Structure and vibrational spectra of sulfur hexafluoride encapsulated α-cyclodextrin

Density functional calculations have been utilized to understand the interactions of sulfur hexafluoride (SF6) with α-cyclodextrin (α-CD) host. An encapsulation of SF6 (guest) within the host cavity and its binding to primary as well as secondary hydroxyl rims of α-CD exhibiting different hydrogen bonding patterns have been analyzed. The present calculations have shown that interactions between the guest and protons within the host cavity yield the minimum energy complex wherein guest penetrates from secondary hydroxyl end of the host. Calculated vibrational spectra reveal frequency downshift of S-F stretching in the guest molecule on complexation. Frequency shift in opposite direction has been noticed for O-H vibrations of secondary hydroxyls of the host those facilitate relatively strong hydrogen bonded interactions. These inferences have been supported by mapping difference molecular electron density on the bond critical points in molecular electron density topography. The electronic structure and vibrational spectra derived from the density functional calculations based on the B3LYP and the X3LYP functional are similar. Use of X3LYP functional however, predicts shortening of O6H⋯O6′ as well as CH⋯F hydrogen bonds and thus points to the strengthened host-guest interactions and in turn binding energies of SF6 within these complexes employing the X3LYP functional turn out to be nearly ∼35-40% higher compared to those obtained within the framework of B3LYP theory.