Selectivity in Gas Adsorption by Molecular Cucurbit[6]uril

This is the authors' abstract. We don't add key points for this paper.

The relative preference in adsorption among 19 common gas molecules, namely, C 2 H 2, C 2 H 4, C 2 H 6, CH 4, X 2, HX (X = F, Cl, Br), CO 2, CS 2, CO, H 2, H 2 O, H 2 S, N 2, NO 2, 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], C 2 H 2 @CB[6] being the most viable system. However, the dissociation free energy change implies that CB[6] is most efficient in accommodating Cl 2 followed by C 2 H 2 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 −C 2 H 5 substitution improves C 2 H 2 and CO 2 adsorption ability of CB[6] the most, the −CN functionalized CB[6] is the best candidate to encapsulate C 2 H 4 and C 2 H 6 among the studied −OH, −C 2 H 5, and −CN substituted analogues. The interaction is mostly of van der Waals type, except in the cases of C 2 H 2, H 2 O, H 2 S, 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.