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 initio and density functional theory methods. Starting from the molecular species CH3OPO3H-, CH3OPO3H-.(H2O), and CH3OPO3H-.(H2O)( 2), gas phase reaction coordinates of the proposed mechanisms were followed . Solvation free energies were evaluated using the polarizable continuum mo del (PCM) at the stationary point geometries. The dissociative mechanism, w hich involves the formation of a metaphosphate ion (PO3-), is found to be m ore favorable than the associative mechanism, which involves a pentacoordin ated intermediate, both in the gas phase and in aqueous solution. In the di ssociative mechanism, the first step is rate determining. The computed free energy of activation in solution is within 1.7 kcal/mol of the experimenta lly determined activation free energy for hydrolysis. The first step and th e second step in the dissociative mechanism are each shown to proceed via a six-centered water-assisted transition state.