Energy barriers for alkaline hydrolysis of carboxylic acid esters in aqueous solution by reaction field calculations

The hydrolysis of six representative alkyl esters in aqueous solution were evaluated by performing ab initio molecular orbital calculations using five different self-consistent reaction field (SCRF) procedures. Energy barriers were obtained for hydrolysis by bimolecular base-catalyzed acyl-oxygen cleavage (B_AC2) and bimolecular base-catalyzed alkyl-oxygen cleavage (B_AL2). Despite strong solute-solvent hydrogen bonding, the calculated solvent shifts of the energy barriers are dominated by electrostatic interactions between solute and solvent, and nonelectrostatic interactions largely cancel out. SCRF calculations that ignore volume polarization or use a charge renormalization scheme usually overestimate the solvent shifts of the energy barriers. A recently developed surface and volume polarization for electrostatic interaction (SVPE) procedure yields results comparable to experimental data when the solute cavity surface is defined as the 0.002 au electron charge isodensity contour. The differences between values from the SVPE calculations with this contour and the corresponding average experimental values for the examined esters are smaller than the range of experimental values reported by different laboratories. The SVPE calculations for the B_AC2 hydrolysis predicted the lowest energy barrier for methyl formate and the highest for tert-butyl acetate, and the remaining four esters grouped closely. These results are consistent with the substituent shifts of the experimental activation energies. The energy barriers predicted for B_AL2 hydrolysis are always considerably higher than those predicted for the B_AC2, consistent with the observation that in aqueous solution B_AL2 hydrolysis is negligible compared to B_AC2 for alkyl esters.