Computational Investigations at the Gas-Surface Interface: Organic Surface Oxidation and Hydrolysis of Chemical Warfare Agents and Simulants
Chapleski Jr, Robert Charles
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Motivated by recent experiments in gas-surface chemistry, we report our results from computational investigations of heterogeneous systems relevant to atmospheric chemistry and protection against chemical weapons. To elucidate findings of ultra-high vacuum experiments that probe the oxidation of carbon-carbon double bonds on model surfaces, we used electronic structure and QM/MM methods to study the reaction of ozone with C60-fullerene and the products of nitrate addition to a vinyl-terminated self-assembled monolayer. In the first system, we followed a reaction pathway beginning with primary ozonide formation through the formation of stable products. Theoretical vibrational spectra were used to identify a ketene product in prior experimental work. Next, through the construction of a multilayer model for the initial addition product of a nitrate radical to a chain embedded within a self-assembled monolayer, we report theoretical spectra that are consistent with experimental results. We then examined the fundamentals of the hydrolysis mechanism for nerve agents by a catalyst of interest in the development of filtration materials for chemical-warfare-agent defense. By following the gas-surface reaction pathway of the nerve agent Sarin on the Lindqvist polyoxoniobate Cs8Nb6O19, we determined that the rate-limiting step is the transfer of a proton from an adsorbed water molecule to the niobate surface, concomitant with the nucleophilic addition of the nascent hydroxide to the phosphorus atom in Sarin. Our results support a general base hydrolysis mechanism, though high product-adsorption energies suggest that thermal treatment of the system is required to fully regenerate the catalyst. We report similar mechanisms for the simulants dimethyl methylphosphonate and dimethyl chlorophosphate, though the latter may serve as a better simulant in studies of this type. Finally, an investigation of Sarin hydrolysis with solvated Cs8Nb6O19 shows an increase in the rate-limiting barrier relative to the gas-surface system, revealing the role of Cs counterions in the reaction. Then, we further increased explicit solvation to model the homogeneous solution-phase reaction, finding a different mechanism in which a water molecule adds to phosphorus in the rate-limiting step and protonation of the niobate surface occurs in a subsequent barrierless step. By examining the rate-limiting barrier for protonation, we suggest that specific base hydrolysis is also likely in the homogeneous system.
- Doctoral Dissertations