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Density functional investigations on the charge distribution, vibrational spectra, and NMR chemical shifts in cucurbit[n]uril (n = 5-12) hosts.

Electronic structure, charge distribution, and vibrational frequencies of cucurbit[n]uril, CB[n] (n = 5-12), hosts have been derived using the density functional methods. CB[n] conformers with different orientations of methylene group connecting glycouril units have been investigated. The conformers that possess uniform CB[n] cavity turn out to be of lowest energy, and molecular electrostatic potential (MESP) herein engender shallow minima near ureido oxygens along the series. MESP topography has been utilized to estimate the cavity height and diameter; the ratio of which governs the shape (circular or elliptical) of the cavity. When this ratio is larger than unity (for CB[n] with n >or= 8), an elliptical host cavity is noticed. Calculated vibrational spectra reveal that carbonyl stretching frequency shift in successive CB[n] homologue decreases steadily from 1760 cm(-1) in CB[5] to 1742 cm(-1) in CB[12]. An increase in glycouril units along the CB[n] series influences significantly the intensity profile of C horizontal lineO and C-N stretching vibrations in the calculated infrared spectra. Furthermore, calculated (1)H chemical shifts predict that one of methylene protons directing outside the host cavity are deshielded, whereas the remaining proton near the carbonyl group exhibits downshifted signal in the NMR spectra.