Infrared spectra of monosubstituted toluene derivatives in cyclodextrin: Orientation of guest molecules in included complexes

Infrared spectroscopy was applied to determine the orientations of guest molecules (m- and p-chlorotoluene, m- and p-tolunitrile, and m- and p-toluidine) in cyclodextrin (α-, β-, or γ-cyclodextrin (CD)). Some bands of the guest molecules were found to shift by several wavenumbers upon formation of inclusion complexes or upon variation of the diameters of CD. The shifts could be explained by the steric hindrance imparted on the guest molecules in the CD cavities. Moreover, the bands were thought to be due to the vibrations of the moieties of guest molecules in the cavity of CD. The bands were assigned with the aid of quantum chemical calculations, which revealed that chlorotoluene, tolunitrile, and p-toluidine were able to enter the CD cavity, respectively, from the Cl, CN, and CH3 sides. On the other hand, both the CH3 and NH2 groups of m-toluidine direct to the outside of the CD ring. The orientation of a guest molecule in a CD cavity was found to be determined by the charge distribution in the guest molecule; the negatively charged moiety of a guest molecule is attracted to the inside of CD.