Anisotropic Hyperfine Interactions in the ESR Spectra of Alkyl Radicals

Abstract

We have studied the effect of anisotropic hyperfine interactions on the electron spin resonance (ESR) spectra of alkyl radicals trapped in polycrystalline matrices. The anisotropy broadens some or all of the hfs components, thus complicating the spectra. In alkyl radicals one has both isotropic and anisotropic hfs interactions with the α protons, but only an isotropic interaction with the β protons. Computed and experimental line shapes for the ethyl and propyl radicals are in qualitative agreement, provided that the hfs interaction is averaged over the various equilibrium orientations of the —CH₂· group. This implies a rapid reorientation of the —CH₂· group. It is noteworthy that when the two α protons are antiparallel the hyperfine anisotropy cancels and sharp intense hfs components result, while parallel orientations of the α protons give broad weak hfs lines. In particular, if the broad weak lines associated with the α proton hfs interactions are overlooked, the propyl radical spectrum appears to be a triplet, implying hfs interactions with the β protons only. In radicals where there is an odd number of α hydrogens (e.g. R′ — ĊH — R) there is no possibility of cancellation, and all the lines will be broad and weak. Our attempts to observe radicals of this type have been unsuccessful, most likely for this reason.