Potential Energy Surfaces for Proton Abstractions from Acetic Acid

The abstractions of hydrogen from both carbon and oxygen in acetic acid by hydride, fluoride, and hydroxide anions have been studied using ab initio electronic structure calculations. Molecular structures were optimized at the Hartree-Fock level of theory using the 6-31++G(d,p) basis set. For energetics, the 6-311++G(d,p) basis set was used, with second- and fourth-order perturbation theory corrections, for both minima and transition states. For the hydride and fluoride ion abstractions of hydrogen from carbon, a small activation energy exists at the Hartree-Fock level, but vanishes when correlation energy corrections are introduced. No other barriers are found for the abstraction reactions, but intermediate minima are found on the F{sup -} + CH{sub 3}COOH {yields} FH + {sup -}CH{sub 2}COOH surface and on the analogous OH{sup -} + CH{sub 3}COOH surface. The calculated heats of formation for both acetic acid anions are in good agreement with the experimental value. The fourth-order perturbation theory calculation of the activation energy for the isomerization of acetate to enolate ion is 50.4 kcal/mol. The G2 values for the gas phase acidities of acetic acid at the OH and CH ends of the molecule are 339.3 and 365.8 kcal/mol, respectively. The former result is in good agreementmore » with the experiment. 26 refs., 5 figs., 5 tabs.« less