Reactivity of phosphate monoester monoanions in aqueous solution.: 2.: A theoretical study of the elusive zwitterion intermediates RO+(H)PO32-

In a previous theoretical study [J. Am. Chem. Soc., in press], using a combination of DFT and continuum solvation (PCM) methods, the anionic zwitterion CH3O+(H)PO32-(2) has been identified as a key intermediate in the mechanism for the dissociative hydrolysis of the methyl phosphate anion CH3OPO3H- (1). To confirm this finding, DFT/B3LYP calculations in which a few solvent molecules are explicitly considered, are reported. Hydrogen-bonded complexes 2(.)(H2O)(n) (n = 2-4) have been fully optimized and characterized on their respective potential energy surfaces. We have shown that only two specifically solvating water molecules are sufficient to reproduce the PCM results previously obtained provided they occupy a critical bridging position between the methanol and metaphosphate fragments. Further, DFT-PCM geometry optimizations of RO+(H)PO32- (R = phenyl and 2,4-dinitrophenyl) agree with predictions, based on the mechanistic picture proposed by Kirby and Varvoglis [J. Am. Chem. Soc. 1967, 89, 415] 35 years ago, according to which anionic zwitterions are assumed to exist as intermediates for the methyl and phenyl esters but not for the 2,4-dinitrophenyl ester. Moreover, biologically important counterions such as Mg2+ can also play a crucial role in stabilizing the zwitterionic structure RO+(H)PO32- in the gas phase.