High pressure chemistry of thioaldehydes: A first-principles molecular dynamics study.

First-principles molecular dynamics simulations are used to investigate the chemical behavior of bulk thioacetaldehyde (MeC(H)S) in response to changes in pressure, P. The simulations show that these molecules oligomerize in response to applied P. Oligomerization is initiated through C-S bond formation, with constrained dynamics simulations showing that the barrier to this reaction step is lowered significantly by applied P. Subsequent reactions involving the formation of additional C-S bonds or radical processes that lead to S-S and C-C bonds lengthen the oligomers. Oligomerization is terminated through proton transfer or the formation of rings. The mechanistic details of all reactions are examined. The results indicate that the P-induced reactivity of the MeC(H)S-based system differs significantly from that of analogous MeC(H)O-based systems, which have been reported previously. Comparison with the MeC(H)O study shows that replacing oxygen with sulfur significantly lowers the P required to initiate oligomerization (from 26 GPa to 5 GPa), increases the types of reactions in which systems of this type can take part, and increases the variety of products formed through these reactions. These differences can be explained in terms of the electronic structures of these systems, which may be useful for certain high P applications.