Azole rings containing carbonyl groups

The carbonyl group is found in a wide variety of situations in five-membered rings accommodating diverse heteroatoms. Carbonyl frequencies to be discussed range from about 1800 to below 1600 cm 1 some such frequencies are so low that they often fail to be recognized as carbonyl absorptions at all, e.g. carbonyl joined to a heterocyclic ring via an exocyclic double bond as in structure (127). Low frequency v(C = O) bands are widespread because five-membered heterocyclic rings are inherently tt-electron donors. The transfer of electrons into the exocyclic double bond [v(C = Z) of 127] is likely to be extensive when both X and Y are electron donor atoms. 

In Table 32 the v(C = O) and other characteristic bands are given for some saturated five-membered heterocycles, and compared with the corresponding absorption frequencies for cyclopentanone. Adjacent NH groups and sulfur atoms have the expected bathochromic effect on v(C = O), whereas an adjacent oxygen atom acts in the reverse direction. The CH2 vibrations of cyclopentanone are repeated to a considerable extent in the heterocyclic analogues. 

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