Nuclear magnetic resonance studies of heterocyclic fluorine compounds. Part 2.—Perfluoro-N-fluoro-piperidine and perfluoro-N-fluoro-morpholine ring systems

The temperature-dependence of the 19F nuclear magnetic resonance spectra of perfluoro-N-fluoro-piperidine and perfluoro-N-fluoro-morpholine has been examined in the range from –74 to 20°C. The changes in appearance of the spectra are interpreted on the basis of the changing rate of chair-to-chair interconversion. There is evidence that, at corresponding temperatures, this interconversion rate is greater for the perfluoro-N-fluoro-morpholine than for the perfluoro-N-fluoro-piperidine ring. The spectra of perfluoro-(N-fluoro-methylpiperidines) are essentially temperature-independent over the range from –90 to 100°C suggesting conformationally rigid structures. The effect of the perfluoromethyl substituent on the vicinal ring fluorines is considered.