A Study Predicting Possibility of Acid-Catalyzed Hydrolysis of Esters Using the MO/MC method

Abstract

We have been trying to use activation energies (Ea) as an index for degradation of esters in the natural conditions. However, it was confirmed that Ea is not the suitable index for all the esters cited in the Chemical Substances Control Law of Japan. On the other hand, activated free energies (ΔG‡) are directly connected with observed reaction rate constants. Instead of Ea, therefore, we intended to use ΔG‡ calculated from molecular orbital (MO) calculations including vibration frequency calculations at the MP2/6-31+G(d,p) level of theory and/or those from the MO/MC simulations (Monte Carlo simulations using MO calculations). In the MO/MC method, we used a droplet model that includes a solute molecule in its center and arranged 32 solvent molecules around the solute with in 6.5 A from the center. The calculated ΔG‡ values for methyl acetate is well consistent with that from the experiments. In addition, we calculated activation free energies (ΔG‡ obs) that come from actual measurements of the acid-catalyzed hydrolysis of esters to compare these values with calculated ones.