The Electron Spin Resonance and Electronic Spectra of the Anion Radicals of 4-Nitropyridine and 4-Nitropyridine 1-Oxide

The ESR and electronic spectra of the 4-nitropyridine and 4-nitropyridine 1-oxide anions have been measured. The nitrogen and hydrogen hyperfine splitting constants of these two compounds have been determined. In order to analyze the ESR spectra exactly, the measurements have also been carried out with respective compounds containing 15N instead of 14N in the nitro group. The spin densities on the nitrogen and carbon atoms of these two anions have been estimated from the splitting constants. It has thus been found that the spin densities on the ring nitrogen and carbon atoms are undoubtedly larger in the 4-nitropyridine 1-oxide anion than in the 4-nitropyridine anion. The electronic spectrum of the 4-nitropyridine anion has been measured and three peaks found at 313, 440 and 640 mμ. According to the molecular orbital calculations carried out with the π-electron system of the 4-nitropyridine anion, it has been concluded that the 440 mμ band may be regarded as the intramolecular charge transfer band, and that the 313 mμ band can be interpreted as the blue shifted band of the pyridine anion.