Selectivity in Gas Adsorption by Molecular Cucurbit[6]uril

Abstract

The relative preference in adsorption among 19 common gas molecules, namely, C2H2, C2H4, C2H6, CH4, X2, HX (X = F, Cl, Br), CO2, CS2, CO, H2, H2O, H2S, N2, NO2, and NO within the cavity of cucurbit[6]uril (CB[6]) is investigated via density functional theory computations. Energies associated with the dissociation of gas@CB[6] producing CB[6] and gas molecules show the order of the efficacy to be encapsulated within CB[6], C2H2@CB[6] being the most viable system. However, the dissociation free energy change implies that CB[6] is most efficient in accommodating Cl2 followed by C2H2 among the considered gas molecules. In general, guest molecules having large surface contact with the host and/or high polarizability and/or having acidic hydrogen to make hydrogen bond with >C═O show larger propensity to be encapsulated within CB[6] cavitand. Functionalized CB[6] are better candidates for gas adsorption than CB[6]. However, the nature of functionalization needed to improve the adsorption ability varies with the change in the guest molecule. While full −C2H5 substitution improves C2H2 and CO2 adsorption ability of CB[6] the most, the −CN functionalized CB[6] is the best candidate to encapsulate C2H4 and C2H6 among the studied −OH, −C2H5, and −CN substituted analogues. The interaction is mostly of van der Waals type, except in the cases of C2H2, H2O, H2S, and HX (X = F, Cl, Br), in which both the electrostatic and dispersion contributions are important owing to the interaction between acidic hydrogen of these guest molecules and oxygen centers of the host moiety.