Modelling the environmental decomposition of nerve agents
Creators: name: Finnigan, Sophie
Publisher: name: Imperial College London
Year: 2021

description: Organophosphorus nerve agents are a highly toxic class of chemical warfare agents, characterised by their physiological effect. VX is one of the most toxic nerve agents. The environmental persistence and slow degradation of VX means that the nerve agent remains in significant quantities days after release. The interactions of VX with the surrounding environment will influence the decomposition mechanism and behaviour of the agent. Understanding how VX interacts with the surrounding environment will aid the design of effective decontaminants that either efficiently degrade VX or aid the safe removal of the agent from the environment. The fundamental decomposition mechanism of VX needs to be initially characterised before considering how the environment could affect VX degradation. Reaction mechanisms for VX hydrolysis have been calculated using Density Functional Theory (DFT) at the M06-2X/6-311++G(d,p) level of theory. The mechanism for neutral hydrolysis of VX was initially calculated in both the gas and the solvated phase, using SMD to implicitly model the aqueous environment. The protonation behaviour of VX was found to be significantly different between the two phases. In the solvated phase, the amino nitrogen preferentially acts as a proton acceptor during the reaction. In contrast, protonation of the amino group is highly unfavourable in the gas phase. The approach mechanism, space of trigonal bipyramidal (TBP) intermediates and cleavage mechanisms differ between the phases as a result of the protonation behaviour. The difference in available approach mechanisms means that VX hydrolysis is only kinetically accessible in the solvated phase and will not occur in the gas phase at ambient temperatures. VX hydrolysis is pH dependant in terms of both reaction rate and product ratio. The effect of pH on VX hydrolysis was investigated by calculating the reaction mechanism for several reaction systems. VX protonates at the amino group with a pKa of 8.6 to form VXH+ and OH  will replace H2O as the predominant nucleophile at basic pH. The reactants were therefore, varied to model the reaction system that would occur at a certain pH. The reaction systems studied were VXH+ + H2O (weakly acidic), VXH+ + OH (neutral) and VX + OH (alkaline), alongside the original neutral VX/H2O system. Important mechanistic features, including proton transfer and the formation of intramolecular interaction, are identified from the calculated reaction mechanisms. Across the acidic to neutral to basic systems, the energy span of the hydrolysis reaction was found to decrease. The result qualitatively agrees with the experimentally observed increase of the reaction rate with pH. However, quantitatively, the differences in the calculated energy spans across the reaction systems are significantly larger than the differences between the experimental reaction rates. The differences are hypothesised to be due to the limitations of the simple, model reaction system used and the implicit treatment of solvation which treats pH as neutral and static. The ongoing parametrisation of a nerve agent force field aims to improve the model system studied by enabling larger scale simulations of VX and other nerve agents. The current status of the nerve agent force field parametrisation is described. The force field will allow future study of the interactions between VX and natural environments, including bulk solvation and natural surfaces. Thus, the work aims to make the future investigation of the environmental effect on the nerve agent decomposition accessible.