Hydration of the carboxylate group: Anab initio molecular orbital study of acetate-water complexes

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.