Water structuring inside the cavities of cucurbit[n]urils (n = 5–8): a quantum-chemical forecast

In this work we report findings of the quantum-chemical examination of water structuring in the cavities of cucurbit[n]urils (CB[n]), n = 5–8 obtained within the density functional theory. The thermodynamically most stable structures of inclusion compounds (H2O)m@CB[n] were determined for different numbers m of H2O molecules inside the cavities. From the viewpoint of thermodynamics, the most probable numbers m of water molecules in the CB[n] homologues are the following: m = 2 for CB[5], m = 4 for CB[6], m = 8 for CB[7] and m = 10 for CB[8]. For the case of CB[6] synthesized in aqueous solution, we compared its experimental IR spectrum with that calculated quantum-chemically for the model inclusion systems (H2O)m@CB[6] where m ranges from 1 to 6. The best agreement between the experimental and theoretical spectra was observed for (H2O)4@CB[6], in complete agreement with the conclusion made based on the thermodynamic estimations. Our results are also in good agreement with other available estimates of the most probable number of water molecules in CB[n].