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

Abstract

Geometry of the CO2–H2O complex and reaction barriers leading to the formation of H2CO3were 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.