Electron spin resonance studies. Part XLI. Investigations of structure and conformation. Part III. Conformational interconversion in some heterocyclic radicals

Abstract

Radicals have been generated in aqueous solution from pyrrolidine, morpholine, 1,4-dioxan, piperidine, and some alkyl-substituted derivatives, by reaction with ·OH at low pH. Analysis of e.s.r. hyperfine splittings and line-widths recorded over the attainable temperature range (ca. 5–45°) yields kinetic and thermodynamic information about conformational interconversion in these species. The five-membered-ring radicals undergo rapid interconversion between half-chair geometries; the conformational preference for a methyl group in this system is 1·4 kJ mol^–1 at 23°. In contrast, the radical from piperidine undergoes chair–chair ring-flipping with rate constants in the range intermediate between fast and slow exchange [e.g.(2·5 ± 0·2)× 10⁹ s^–1 at 39°]; activation parameters for this interconversion are E_a= 30·4 ± 3·0 kJ mol^–1 and log₁₀A= 14·5 ± 0·2.