High-Level Ab Initio Predictions of the Energetics of mCO2•(H2O)n (n=1-3, m=1-12) Clusters

Electronic structure calculations at the correlated molecular orbital theory and density functional theory levels have been used to generate a reliable set of clustering energies for up to three water molecules in carbon dioxide clusters up to n = 12. The structures and energetics are dominated by Lewis acid-base interactions with hydrogen-bonding interactions playing a lesser energetic role. The actual binding energies are somewhat larger than might be expected. The correlated molecular orbital MP2 method and density functional theory with the omega B97X exchange-correlation functional provide good results for the energetics of the clusters, but the B3LYP and omega B97X-D functionals do not. Seven CO2 molecules form the first solvent shell about a single H2O with four CO2 molecules interacting with the H2O via Lewis acid-base interactions, two CO2 interacting with the H2O by hydrogen bonds, and the seventh CO2 completing the shell. The Lewis acid-base and weak hydrogen bond interactions between the water molecules and the CO2 molecules are strong enough to disrupt the trimer ring configuration for as few as seven CO2 molecules. Calculated C-13 NMR chemical shifts for mCO(2)center dot(H2O)(n) show little change with respect to the number of H2O or CO2 molecules in the cluster. The O-H stretching frequencies do exhibit shifts that can provide information about the interactions between water and CO2 molecules.