The structure and stability of putative zwitterionic complexes in the aminolysis of phenyl acetate with ammonia are examined using density functional and ab initio methods by applying the explicit, up to 7 H2O, and implicit, PCM, solvation models. The stability of the zwitterionic tetrahedral intermediate required an explicit solvation by at least five water molecules with stabilization energy of approximately 35 kcal mol(-1). The explicit water molecules are inter-connected by cyclic hydrogen bonding networks terminating at the negatively charged carbonyl oxygen and one of the hydrogen atoms on the positively charged ammonium ion. (c) 2006 Elsevier B.V. All rights reserved.