E.S.R. spectra of substituted pyridine and pyrimidine anion radicals

Abstract
Electron spin resonance spectra were obtained of the 3-nitropyridine, 3,5-dinitropyridine, 2-chloro-5-nitropyridine, 5-nitropyrimidine, 3-acetyl pyridine anion radicals produced electrolytically in dimethyl-sulphoxide solutions. Hyperfine splitting constants were assigned in some cases with the help of molecular orbital calculations. The 3-acetylpyridine anion exists in two isomeric forms, the spectra of which were observed. Ionization of chloride from 2-chloro-5-nitropyridine, 2-chloro-3-nitropyridine, and 2-chloro-5-nitropyrimidine occurred on reduction, yielding the 3-nitropyridine anion from the first two compounds, and the 5-nitropyrimidine anion from the third. Analysis of the data suggests that, for this series of radicals, the σ-π parameter Q cn n is in the range 0 ⩾ Q cn n ⩾ -2·5 gauss.