On the Possible Roles of the Zn Cation in the Carbon Dioxide Hydration Reaction: An Ab Initio RHF‐SCF MO Study of Transition Structures on the [OH 2 + CO 2 ] [ZnOH 2 CO 2 ] 2+ and [Zn(NH 3 ) 3 OH 2 CO 2 ] 2+ Reactive Hypersurfaces

Transition structures and related minima on the reactive energy hypersurfaces for the hydration of carbon dioxide in the presence of a bare zinc ion and with the cation liganded with three ammonia molecules are determined in the RHF MO SCF framework at a relatively high level of basis‐set representation. For the sake of comparison, the standard intramolecular proton transfer model in absence of zinc is revisited and the corresponding transition structure (TS) located. In the coordination sphere of zinc, the standard mechanism of hydration in vacuo is modified: a nucleophilic attack of water onto zinc‐activated carbon dioxide. The reactive path goes via TS signaling synchronous movements in the coordination sphere of zinc: Water goes away from and carbon dioxide toward the metal. For the model systems [ZnOH 2 CO 2 ] 2+ , this TS connects with a valley having a geminal carbonic acid (gCA) as product; the carbon–oxygen interaction of the in vacuo complex H 2 O···CO 2 is transformed into a covalent bond by its binding to zinc: H 2 O—CO 2 ‐Zn is a minimum on this energy hypersurface. The standard path for intramolecular proton transfer, namely, H 2 O—CO 2 —Zn changing into (HO) 2 —CO—Zn, is not catalyzed by the metal. For the ammonia‐ligand model system, the carbon dioxide hydration follows the same pathway as in the bare‐zinc case. A possible irreversible mechanism of carbon dioxide hydration catalyzed by carbonic anhydrases at pH lower than 6 can be suggested based on the present study; here, a central role is played by an intermolecular deprotonation of gCA by water found at the active‐site cleft around the metal center. This zinc–water mechanism is extrapolated to include a general acid catalysis of bicarbonate/carbon dioxide interconversion in water. Results obtained with a hydronium ion replacing zinc and an ancillary water acting as a proton acceptor for the gCA strongly suggest that, in water at pHs lower than 7, direct deprotonation of gCA offers a low‐activation channel to produce carbonic acid; in the reverse direction, protonation of the hydroxyl oxygen in bicarbonate leading to gCA offers a reasonable answer to the instability of this anion in solution at low pH. This picture agrees with the one reported by Paneth and O'Leary. [J. Am. Chem. Soc. 107, 7381 (1985)] based on experimental kinetic information.