Pyridazine ultraviolet spectra

However, in 2,3,5-trifluoro-6-hydroxypyrazine (47), the infrared spectrum of the solid was reported to show no absorption attributable to the carbonyl group, and its ultraviolet spectrum (in ethanol) was similar to that of 23,5-trifluoro-6-methoxypyrazine, thus indicating that it was a true hydroxy compound (851). A similar phenomenon has been observed in the pyridine (1087-1089) and pyrimidine (1090) series, but differs from observations in the pyridazine series (1091). 

The spectra of pyrazine, pyrimidine, and pyridazine, all in cyclohexane, are compared by Albert (1462). They show two bands with associated fine structure, the peaks for pyrazine center around 260 and 328 nm. The near-ultraviolet spectrum of pyrazine has been measured in several solvents [and at various pH values (1463)] and the transitions assigned (1474, 1482). The diffuse system at 260 nm has been attributed to rr->-7r transitions whereas the sharp system at 328 nm has been ascribed to n rr transitions (1467, 1474). Semiempirical calculations have been made on the electronic structure of pyrazine with reference to its -> rr transition (1483-1486), calculations have been made of transition energies in N-... 

Amino-pyrazines and -pyridazines have been shown to exist predominantly in the amino form by infrared spectroscopic studies (cf. Table VI). Ultraviolet spectral data have been interpreted to indicate that 4-aminocinnoline exists predominantly in the imino form 256, but this conclusion, which was based on comparison of its spectrum with those of cinnolin-4-one and 4-ethoxycinnoline, is probably incorrect. Ultraviolet spectroscopic data strongly support the predominance of amino structures for 2-aminopyrazine (257) and 2-aminoquin-oxaline how ever, the former compound was at first erroneously concluded to exist in the imino form from ultraviolet spectral evidence. Isolation of two isomers of 2-amino-8-dimethylamino-3-methylphenazine, assigned the amino and imino structures 258 and 259, respectively, has been claimed, but it is very unlikely that these assignments are correct. 

In the ultraviolet photoelectron spectrum, the most readily ionized level of pyridine is the nonbonding orbital (with contributions from the o -framework). The three diazines show two lone-pair levels, with the greatest splitting in the case of pyridazine but considerable also in pyrimidine and pyrazine. These long-distance splittings are attributed to both through-space and through-bond interactions, particularly the latter. 

Several spectroscopic investigations concerning pyridazine have been performed. Pyridazine is isoelectronie with benzene and replacement of CH groups in the latter by the more electronegative nitrogen atoms causes a marked dilferenee, alike with the other two diazines, in the near ultraviolet speetrum. It has a highly aromatic type of absorption spectrum. The ultraviolet spectra of pyridazine, its anion, and cation have been measured in different solvents, calculated, and interpreted. A detailed discussion... 

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