The conformational analysis of saturated heterocycles. Part XVIII. The orientation of NH-groups in piperidines and morpholines from infrared spectroscopy

Abstract

Piperidine, morpholine, and ten substituted derivatives show, in the gas-phase, two resolved bands in the first NH-overtone region which are assigned to the NH-equatorial and NH-axial conformers by qualitative and quantitative comparisons of their contours with those predicted by theory. A temperature-dependence study of the intensity ratios indicates that the NH-equatorial conformer predominates for both piperidine and morpholine, with ΔH 0·5 ± 0·1 kcal./mole. Examination of piperidine, morpholine, and cis-2,6-dimethylpiperidine in carbon tetrachlorid egives ΔH 0·6 ± 0·2 kcal./mole preference for NH-equatorial. ΔG Values probably differ little from ΔH values.