Small heterocyclic systems, spectra

Saturated three- and four-membered heterocyclics absorb little in the readily accessible regions of the UV spectrum. Sulfur-containing rings are an exception, as can be seen in Table 9, Despite the lack of absorption of most parent compounds, there is a wealth of photochemistry of small heterocyclics. Light absorption by substituents, and energy transfer from photoexcited molecules present in the photoreactive system make photoconversion of the heterocycles practical. On the other hand, the lack of substantial absorption of their own can be exploited in the preparation of small heterocycles, by designing the system to be unsuitable for destructive energy transfer. 

The unusual N—S—N bond system in 1,2,5-thiadiazoles poses interesting theoretical questions. Some insight into the structure and properties of 1,2,5-thiadiazoles was gained through studies of electron and X-ray diffraction, the microwave spectrum, and the Baman and infrared spectra of 4 and its derivatives. The iso-iT-electronic relationship between the 1,2,5-thiadiazoles and the pyrazines was examined in detail and a comparative study of the four isomeric thiadiazoles using the MO method in the LCAO approximation for small heterocyclic molecules was reported. 

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