Chiral recognition by fullerenes: CHFClBr enantiomers in the C82 cage

Abstract

Density-functional theory and symmetry-adapted perturbation theory calculations on complexes of the enantiomers of CHFClBr with the most stable isomer of C₈₂-3 fullerene show that despite the guests being too large for the host cage, they are nevertheless stabilized by electrostatic interactions. The complexation leads to considerable strain on the cage and the guests accompanied by compression of the bonds of the guest molecule, resulting in considerable complexation-induced changes in the infrared (IR), vibrational circular dichroism (VCD), nuclear magnetic resonance (NMR), and UV-vis spectra. The effect of chiral recognition is pronounced only for the ¹⁹F signal in the NMR spectra and in a sign reversal of the rotational strength of the n_CH stretching vibration of S-CHFClBr@C₈₂-3 in the VCD spectrum as compared to that of the free guest, making the sign of this band for the C₈₂ complexes with the S- and R-guest enantiomers the same. This is a surprising result since vibrational circular dichroism is considered a reliable method for determining the absolute chirality of small molecules and for establishing dominant conformations in biopolymers.