A mechanism for carbon isotope exchange between aqueous acetic acid and CO2/HCO3-: An ab initio study

The aqueous exchange of carbon atoms between organic acids, such as acetic acid, and dissolved inorganic carbon is an important topic in the chemistry of oil-field waters. We propose that the exchange of carboxylic carbon atoms between acetic acid/acetate and CO2/HCO3- passes through a malonic acid intermediate or a hydroxylated malonic acid intermediate. Model reactions to and from these intermediates were investigated using molecular orbital and transition state theory (MO-TST) modeling at the B3LYP/6-31+G(d,p) level. Zero-point energy corrected barriers (ZPECB) and solvation-corrected Gibbs free energies of activation predict that the slow steps are the isomerization reactions from acetic acid and acetate to CH2C(OH)2 and CH2C(OH)O−, respectively. Using these reactions to represent the reactant under acid and basic conditions, fractionation factors and rate constants were computed. The computed fractionation factors are generally consistent with previous hydrous pyrolysis experiments performed by Dias et al. [Dias, R.F., Freeman, K.H., Lewan, M.D., Franks, S.G., 2002a. Delta C-13 of low-molecular-weight organic acids generated by the hydrous pyrolysis of oil-prone source rocks. Geochimica et Cosmochimica Acta 66, 2755-2769]. The involvement of water molecules in the transport of protons significantly lowers the ZPECB of the slow steps. Rate constants from 298 to 673 K for the reaction under “basic” conditions are about 0-1 orders of magnitude higher than rate constants of the reaction under “acidic” conditions and therefore indicate that the basic reaction is faster, assuming equal concentrations of acetic acid and acetate. Regression analyses yield activation energies of 176 kJ/mol for the acidic case and 165 kJ/mol for the basic case. For an aqueous system of acetic acid or acetate and dissolved inorganic carbon at or above 473 K, the results suggest that malonic acid may be present and isolable because malonic acid is a reaction intermediate in a relatively deep potential energy well.