13C solid-state NMR study on populations, conformations, and molecular motions of γ-valerolactone enantiomers enclathrated in the chiral cholic acid host

Abstract

¹³ C Solid-state NMR spectra were measured for γ-valerolactone in the cholic-acid host which exhibits efficient optical resolution. Both the more-favoured (S)-(–)-enantiomer [(S)-1] and the less-favoured (R)-(+)-enantiomer [(R)-1] of γ-valerolactone were found to coexist microscopically in the cholic-acid channels by two-dimensional exchange NMR. Their populations were non-destructively determined from the intensities of their methyl-carbon signals. The principal values of the ¹³C chemical-shift tensors for the methyl carbons were determined by the one-dimensional switching-angle sample spinning (1D-SASS) NMR method. By comparing the ¹³C chemical-shift tensors with those calculated by the ab initio GIAO method, the conformation of each enantiomer was determined as follows: (S)-1 has the methyl group in the equatorial direction to the five-membered ring of γ-valerolactone, while (R)-1 has it in the axial direction. The optical resolution of γ-valerolactone by the inclusion method using the cholic-acid host is mainly ascribed to the energy difference in the conformations of γ-valerolactone. Temperature variation of the ¹³C chemical-shift powder pattern for the methyl carbons of both enantiomers revealed the presence of an overall reorientation among a finite number of isotropically distributed sites accompanying slow rotational diffusion. The rate of the reorientation for (S)-1 is higher than that for (R)-1; moreover, the corresponding barrier for (S)-1 is notably lower than that for (R)-1.