An electron spin resonance study of conformational inversion in 4-alkyl-piperidine nitroxides

Abstract

The advantages of e.s.r. spectroscopy for the investigation of conformational changes involving low energy barriers have been exploited in a detailed study of a series of substituted piperidine nitroxides. The nitroxides were generated in methylene chloride solution and their e.s.r. spectra were recorded from –100 to +35°. Detailed analysis of the spectra confirms that rapid chair–chair inversion takes places and computer simulation of the spectra obtained at different temperatures leads to accurate values for the associated activation parameters, ΔG^‡, ΔH^‡, and ΔS^‡. The observed variation with temperature of the coupling constants of the C(2) and C(6) axial hydrogens is rationalised in terms of rapid inversion at the nitrogen atom.