Abstract
The hydration of the carboxylate group in the acetate anion has been investigated by performingab initio molecular orbital calculations on selected conformers of complexes with the form CH3CO2− ·nH2O·mH2O, wheren andm denote the number of water molecules in the first and second hydration spheres around the carboxylate group, andn + m ≤ 7. The results of RHF/6–31G* optimizations for all the complexes and MP2/6–31+G** optimizations for several one-water complexes are reported. The primary consequence of hydration on the structure 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 MP2/6–311+ +G** levels are reported for the reactions CH3CO2− + [H2O] P → CH3CO2− ·nH2O ·mH2O where [H2O] P is a water cluster containingp water molecules andp=n+m ≤ 7. The calculations show that conformers with the lowest enthalpy change on complex formation are often not those with the lowest free energy change, due to a greater entropic loss in complexes with tighter and more favorable enthalpic interactions. Hydrogen bonding of six water molecules directly to the carboxylate group in CH3CO2− is found to account for approximately 40% of the enthalpy change and 37% of the free energy change associated with bulk solvation.
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