Hydrolysis versus aminolysis of a potential nerve agent tabun: a computational reaction mechanism study
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Hydrolysis versus aminolysis of a potential nerve agent tabun: a computational reaction mechanism study Debasish Mandal1 Received: 27 July 2020 / Accepted: 12 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The mechanism for the hydrolysis and aminolysis reactions of potential nerve agent tabun have been investigated both at DFT/M062X level and at ab initio MP2 level of theory. Effect of solvent water has also been incorporated using Minnesota SMD model. Based on leaving groups, there are three probable dissociation pathways of tabun: (1) elimination of HCN, (2) elimination of dimethyl amine and (3) elimination of ethanol. Each of the pathways follows a stepwise neutral route through two transition states. From the mechanism, it has been established that the HCN removal pathway is most favorable than the other two due to greater leaving tendency of the cyanide group. The reaction possesses two four-membered transition state in each path, i.e., for the addition of ammonia/water and for the release of leaving groups. As the four-membered cyclic TSs are not so favorable, the investigated reactions were found quite a bit high energetic. The first addition step of the H2O or N H3 is found as the rate-determining step. The reactivity of ammonia toward tabun has been found highly favorable (by ~ 7.6 kcal mol−1) than the water. This can be easily explained using the fact that the nucleophilicity of NH3 is higher than the H2O. Further, we have seen that water can be a very effective catalyst in the aminolysis reaction as it reduces the barrier of activation of the RDS by ~ 10 kcal mol−1. Self-catalyzed hydrolysis of water has also been performed, and it decreases the activation of ~ 11 kcal mol−1 than the uncatalyzed pathways. The significant reduction of energy of activation takes place due to the formation of six-membered transition states that makes much easier hydrogen transfer compared to the four-membered transition state formed in case of uncatalyzed aminolysis reactions. Keywords Nerve agent · Organophosphorus · Tabun · Hydrolysis · Aminolysis · Reaction mechanism
1 Introduction Organophosphorus compounds (OPCs), phosphoric acid ester derivatives, are extremely neurotoxic as they exhibit potential acetylcholinesterase (AChE) inhibiting properties [1–9]. These are mostly used as chemical warfare agents (CWA) or nerve agents due to their high lethality. Nerve agents were generally classified into two types, e.g., G agents and V agents. G type nerve agents (G stands for German) are more volatile, whereas V-type nerve agents (V stands Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00214-020-02688-8) contains supplementary material, which is available to authorized users. * Debasish Mandal [email protected] 1
School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004, India
for venomous) are more persistent [10]. Understanding the nature, reactivi
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