A computational study of interactions between acetic acid and water molecules.

Density functional theory calculations were performed for the title reactions to elucidate the difference between the strong cyclic hydrogen bond of (Me-COOH)(2) and the electrolytic dissociation, MeCOOH <==> Me-COO(-) + H(+), as a weak acid. The association of water clusters with acetic acid dimers strengthens the cyclic hydrogen bond. A nucleophilic attack of the carboxylic carbon by a water cluster leads to a first zwitterionic intermediate, MeCOO(-) + H(3)O(+) + (HO)(3)C-Me. The intermediate is unstable and is isomerized to a neutral interacting system, MeCOOH...(HO)(3)C-Me + H(2)O. The ethanetriol, (HO)(3)-CMe is transformed to an acetic acid monomer. The monomer may be dissociated to give a second zwitterionic intermediate with reasonable proton-relay patterns and energy changes. In proton relay reaction channels, H in MeCOOH is not an acidic proton but is always a hydroxy proton. Copyright 2003 Wiley Periodicals, Inc. J Comput Chem 24: 939-947, 2003