A study by ultraviolet spectroscopy on the self-association of diazines in aqueous solution

The self-association of pyridazine and pyrazine was studied in aqueous solution at acidic, neutral and basic pH values, by ultraviolet spectroscopy. The spectra of pyridazine in the mid-ultraviolet region did not show any variation in molar absorptivity upon concentration of this compound, indicating that self-association is not important. By contrast, deviations from Beer-Lambert law with increasing concentration were found in pyrazine at the pH values studied. A single hypochromic effect was detected at pH<0, 0.6 and 11.0, while a double hypochromic effect was observed at pH 6.9. These results were interpreted in terms of self-association of pyrazine leading to the formation of dimers at acidic and basic pH, and of dimers and polymers at neutral pH. From the fitting of the experimental curves of hypochromic effects, self-association constants were calculated. The self-association of 3-methylpyridazine, 2-methylpyrazine, and 2,3-dimethylpyrazine was also analyzed, concluding that the introduction of a methyl group has no influence upon the self-association of the pyridazine ring, but the self-association of pyrazine is enhanced by the introduction of two methyl groups. A comparative discussion of the self-association behavior of pyridazine, pyrazine and pyrimidine in aqueous solution has been carried out in terms of the differences in the molecular structure of the three diazines.