Carbon kinetic isotope effects on the hydration of carbon dioxide and the dehydration of bicarbonate ion

13C kinetic isotope effects on the hydration of CO2 and the dehydration of HCO3- in aqueous solution have been measured by rapid removal of the product and conversion to an isotopically stable form, followed by isotope-ratio analysis. The isotope effect on hydration is k12/k13 = 1.0069 +/- 0.0003 at 240C. The isotope effect on dehydration is 1.0147 +/- 0.0007. The ratio of these two values gives an equilibrium isotope effect of K12/K13 = 1.0077, in good agreement with previously measured values. The small magnitudes of the kinetic isotope effects indicate that the transition state for conversion of HCO3- to CO2 is very similar to that of HCO3-. This information together with previously measured solvent isotope effects indicates that the mechanism of HCO3- dehydration probably involves general-acid-catalyzed donation of a proton from H3O+ to the departing oxygen, rather than unimolecular decomposition of the zwitterion H2O+-CO2- in the rate-determining step. 21 references, 2 figures