CHEMISTRY OF LARGE HYDRATED ANION CLUSTERS X-(H2O)N, 0-LESS-THAN-OR-EQUAL-TO-N-ALMOST-EQUAL-TO-50 AND X = OH, O, O2, AND O3 .1. REACTION OF CO2 AND POSSIBLE APPLICATION IN UNDERSTANDING OF ENZYMATIC-REACTION DYNAMICS

The reactions of CO2 with large hydrated anion clusters X-(H2O)n = 0- 59, X = OH, O, O2, and O3, were studied in a fast-flow reactor under thermal conditions. Interesting solvation effects were found in terms of the observed mechanisms and reaction kinetics. Hydration by only a few water molecules makes O2- and O3- very inert to attack by CO2 due to the change in sign of the reaction enthalpy from negative to positive caused by the solvation. In contrast, reactions of hydrated OH- with CO2 are exothermic at all degrees of solvation, and the large discrepancy between the experimentally measured rate constants and the theoretically calculated values, also attributable to solvation, can be explained by a consideration of the reaction kinetics in which the unimolecular dissociation rate constant of the reaction intermediate increases and the intermolecular conversion rate constant decreases at progressively larger cluster sizes. An application of this work is found in contributing to a further understanding of the reaction dynamics of enzymatic hydration of CO2 in biological systems. The present work gives experimental evidence, by bridging the gas and the condensed phase via reactivity studies at different degrees of hydration, to support a proposed mechanism that attributes the enzymatic effect to a decrease in the hydration of OH- by carbonic anhydrase.