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Reaction dynamics of cyanohydrins with hydrosulfide in water

We studied the reaction dynamics of a proposed prebiotic reaction theoretically. The chemical process involves acetone cyanohydrin or formalcyanohydrin reacting with hydrosulfide in an aqueous environment. Rate constants and populations of reactant and product bimolecular geometric orientations for the reactions were obtained by using density functional theory for the energies, transition state theory for the rates and matrix exponentiation as well as the hybrid tau-leaping algorithm for the population dynamics. The role of including the solvent explicitly versus implicitly was also investigated. We found that adding water or hydrogen sulfide molecules explicitly lowers the activation energy barrier and leads to a more efficient reaction pathway. In particular, hydrogen sulfide was a better proton donor. Further, when adding the solvent explicitly a synchronous reaction mechanism was observed while a concerted reaction mechanism was seen when using an implicit solvent model. Finally, we studied the role of including more than one reactant and product bimolecular orientation geometry in the dynamics. Including all pathways, reactant to reactant, product to product, reactant to product and product to reactant reduced the reaction half-time by two orders of magnitude and was therefore determined to be fundamental for an accurate description of the dynamics. Overall, we found that most reactions which involve formalcyanohydrin occur more rapidly or at the same speed as reactions which involve acetone cyanohydrin at room temperature.