Peral Fernando, Gallego Ernesto
Departamento de Ciencias y Técnicas Fisicoqui;micas, U.N.E.D., Paseo Senda del Rey 9, E-28040, Madrid, Spain.
Spectrochim Acta A Mol Biomol Spectrosc. 2003 Apr;59(6):1223-37. doi: 10.1016/s1386-1425(02)00304-9.
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.
通过紫外光谱法研究了哒嗪和吡嗪在酸性、中性和碱性pH值的水溶液中的自缔合作用。哒嗪在中紫外区域的光谱并未显示出该化合物浓度变化时摩尔吸光率有任何变化，这表明自缔合并不重要。相比之下，在所研究的pH值下，吡嗪随着浓度增加出现了偏离比尔-朗伯定律的情况。在pH<0、0.6和11.0时检测到单一的减色效应，而在pH 6.9时观察到双重减色效应。这些结果可解释为吡嗪在酸性和碱性pH条件下自缔合形成二聚体，在中性pH条件下形成二聚体和聚合物。通过对减色效应实验曲线的拟合计算出自缔合常数。还分析了3-甲基哒嗪、2-甲基吡嗪和2,3-二甲基吡嗪的自缔合作用，得出结论：甲基的引入对哒嗪环的自缔合没有影响，但两个甲基的引入增强了吡嗪的自缔合。根据三种二嗪分子结构的差异，对哒嗪、吡嗪和嘧啶在水溶液中的自缔合行为进行了比较讨论。
