Other Aromatic Six-Membered Ring Heterocyclics

Assignments for pyridazine and pyrazine were suggested by Ito et al. [40], who noted that the latter differed from other heteroaromatics in having no infrarred absorptions above 1480 cm . However, this is a consequence of its symmetry and the usual four bands in the 1600—1400 cm range are shown in the Raman spectrum. Several bands also appear in the infra-red when the symmetry is lost by substitution. Alkyl pyrazines show three or four bands in this range. Pyridazines have been studied as 3 6-substituted compounds and show two bands between 1600—1540 cm and 1430— 1395 cm [63]. Both series show medium to weak bands between 1300 and 1000 cm from in plane CH bends and ring modes. These are better seen in the Raman spectra [44]. Below 1000 cm the out of plane CH bends appear. Pyridazine has its as a band at 760 cm which would correspond to four adjacent hydrogens, and pyrazine at 804 cm .  

5-Triazines have been studied by Lancaster et al. [7]. All symmetrically substituted derivatives show only two ring stretching bands near 1560 and 1410 cm In asymmetrical compounds the degeneracy is split but is still small, so that two bands are still usual. All 5-triazines absorb in the 800 cm region. The position of the band depends upon the resonance characteristics of the substituents and the overall range is 817—736 cm .  

The infra-red spectrum of tetrazine has been discussed by Spencer et al. [8] but not much is known of its derivatives. The CH stretch is rather higher than usual (3090 cm ) and the ring stretches are at 

The spectra of polycyclic heteroaromatics are complex and cannot be covered adequately in the space available here. Katritzky and Taylor [43] summarise the characteristic frequencies of many such systems such as quinoxalines, naphthyridines, pyridopyridazines and pyrimidotriazines. 

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