An ab initio and density functional study of microsolvation of carbon dioxide in water clusters and formation of carbonic acid

Geometry of the CO2-H2O complex and reaction barriers leading to the formation of H2CO3 were studied at the RHF/6-311++G**, MP2/6-311++G**, B3LYP/AUG-cc-pVDZ, B3LYP/AUG-cc-pVTZ, MP2/AUG-cc-pVDZ and CCD/AUG-cc-pVDZ levels of theory. The rotational barrier of the CO2-H2O complex and the reaction barrier leading to the formation of H2CO3-H2O from CO2-(H2O)(2) were studied using the first three of the above-mentioned methods. Microsolvation of CO2 in water clusters having upto eight water molecules was studied using the B3LYP/AUG-cc-pVDZ method. Various methods except MP2/AUG-cc-pVDZ predict the equilibrium structure of the CO2- H2O complex to be symmetric while the MP2/AUG-cc-pVDZ method predicts it to be unsymmetric. Formation of H2CO3 from CO2-H2O is strongly catalyzed by the presence of a second water molecule. Atomic orbitals are strongly rehybridized in going from the equilibrium structures of the CO2-H2O and CO2-(H2O)(2) complexes to the transition states involved in the formation of H2CO3 and H2CO3-H2O, respectively, as shown by hybridization displacement charges.