Effects of atom pairs O and S on the stability of zwitterionic tetrahedral intermediate: A theoretical study

Ab initio and DFT studies have been carried out on the structure and stability of the zwitterionic complexes formed in water in the aminolysis of phenyl acetate, CH3C(O)–OAr (O, O), and its thio analogues (O, S), (S, O) and (S, S) with ammonia by explicitly solvating 0–5 water molecules. Formation of the zwitterionic tetrahedral structure in water was found to be more facile and more stable for the thio derivatives in the order (O, O) < (O, S) < (S, O) < (S, S). This is in good agreement with the experimental rates of the aminolysis in water, which indicates that the proclivity to form, and the stability of, the zwitterionic intermediate constitute major factors in determining the reaction rate.