Electronic structure, vibrational spectra and 1H NMR of halide ion (F−, Cl− and Br−) encapsulated bambus[6]uril from density functional theory

Molecular Interactions between novel Bambus[6]uril host (BU[6]) and halide ion (F−, Cl−, Br−) have been studied using the density functional theory. Molecular electrostatic potential (MESP) investigations in BU[6] have shown large electron-rich regions near carbonyls on portals than those near cavity of the host. Further MESP minima within cavity are shallow compared to those near portals of BU[6]. Vibrational spectra of isolated BU[6] and halide ion encapsulated BU[6] are derived. Normal mode analyses reveal that on encapsulation of halide ion within BU[6] cavity engenders a downshift of 12 cm−1 for 1757 cm−1 band assigned to stretching of carbonyls at host portals. Likewise the 1714 cm−1 vibration of isolated BU[6] arising from carbonyl stretching near host cavity shows a shift up to 5 cm−1 on complexation. 1H NMR chemical shifts in DMSO from B3LYP/6-31+G(d,p) theory predict large shielding for methyl protons. On the other hand methine protons directing toward the cavity are deshielded and follow the order: Hc (methine protons) > Hb (methylene) > Ha (methyl) in the free host; which remains unchanged on complexation. These inferences are in consonant with the measured 1H NMR spectra (Svec et al., Angew. Chem. Int. Ed. 49 (2010) 2378). Subsequent structure and NMR chemical shifts in Cl−@BU[6] as well as those of isolated BU[6] obtained from a variety of functionals viz., B3LYP, X3LYP, PBE0, BMK and M-06L in density functional calculations, are compared. Structure and 1H NMR chemical shifts in DMSO from the calculations incorporating B3LYP or X3LYP functional generally agree better with experiment.