The effect of leaving group on mechanistic preference in phosphate monoester hydrolysis

We present 2-dimensional potential energy surfaces and optimised transition states (TS) for water attack on a series of substituted phosphate monoester monoanions at the DFT level of theory, comparing a standard 6-31++g(d,p) basis set with a larger triple-zeta (augmented cc-pVTZ) basis set. Small fluorinated model compounds are used to simulate increasing leaving group stability without adding further geometrical complexity to the system. We demonstrate that whilst changing the leaving group causes little qualitative change in the potential energy surfaces (with the exception of the system with the most electron withdrawing leaving group, CF₃O⁻, in which the associative pathway changes from a stepwise A_N + D_N pathway to a concerted A_ND_N pathway), there is a quantitative change in relative gas-phase and solution barriers for the two competing pathways. In line with previous studies, in the case of OCH₃, the barriers for the associative and dissociative pathways are similar in solution, and the two pathways are equally viable and indistinguishable in solution. However, significantly increasing the stability of the leaving group (decreasing proton affinity, PA) results in the progressive favouring of a stepwise dissociative, D_N + A_N, mechanism over associative mechanisms.