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Relationships between the HOMO energies and pKa values in monocyclic and bicyclic azines

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Abstract

Ab initio SCF-MO calculations have been used to interpret the observed basicities of certain monocyclic and bicyclic azines. We studied (A) pyridine and the single ring diazines pyridazine, pyrimidine and pyrazine; (B) the benzodiazines cinnoline, quinazoline, quinoxaline and phthalazine, and (C) the naphthyridines substituted at positions 1,5, 1,6,1,7, 1,8 and 2,7, in addition to quinoline and isoquinoline. We report that a good linear relationship exists between the experimental pK values and the HOMO energy for each of the separate series (A), (B) and (C). There is also a good linear relation between the HOMO energy and the first π -electron ionization energy. The basicities of these azines may therefore be directly interpreted in terms of the HOMO energies, as predicted by frontier orbital theory.

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