Water clustering in the presence of a CO2 molecule

The gas-phase geometries of the complexes CWn formed between one CO2 molecule and various numbers n of water molecules (n = 1-8) have been examined using the Møller-Plesset perturbation (MP2) and the density functional methods coupled with the aug-cc-pVDZ basis set. For a given number of water molecules, the latter aggregate in a stable complex with strong hydrogen bonds on one side of the CO2 molecule. The reported stabilization energies associated with the clustering of CO2 and water molecules have been corrected for the basis set superposition error. van der Waals and OH⋯OC interactions bind the water and CO2 molecules in the complexes. It was also found that the water moieties of some of the optimized structures of the CW4CW8 complexes resemble the water moieties of the well known gas hydrates. The Gibbs free energy changes for clustering of molecules as stable complexes were calculated at various temperatures. These calculations revealed that their formation is spontaneous at sufficiently low temperatures. The stretching frequency of the CO2 in the complex increases and that of the water molecules attached to the CO2 molecule decreases compared to their free states. These shifts depend on the strength of the OH⋯OC interaction. The comparison between the stabilization energies of a number of complexes CO2(H2O)n, indicated that their stabilization energies are approximately equal to the sum of the stabilization energies of two moieties of CO2(H2O)n/2 provided that their (H2O)n/2 are located on the opposite sides of CO2.