Viviane S. Vaiss

Theoretical study of the reaction between HF molecules and hydroxyl layers of Mg(OH)2.

The reaction of HF molecules with brucite, Mg(OH)(2), leading to the formation of Mg(OH)(2-x)F(x), was theoretically studied by ab initio density functional theory (DFT) with periodic boundary conditions. We proposed as mechanism for this reaction four elementary steps: adsorption of the HF molecule, OH(-) liberation from brucite as a water molecule, desorption of the newly formed H(2)O, and rearrangement of the F(-) anion into a hydroxyl position. For the Mg(OH)(2-x)F(x) formation, with x = 1/9, the final product, outcome from an initially adsorbed HF molecule, we computed the Helmholtz free energy variation DeltaF = -23 kcal/mol. The calculated frequency for the most intense infrared band, a Mg-F stretching mode, was 342 cm(-1). Two transition states, corresponding to the hydroxyl reacting with a proton forming a water molecule and migration of a fluoride anion into a hydroxyl vacancy, were computed. The calculated reaction barriers indicate that the reaction between Mg(OH)(2) layers and HF molecules is slow and irreversible.

Combining Nuclear Magnetic Resonance Spectroscopy and Density Functional Theory Calculations to Characterize Carvedilol Polymorphs

The experiments of carvedilol form II, form III, and hydrate by (13)C and (15)N cross-polarization magic-angle spinning (CP MAS) are reported. The GIPAW (gauge-including projector-augmented wave) method from DFT (density functional theory) calculations was used to simulate (13)C and (15)N chemical shifts. A very good agreement was found for the comparison between the global results of experimental and calculated nuclear magnetic resonance (NMR) chemical shifts for carvedilol polymorphs. This work aims a comprehensive understanding of carvedilol crystalline forms employing solution and solid-state NMR as well as DFT calculations.

Theoretical Chemistry at the Service of the Chemical Defense: Degradation of Nerve Agents in Magnesium Oxide and Hydroxide Surface

The Pró-Defesa funding of projects was a joint initiative of CAPES and the Ministry of Defense to fund research in defense. In this article we review the main scientific results obtained in the project “Formação de Pessoal Qualificado em Química Quântica Computacional para Atuação na Área de Defesa Química”, funded by the first edition of Pró-Defesa in 2005, which involved a collaboration between the Departments of Chemistry of the Federal University of Juiz de Fora and of the Military Institute of Engineering. The main goal of this project was to study, using theoretical-computational methods, the potential of magnesium oxide and hydroxide surfaces to degrade organophosphorus (OP) substances used as warfare agents, also known as neurotoxics. These substances inhibit the acetylcholinesterase, enzyme of fundamental role in the central nervous system. The VX (O-ethyl-S-(2-diisopropylethylamino) ethyl methylphosphonothioate) and sarin (isopropyl methylphosphonofluoridate) agents are some of the main neurotoxic agents, thus the search for ways to degrade them is considerably important. In this paper, we present theoretical investigations of two chemical degradation processes of organophosphates: the hydrolysis of a VX-like OP compound (O,S-dimethyl methylphosphonothioate, DMPT) by the dissociative chemisorption on the MgO(001) surface and the degradation of sarin by a surface of magnesium hydroxide Mg(OH)2, known as brucite. These processes were studied by the combination of density functional theory (DFT) with periodic boundary conditions, an approach that is the state of the art of the theoretical methods for this type of problem. In the case of DMPT, we proposed a degradation mechanism that involves degradation reactions of DMPT and water molecules in the presence of two types of MgO(001) surface models: terrace and Al-doped. Conformations, free energy differences, transition states and reaction barriers were calculated. We showed that the MgO(001) surface acts as a possible catalyst for the degradation of VX, but with higher selectivity than the Al-doped surface when compared with sites without defects on the terrace. These results may have important applications as well as to serve as reference for further studies on the decomposition of VX. In the degradation process of sarin using brucite, we proposed four elementary reactions for the global mechanism. We analyzed the adsorption of sarin on the brucite layer through the fluorine and the phosphonyl oxygen atoms (P=O). The two transition states correspond to a hydroxyl anion motion toward the phosphorus atom and to a fluoride moving toward the hydroxyl vacant position; the activation barrier for the rate-limiting step, corresponds to adsorbed sarin attacked by the brucite hydroxyl. The products of the global reaction were an isopropyl methylphosphonate molecule and [Mg(OH)2−x]Fx. The results of the deactivation process of sarin using a brucite surface show the potential of layered hydroxides to degrade OP compounds. Also the results of this research have a dual character because several pesticides are organophosphorous compounds. Furthermore, the theoretical approach employed can be used for various chemicals processes to get quite