Hydrogen-bonding Effects on the Electron Spin Resonance Spectra of the Anion Radicals of Several Aromatic Amine <i>N</i>-Oxides

Abstract

Abstract The hydrogen-bonding effects of ethanol, phenol, and water on the electron-spin-resonance spectra of the anion radicals of 4-nitro- and 4-cyano-pyridine N-oxides, pyrazine mono- and di-N-oxides, and 4-nitropyridine electrochemically generated in dimethylformamide have been studied quantitatively. The equilibrium constant, K, pertinent to the hydrogen-bonding systems was determined on the basis of analyses of the variation in the hyperfine coupling constant caused by the addition of a proton donor. The active site in a radical anion participating in the hydrogenbonding interaction and the K values were then discussed on the basis of the theoretical calculation of the spin density and the coupling constant of the radicals in the hydrogen-bonding interaction. It has been pointed out that this sort of experiment in hydrogen bonding interaction is also useful for assigning the hyperfine coupling constant to a definite atom in a radical. Last, these K values are used to explain the mechanism of radical formation by the controlled potential electrolysis at different reduction potentials of pyrazine mono- and di-N-oxides in dimethylformamide-ethanol mixed solvents.