Theoretical Studies of the Hydrolysis of the Methyl Phosphate Anion

The mechanism for the hydrolysis of the methyl phosphate anion was studied using high-level ab itiitio and density functional theory methods. Starting from the molecular species CH₃OPO₃H^-, CH₃OP₃H^-·(H₂O), and CH₃OPO₃H^-(H₂O)₂, gas phase reaction coordinates of the proposed mechanisms were followed. Solvation free energies were evaluated using the polarizable continuum model (PCM) at the stationary point geometries. The dissociative mechanism, which involves the formation of a metaphosphate ion (PO₃^-), is found to be more favorable than the associative mechanism, which involves a pentacoordinated intermediate, both in the gas phase and in aqueous solution. In the dissociative mechanism, the first step is rate determining. The computed free energy of activation in solution is within 1.7 kcal/mol of the experimentally determined activation free energy for hydrolysis. The first step and the second step in the dissociative mechanism are each shown to proceed via a six-centered water-assisted transition state.