Binding of viologen derivatives to cucurbit[8]uril

Abstract

Molecular interactions between cucurbit[8]uril host and methyl -(MV2+), ethyl -(EV2+), propyl -(PV2+) and butyl -(BV2+) viologen have been analyzed employing the density functional theory. The complexation of viologen guests with CB[8] favors complete encapsulation of guest within host cavity over lateral interactions from host portals. The lowest energy complex reveals that both α- and methylene protons of alkyl substituent interact with hydrophilic exterior of CB[8] via hydrogen bond interactions. These host–guest interactions are analyzed using quantum theory of atoms in molecules. It has been shown that amongst viologen derivatives, methyl viologen binds strongly to CB[8] and strength of binding decreases steadily with increasing aliphatic chain length of the substituent. Normal vibration analyses reveal that CO stretching vibrations from CB[8] (1788cm−1) downshifts to 1754cm−1 when ureido oxygens interact with viologen guest. The CO vibrations from hydrogen bonded interactions (1754cm−1) and those from noninteracting ones (1792cm−1) engender distinct vibrations in the complex. The CN stretching vibration of viologen cation exhibits a blue shift of 22cm−1 on encapsulation within CB[8] cavity. The direction of frequency shift of CO and CN stretching vibrations has been explained by combining MED topography with difference electron density maps and further been supported by natural bond orbital analysis. Calculated 1H NMR chemical shifts predict that β-protons of bipyridinium moiety are shielded due to encapsulation within the host cavity unlike α- or methylene protons participating in CO⋯H interactions which exhibit deshielding. These inferences are in consonant with 1H NMR experiments.