Pyridazines. IX. Proton Magnetic Resonance Studies of Pyridazine, Pyrazine, and Substituted Pyridazines.

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Pyridazine, pyrazine and substituted pyridazines have been studied by proton magnetic resonance spectroscopy. The spectra of pyridazine and pyrazine were analyzed completely with the aid of information obtained from the C¹³-satellites (in natural isotopic abundance), whereas the spectra of substituted pyridazines were analyzed easily by the usual treatments. The proton magnetic resonance parameters of diazines were discussed in connection with the electronegativity of the nitrogen atoms. Effects of some substituents upon the ring proton shifts of pyridazine were discussed. The total methyl effect on the ring protons of pyridazine (0.54 p.p.m.) falls in the normal aromatic range. Electronic charge distributions in diazine molecules were discussed by comparing the π-electron densities calculated by the Huckel MO method with those obtained from the ring proton chemical shifts. Discrepancies in these electronic densities were ascribed to the possible paramagnetic anisotropy effect arising from the n-π^* transition of the lone-pair electrons of the nitrogen atoms.