MOLECULAR FLEXIBILITY OF N-ACYL HETEROCYCLES STUDIED USING C-13 NMR-SPECTROSCOPY AND COMPUTATIONAL CHEMISTRY

The C-13 NMR spectra of a series of N-acyl six-member ring nitrogen he terocycles have been measured as a function of temperature and the res ults interpreted in terms of restricted amide rotation and ring invers ion. Slow rotation is observed on the NMR time scale around the N-CO b ond whilst the ring inversion is; fast at ambient temperature. High te mperature experiments have been undertaken to investigate the free ene rgy of activation for the amide rotation and low temperature NMR exper iments were used to probe the ring inversion process. The experimental results have been compared with theoretical calculations using molecu lar orbital, molecular mechanics and molecular dynamics approaches and good agreement is observed in all cases.