Paramagnetic resonance of methyl- and chloro-substituted p-benzosemiquinones

Abstract

Detailed measurements, made under conditions of high resolution, of the paramagnetic resonance spectra of the negative radical ions of all the methyl-and chloro-substituted p-benzosemiquinones are reported. All the spectra exhibited hyperfine structure which is accounted for in terms of interactions between the unpaired electron and all the protons in the radicals; no chlorine hyperfine splittings were observed. In some of the chlorine compounds anomalous lines, not attributable to the chloro-substituted p-benzosemiquinone ion, appeared; these presumably arise from radicals produced by side reactions. Microwave power saturation was encountered in all the spectra examined. Line widths of all the spectra were observed to depend markedly on the microwave power and on the concentration of the hydroquinone from which the semiquinone was prepared. Separations of the hyperfine components were measured to a high degree of accuracy and splitting constants for individual protons were calculated from the measurements using the method of least squares. Proton splitting constants ranged from a minimum of 1.714 gauss for the methyl group protons in 2,3-dimethyl semiquinone ion to a maximum of 2.595 gauss for the ring protons in that same compound. The variation of the splitting constants with chlorine and methyl group substitutions are shown to obey a nearly additive relation.