Electron Spin Resonance Spectra of Methyl-Substituted Dihydropyrazine Cations and Related Radicals

Abstract

Electron spin resonance spectra have been obtained for the cation radicals of dihydropyrazine and its methyl‐substituted derivatives, and of dihydroquinoxaline, 2,3‐dimethyldihydroquinoxaline, dihydrophenazine, and N,N′‐dihydro‐4,4′‐dipyridyl. It was found that the parameter which determines the effect on ¹⁴N splittings of the spin density on an adjacent carbon atom has the value Q_CN^N=2.62 G. This result was obtained without making any assumptions about the magnitudes of the sigma—pi parameters and without using valence‐theory calculations of pi‐electron wavefunctions. The positive sign for Q_CN^N is opposite to that expected from the analogous parameters Q_CH^H and Q_{C C}^C. The small value of Q_CN^N, and the approximate constancy of the ratio (sum of the spin densities on atoms adjacent to the nitrogen atom) to the (spin density on the nitrogen atom) in most of the compounds for which data are available, makes it possible to represent the nitrogen splittings by the expression a^N=—0.940 a_NH^H, where a_NH^H is the NH—proton splitting. In some radicals with a large adjacent spin density, however, Q_CN^N may make a significant contribution to the nitrogen splitting. Values of the NH—proton parameter Q_NH^H, and of the C–CH₃ parameter Q_Me^H, were obtained as a function of an assumed value of the parameter Q_CH^H for CH protons. With Q_CH^H=—23.7 G, it was found that Q_NH^H=—33.7 G and Q_Me^H=18.4 G. The latter value is considerably smaller than usually assumed for methyl‐proton splittings. Molecular orbital calculations for the methyl‐substituted pyrazines, as well as for the other cations studied, were in good agreement with experiment.