Long-range proton hyperfine splittings in the epr spectra of cyclohexyl and alkylcyclohexyl radicals: ring inversion in the cyclohexyl radical

Abstract

The cyclohexyl radical has been studied by EPR spectroscopy in liquid cyclopropane at low temperatures and hyperfine splittings from the γ-protons have been observed for the first time. The temperature-dependent EPR spectra have been computer simulated, taking account of β-, γ- and δ-proton exchange, to afford a new Arrhenius rate expression [eqn. (A)] for chair-to-chair interconversion in this radical (θ= 2.303 RT kJ mol–1). log10(kexch/s–1)=(13.5 ± 0.3)–(23.5 ± 1.1)/θ(A)The 4-tert-butylcyclohexyl radical is conformationally locked by the equatorial tert-butyl group and its EPR spectrum does not exhibit temperature-dependent lineshape effects or resolvable splitting from the axial δ-proton. The cis-3,5-dimethylcyclohexyl radical is also locked in a chair conformation and its EPR spectrum shows long-range splittings from two axial γ-protons and from the equatorial δ-proton. Ab initio and semi-empirical (AM 1) molecular orbital calculations predict a slightly non-planar equilibrium configuration at Cα in the cyclohexyl radical; Hα prefers equatorial placement, but the energetic cost of planarisation is very small.