Hydration in solution is critical for stable oxygen isotope fractionation between carbonate ion and water

Various isotope studies require accurate fractionation factors (α's) between different chemical compounds in thermodynamic equilibrium. Although numerous isotope systems involve aqueous solutions, the conventional theory is formulated for the gas-phase and predicts incorrect α's for many compounds dissolved in water. Here I show that quantum-chemistry calculations, which take into account solute-water interactions, accurately predict, for instance, oxygen isotope fractionation between dissolved CO32- and H2O (hereafter α(CO32--H2O)). Simple force field and quantum-chemistry calculations for the 'gas-phase' ion CO32- predict α(CO32--H2O)≃1.015 (15‰) at 25 °C. However, based on CO32-·(H2O)n-clusters with up to 22 H2O molecules, I calculate a value of 25‰, which agrees with the experimental value of 24.5 ± 0.5‰. Effects of geometry and anharmonicity on the calculated α were also examined. The calculations reveal the critical role of hydration in solution, which is ignored in the gas-phase theory. The approach presented provides an adequate framework for calculating fractionation factors involving dissolved compounds; it may also be used to predict α's that cannot (or have not yet been) determined experimentally.