HYDRATION OF THE CARBOXYLATE GROUP - AN AB-INITIO MOLECULAR-ORBITAL STUDY OF ACETATE-WATER COMPLEXES

The hydration of the carboxylate group in the acetate anion has been i nvestigated by performing ab initio molecular orbital calculations on selected conformers of complexes with the form CH3CO2-. nH(2)O . mH(2) O, where n and m denote the number of water molecules in the first and second hydration spheres around the carboxylate group, and n + in les s than or equal to 7. The results of RHF/6-31G optimizations for all the complexes and MP2/6-31+G* optimizations for several one-water com plexes are reported. The primary consequence of hydration on the struc ture of the acetate anion is a decrease in the length of the C-C bond. Enthalpy and free energy changes calculated at the MP2/6-31+G* and M P2/6-311++G* levels are reported for the reactions CH3CO2- + [H2O](p) --> CH3CO2- . nH(2)O . mH(2)O where [H2O](p) is a water cluster conta ining p water molecules and p = n + m less than or equal to 7. The cal culations show that conformers with the lowest enthalpy change on comp lex formation are often not those with the lowest free energy change, due to a greater entropic loss in complexes with tighter and more favo rable enthalpic interactions. Hydrogen bonding of six water molecules directly to the carboxylate group in CH3CO2- is found to account for a pproximately 40% of the enthalpy change and 37% of the free energy cha nge associated with bulk solvation.