Carbonic acid formation from reaction of carbon dioxide and water coordinated to Al(OH)3: a quantum chemical study.

Density functional and ab initio calculations have been performed on CO(2)-nH(2)O and Al(OH)(3)-CO(2)-nH(2)O (where n = 1, 2, 3) cluster models to elucidate the catalytic effect of a hydroxylated metal center on the formation of carbonic acid (H(2)CO(3)). B3LYP/6-311++G(d,p)-calculated geometries and RI-SCS-MP2/aug-cc-pVTZ//B3LYP/6-311++G(d,p)-calculated energies with respect to isolated gas-phase molecules and various H(2)O, CO(2), and H(2)CO(3)-Al(OH)(3) complexes are presented. It is shown here that H(2)CO(3) formation proceeds via direct CO(2) and nH(2)O reaction with very high activation barriers in the gas phase, 51.40, 29.64, and 19.84 kcal/mol for CO(2)-H(2)O, CO(2)-2H(2)O, and CO(2)-3H(2)O clusters, respectively, decreasing in magnitude with an increase in the number of H(2)O molecules. The energetics as well as the reaction mechanism and energy landscape change significantly when carbonic acid is formed from CO(2) and nH(2)O in the presence of Al(OH)(3), a hydroxylated metal center. Results presented here show important details of the influence of the coordinating metal center in the formation of H(2)CO(3).