On the binding of SF6 to cucurbit[6]uril host: density functional investigations.

Interactions of sulfur hexafluoride (SF(6)) with cucurbit[6]uril (CB[6]) have been investigated using the density functional calculations. An encapsulation of guest within CB[6] cavity as well as its binding to either exterior host protons or portal uredio oxygens have been analyzed. The present calculations predict that the complexes with the complete inclusion of SF(6) are favored over those possessing lateral or external interactions. The interactions between fluorine and ureido carbons (F---C) of the host contribute to lowering of energy of the complex. Normal mode analyses from the calculated vibrational spectra show a red-shifted stretching (approximately 928 cm(-1)) of S-F bonds perpendicular to the CB[6] cavity axis when SF(6) is encapsulated in the host cavity. On the other hand, S-F bonds parallel to the cavity axis exhibit a blue shift as compared to the corresponding vibration of the isolated guest. These frequency shifts of S-F bonds have further been analyzed by mapping the difference electron density on the bond critical point(s) (bcp) in molecular electron density (MED) topography and natural bond orbital (NBO) analyses. A depletion of electron density at the bcp along with an enhanced electron density in antibonding S-F* orbital engender weakening of the bond. Concomitant redistribution of electron density leads to the strengthening of S-F bonds parallel to the cavity axis (directing toward either portals). (1)H NMR reveals that the protons directing toward CB[n] portals are not influenced by encapsulation of the guest, which is in consonant with experimentally measured NMR spectra.