Signs of the Isotropic Hyperfine Splittings in the Electron Spin Resonance Spectrum of the Dihydropyrazine Cation Radical

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Linewidth studies of the electron spin resonance spectrum of the dihydropyrazine cation radical have been employed to determine the signs of the isotropic hyperfine splittings. It was found that the nitrogen splitting aN is positive and the proton splitting aNHH of the protons bonded to the nitrogen atoms is negative. The sigma—pi parameter QNHH in McConnell's relation for NH bonds (aNHH=QNHHρNπ) was found to be negative.