ISOTOPIC FRACTIONATION AND CLUMPING IN THE CO2 HYDRATION AND HYDROXYLATION REACTIONS: EVALUATING THE INFLUENCE OF HYDROGEN-BONDING VIA QUANTUM MECHANICAL CALCULATIONS

Stable isotope ratios in carbonate minerals are known to reflect the influence of several climatological and biogeochemical processes, including ocean temperatures and carbon cycling. Carbonate formation from carbon dioxide via the unequilibrated CO₂ hydration and hydroxylation reactions may impact stable C, O and clumped C-O isotope ratios in carbonate minerals, particularly in corals where the reactions may be a source of “vital effects”. Quantum mechanical calculations under a transition state theory framework were carried out on aqueous clusters of dissolved inorganic carbon species to predict the reactions’ kinetic fractionations. The CO₂ hydration reaction is predicted to discriminate against ¹³C by 10-12‰ and against ¹⁸O by 11‰ at 25°C. The CO₂ hydroxylation reaction is predicted to discriminate against ¹³C by 13-16‰ and against ¹⁸O by 20-21‰ at 25°C. Hydration and hydroxylation disequilibrium can increase C-O clumping by ≤0.10‰ and 0.12‰ respectively. Kinetic fractionation is consistent with the increased C-O clumping observed in some shallow-water corals relative to other carbonates, and is able to explain most but not all of the individual C and O isotopic disequilibria in corals.

Quantum mechanical methods were selected by comparing their predictions against experimental energies, bond lengths, vibrational frequencies, and equilibrium isotope fraction factors. Methods which closely predicted experimental vibrational frequencies extrapolated to the harmonic limit also predicted equilibrium fractionation factors well. By modeling the second solvation shell around bicarbonate product ions and considering the H-bond pattern, the accuracy and precision of modeled equilibrium fractionation values is greatly improved. H-bond effects on clumping and single stable isotope fractionation are not consistent with a simple model of stiffer bonds favoring incorporation of both heavy isotopes and clumped isotopologues. Regressions against H-bond pattern may improve quantum mechanical calculations of isotope fractionation in other aqueous systems.