ESR Study of Interconversion in Substituted Piperidine Iminoxyls

Abstract

ESR spectra of iminoxyl radicals formed photochemically from the following six‐membered alicyclic hydroxylamines, 1‐hydroxypiperidine, 1‐hydroxy‐4‐methylpiperidine, 1‐hydroxy‐4‐phenylpiperidine, and 4‐hydroxymorpholine, have been obtained in methylene chloride over a temperature range of −130° to +120°C. The main features of the ESR spectra result from hyperfine coupling to the nitrogen nucleus and to the four $\beta$ protons. The nitrogen hyperfine interaction is independent of temperature over the range studied in contrast to the β‐hyperfine interaction which depends upon the conformational behavior of these radicals. From the variations of the ESR spectra with temperature the barrier to ring inversion, the rate of interconversion, the conformational preferences, and structural parameters for these molecules are calculated. It is concluded that these radicals are in the chair conformation. At low temperature, hyperfine coupling to $\gamma$ protons are detected and are shown to result from coupling to the axial pair. For 1‐hydroxypiperidine coupling to one $\delta$ proton is found. This coupling is attributed to the equatorial $\delta$ proton and is equal in magnitude to the γ‐proton hyperfine interaction in this radical.