Kinetic fractionation of carbon and oxygen isotopes during hydration of carbon dioxide

Abstract

Kinetic isotope effects (KIEs) during the inorganic hydration of carbon dioxide (CO2) in aqueous solution cause reduced stable carbon and oxygen isotope ratios (13C/12C and 18O/16O) in the reaction product carbonic acid (H2CO3) or bicarbonate ion (HCO3-), relative to CO2. While such KIEs are of importance in various physicochemical, geochemical, and biological systems, very few experimental and theoretical studies have attempted to determine the magnitude of the carbon and oxygen kinetic isotope fractionation (KIF) during hydration of CO2. Here I use transition state theory (TST) and quantum chemistry calculations to investigate the reaction rates of isotopic reactants CO2+nH2O (n=1–8) along the hydration pathway to H2CO3 or HCO3-. Locating transition states is difficult and the quantum chemistry calculations time-consuming at large n. My results suggest that the hydration mechanism for n=1–3 is unlikely to be the dominant pathway producing KIFs during CO2 hydration in aqueous solution; hydration mechanisms for n⩾4 appear more likely. For n=4–8, the predicted KIF based on MP2/aug-cc-pVDZ calculations at 25°C is ∼1.023–1.033 and ∼1.013–1.015, for carbon and oxygen, respectively. However, these values are uncertain and the results of the present study suggest that new experimental work is required to accurately determine the KIF of carbon and oxygen during CO2 hydration.