Electronic structure and (1)H NMR chemical shifts in host-guest complexes of cucurbit[6]uril and sym-tetramethyl cucurbit[6]uril with imidazole derivatives.

Binding patterns and (1)H NMR chemical shifts in the complexes of protonated N-(4-hydroxylphenyl)imidazole (g1), N-(4-aminophenyl)imidazole (g2), 2-phenylimidazole (g3) guests with cucurbit[6]uril (CB[6]), and sym-substituted tetramethyl cucurbit[6]uril (TMeCB[6]) in the gas phase as well as in water have been investigated using the density functional theory. It has been shown that the inclusion complexes of g1 and g2 with CB[6] or TMeCB[6] exhibit selective encapsulation of the phenyl moiety with its substituents binding to portal oxygens on the lower rim of the host and imidazole protons facilitate C-H···O interactions externally with upper rim ureido oxygens. On the other hand, the lowest-energy g3 complex encapsulates the imidazole ring within the host, engendering N-H···O interactions with portal oxygens on the upper rim of the host with the phenyl ring residing outside the cavity owing to an absence of para-substituent and show qualitatively different host-guest binding patterns. Calculated (1)H NMR spectra of the complexes in water reveal shielding of phenyl ring protons within the host cavity which exhibit signals at 0.2-0.5 ppm, whereas the protons of the imidazole ring participating in hydrogen bonded interactions exhibit deshielding, and the corresponding (1)H NMR signals are downshifted by 1.1-1.5 ppm in the spectra compared to those in the unbound guest. (1)H NMR chemical shifts of inclusion complexes thus obtained are in consonant with δ(H) patterns observed in experiments reported in the literature.