Assignment of Hyperfine Splittings in Electron Spin Resonance Spectra by Linewidth Analyses

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Abstract
A method is developed for the assignment to nuclear positions of the hyperfine splittings in the electron spin resonance spectra of free radicals in solution. The procedure is applicable to the splittings from the nuclei of atoms with a nonvanishing pi-electron spin density, and it is thus useful for 13C and 14N splittings, but cannot be employed for proton splittings. The technique used depends on linewidth effects. It is shown that one contribution to the linewidths is approximately proportional to the square of the spin density on the atom containing the nucleus of interest (the local spin density), and that, under appropriate conditions, splittings from nuclei at positions with high spin density show greater linewidth effects than those from positions with low spin density. The technique is applied to two of the 13C splittings from nuclei present in natural abundance in the spectrum of the anthracene negative ion radical.