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

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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 QCNN=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 QCNN is opposite to that expected from the analogous parameters QCHH and QC CC. The small value of QCNN, 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 aN=—0.940 aNHH, where aNHH is the NH—proton splitting. In some radicals with a large adjacent spin density, however, QCNN may make a significant contribution to the nitrogen splitting. Values of the NH—proton parameter QNHH, and of the C–CH3 parameter QMeH, were obtained as a function of an assumed value of the parameter QCHH for CH protons. With QCHH=—23.7 G, it was found that QNHH=—33.7 G and QMeH=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.