociative quimissorption of water molecules on the MgO(001) surface, it was found that the ions formed are stable only if a minimum distance of 4.70 Å. Any distance below this will lead to formation of the water molecule again. In the process involving two water molecules, only one is dissociated, while the other ions stabilizes by the hydrogen bonding formed. The non­dissociated molecule also interacts with a surface site of magnesium. This result was confirmed by the calculations of load density difference of the system, determining the reaction path, where he obtained a energy barrier calculated at 5.55 kcal/mol for the direct reaction and 7.53 kcal/mol for the reverse reaction and by partial decoupling using a trimer of water molecules, with energy calculated at ­5.40 kcal/mol. The results show that the proposed mechanism for the catalyzed hydrolysis of the neurotoxic agent VX by MgO is possible. The models constructed can be modified for testing of new catalysts with MgO type structure, via addition of doping or defects on the surface of the crystal structure, to the development of more efficient catalysts for the hydrolysis reaction with the same mechanism.