How do the polarity and size of guest molecules influence the complexation thermodynamics of β-cyclodextrin (β-CD) with gaseous substances?

β-cyclodextrin (β-CD) is a natural cyclic oligosaccharide with a hydrophobic central cavity and a hydrophilic outer surface. It can form inclusion complexes with various guest molecules through non-covalent interactions. The study investigates the thermodynamic aspects of complex formation between β-CD and gaseous substances such as N2O, CO2, HCN, NO2, SO2, CH4, and CH3CH2CH3 using density functional theory (DFT) calculations.

From: "Inclusion Complexes between β-Cyclodextrin and Gaseous Substances—N2O, CO2, HCN, NO2, SO2, CH4 and CH3CH2CH3: Role of the Host’s Cavity Hydration", Inorganics 2024, 12(4), 110

The polarity and size of the guest molecules significantly influence the complexation thermodynamics. Polar gases like N2O and HCN, which have higher dipole moments, form more stable complexes with β-CD. The study found that the Gibbs free energy of complex formation (∆G) is more negative for polar gases compared to non-polar gases. For example, the ∆G for N2O binding with β-CD is −10.9 kcal/mol for the first water displacement, indicating strong complexation. In contrast, non-polar gases like CH4 and CH3CH2CH3 show weaker interactions, with ∆G values indicating less favorable complexation. The size of the guest molecule also plays a role; larger molecules like propane cannot fit well into the β-CD cavity, resulting in unfavorable complexation.