Equilibrium and kinetic isotopic fractionation in the CO2 hydration and hydroxylation reactions: Analysis of the role of hydrogen-bonding via quantum mechanical calculations

Abstract

The formation of H2CO3 and HCO3− from CO2 is common in aqueous environmental systems and occurs via the CO2 hydration and hydroxylation reactions. The reactions are especially important in the context of CaCO3 mineral precipitation. The reactions carry with them equilibrium and kinetic isotope fractionations that impact paleoclimate proxy records. Herein, quantum mechanical calculations of dissolved inorganic carbon species embedded in H2O clusters are analyzed under a transition state theory framework to predict kinetic isotope fractionations associated with the reactions. Experimental measurements of reaction energetics and equilibrium isotope fractionations are well-reproduced by the models which suggests the computational methods are justified.The CO2 hydration reaction discriminates against 13C by 19–23‰ and against 18O by 14–15‰ at 25 °C. The CO2 hydroxylation reaction discriminates against 13C by 26–31‰ and against 18O by 27–30‰ at 25 °C. Our analyses of the model isotopic fractionations lead to the conclusion that hydrogen-bonds and C(CO2)-O(H2O or OH−) distances influence the calculated values. We infer that when the model H-bond patterns more realistically represent the aqueous species, the accuracy and precision of equilibrium fractionation factor prediction is improved.Implications for coral carbonate paleothermometry are discussed. Results indicate that hydration and hydroxylation δ13C-δ18O slopes bound the values of slopes form corals and abiotic precipitation experiments, albeit with key uncertainties related to precipitation fractionations. The process of regressing molecular model fractionation results against H-bond patterns as presented here is a promising way to improve isotope fractionation calculations in aqueous systems.