Influence of Molecular Structure on Paramagnetic Resonance Line Widths

This is the authors' abstract. We don't add key points for this paper.

Paramagnetic resonance absorption has been observed in a series of free radical triarylaminium salts and in a series of substituted 1-picryl-2,2-diarylhydrazyls. Within each series, the members differ from each other only in the substituents placed in certain aromatic ring positions in the molecules. Line-width measurements in fields of 3080 and 2 oersteds are reported for powder samples of these radicals, together with ``g'' values measured in the larger field. A correlation has been established between the line widths of the paramagnetic resonance absorption lines of the radicals and the Hammett sigma values of their substituent groups. In general, the line widths for both series of radicals decreased with increasingly negative values of the Hammett sigma constants of the substituents. Few compounds containing substituents with positive sigma constants have been observed. Relationships between sigma and line widths for this range of sigma values have been proposed on the basis of the expected electronic effects of such substituents, and are consistent with the available data except for halogen-substituted compounds. These results are interpreted in terms of the degree of delocalization of the unpaired electrons in the various free radicals studied. The effect of delocalization upon the exchange interaction of the unpaired electrons is also discussed in the light of the theories of exchange narrowing of paramagnetic resonance absorption lines.