RADICALS AND IONS OF FORMIC AND ACETIC-ACIDS - AN AB-INITIO STUDY OF THE STRUCTURES AND GAS AND SOLUTION-PHASE THERMOCHEMISTRY

The structures of HCOO., COOH., HCOO-, HCOOH.+, HCOOH, CH2COO.-, CH3CO O., CH2COOH., CH3COO-, CH3COOH.+ and CH3COOH were optimized at HF/6-31 G(D) and MP2/6-31G(D) levels. The vibrational frequencies were calcula ted at the HF/6-31G(D) level and the total energies of these molecules were evaluated at the G2(MP2) level. Gas phase thermodynamic properti es, C-p(-) and S-, H- - H-0(-), Delta(f)H(-) and Delta(f)G(-) were cal culated as functions of temperature using standard statistical thermod ynamic methods. For HCOO. COOH., CH3COO. and CH2COOH., the method of i sodesmic reaction was used. The following are recommended values of De lta(f)H(-) at 298 K in kJ mol(-1): COOH. -193, CH2COOH. -243, CH2COO.- -322, HCOO. -127, CH3COO. -190, all with an uncertainty of +/- 7 kJ m ol(-1). Heats of formation of the RCOO(-) and RCOOH(.+) ions were in e xcellent agreement with those in ref. 1. On the basis of the structura l information from the ab initio calculations and an analysis of the s olution free energies of the parent compounds, aqueous solution free e nergies and free energies of formation in solution were calculated for the radicals. The values of E(-)(RCOO(.)/RCOO(-)) and other calculate d reduction potentials for formate and acetate were shown to be in acc ord with rates of known redox reactions. Also the RCOO(.) radicals wer e predicted to have abnormally low (actually negative) pK(a)s for the loss of C-H protons.