Electron spin resonance spectra have been obtained for the cation radicals of dihydropyrazine and its methylhyphen;substituted derivatives, and of dihydroquinoxaline, 2,3hyphen;dimethyldihydroquinoxaline, dihydrophenazine, andN,Nprime;hyphen;dihydrohyphen;4,4prime;hyphen;dipyridyl. It was found that the parameter which determines the effect on14N splittings of the spin density on an adjacent carbon atom has the valueQCNN=2.62 G. This result was obtained without making any assumptions about the magnitudes of the sigmamdash;pi parameters and without using valencehyphen;theory calculations of pihyphen;electron wavefunctions. The positive sign forQCNNis opposite to that expected from the analogous parametersQCHHandQC CC. The small value ofQCNN, 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 expressionaN=mdash;0.940aNHH, whereaNHHis the NHmdash;proton splitting. In some radicals with a large adjacent spin density, however,QCNNmay make a significant contribution to the nitrogen splitting. Values of the NHmdash;proton parameterQNHH, and of the Csngbnd;CH3parameterQMeH, were obtained as a function of an assumed value of the parameterQCHHfor CH protons. WithQCHH=mdash;23.7 G, it was found thatQNHH=mdash;33.7 G andQMeH=18.4 G. The latter value is considerably smaller than usually assumed for methylhyphen;proton splittings. Molecular orbital calculations for the methylhyphen;substituted pyrazines, as well as for the other cations studied, were in good agreement with experiment.
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