Probing potential medium effects on phosphate ester bonds using 18O isotope shifts on 31P NMR

Dipolar aprotic cosolvents, such as DMSO and acetonitrile, accelerate the rates of hydrolysis of phosphate monoester dianions. It has been speculated that the rate acceleration arises from the disruption of hydrogen bonding to the phosphoryl group. An aqueous solvation shell can stabilize the dianionic phosphoryl group by forming hydrogen bonds to the phosphoryl oxygens, whereas solvents such as DMSO are incapable of forming such bonds. It has been proposed that the loss of stabilization could result in a weakened P-OR ester bond, contributing to the observed faster rate of hydrolysis. Computational results support this notion. We have used the 18O-induced perturbation to the 31P chemical shift to ascertain whether solvent changes result in alterations to the P-O(R) bond. We have studied 16O18O-labeled methyl, ethyl, phenyl, p-nitrophenyl, diethyl p-nitrophenyl, triphenyl, and di-tert-butyl ethyl phosphate in the solvents water, methanol, chloroform, acetonitrile, dioxane, and DMSO. The results suggest no significant solvent-induced weakening of the phosphate ester bonds in any of the solvents tested, and this is unlikely to be a significant source for the acceleration of hydrolysis in mixed solvents.