Inclusion Complexes between β-Cyclodextrin and Gaseous Substances—N2O, CO2, HCN, NO2, SO2, CH4 and CH3CH2CH3: Role of the Host’s Cavity Hydration

The thermodynamic aspects of the process of inclusion complex formation between β-cyclodextrin (acting as a host) and gaseous substances (guests; N2O, CO2, NO2, SO2, HCN, CH4, CH3CH2CH3) are studied by employing well-calibrated and tested density functional theory (DFT) calculations. This study sheds new light on the intimate mechanism of the β-cyclodextrin/gas complex formation and answers several intriguing questions: how the polarity and size of the guest molecule influence the complexation thermodynamics; which process of encapsulation by the host macrocycle is more advantageous—insertion to the central cavity without hydration water displacement or guest binding accompanied by a displacement of water molecule(s); what the major factors governing the formation of the complex between β-cyclodextrin and gaseous substances are. The special role that the cluster of water molecules inside the host’s internal cavity plays in the encapsulation process is emphasized.