Spin Densities in Organic Free Radicals

Abstract

The proton hyperfine constants obtained from protonmagnetic resonance spectra of polycrystalline free radicals and from valence bond calculations of π‐orbital spin densities are compared for the free radicals α,α′‐diphenyl‐β‐picryl hydrazyl, and tris‐p‐chlorophenylaminium perchlorate. Two lines are observed in the protonmagnetic resonance spectra, one shifted upfield and one downfield from the normal resonance frequency. These are assigned to the ortho and paraprotons, and the metaprotons of the free radicals, respectively. Though the agreement between theory and experiment is not quantitative for the hyperfine constants, the theoretical and experimental ratios of hyperfine constants agree within experimental error. This serves to verify the existence of positive and negative spin densities and the signs of some of the relationships involved.