Radicals and ions of formic and acetic acids: an ab initio study of the structures and gas and solution phase thermochemistry

Abstract

The structures of HCOO˙, COOH˙, HCOO^–, HCOOH˙⁺, HCOOH, CH₂COO˙^–, CH₃COO˙, CH₂COOH˙, CH₃COO^–, CH₃COOH˙⁺ and CH₃COOH were optimized at HF/6-31G(D) and MP2/6-31G(D) levels. The vibrational frequencies were calculated at the HF/6-31G(D) level and the total energies of these molecules were evaluated at the G2(MP2) level. Gas phase thermodynamic properties, C_p, S, H–H₀, Δ_fH and Δ_fG were calculated as functions of temperature using standard statistical thermodynamic methods. For HCOO˙, COOH˙, CH₃COO˙ and CH₂COOH˙, the method of isodesmic reaction was used. The following are recommended values of Δ_fH at 298 K in kJ mol^–1: COOH˙–193, CH₂COOH˙–243. CH₂COO˙–322, HCOO˙–127, CH₃COO˙–190, all with an uncertainty of ±7 kJ mol^–1. Heats of formation of the RCOO^–, and RCOOH˙⁺ ions were in excellent agreement with those in ref.1.

On the basis of the structural information from the ab initio calculations and an analysis of the solution free energies of the parent compounds, aqueous solution free energies and free energies of formation in solution were calculated for the radicals. The values of E(RCOO˙/RCOO^–) and other calculated reduction potentials for formate and acetate were shown to be in accord with rates of known redox reactions. Also the RCOO˙ radicals were predicted to have abnormally low (actually negative) pk_as for the loss of C–H protons.