in the interstellar medium; their review focusses on C 5 and C 6 aromatic species because cyanobenzene had been previously detected. Subsequently a five membered ring has been found, 1-cyano-1,3-cyclopentadiene, its highly polar nature µ a = 4.15 D no doubt contributing to its discovery. Speculations as to the presence of three-membered rings include a laboratory study of cyclopropenone as well as its actual detection following on from the first interstellar organic ring, cyclopropenylidene or cyclo-C 3 H 2 , discovered. In a laboratorty study of the rotational spectrum of furan Barnum et al. have concluded that heterocyclics are peculiarly less abundant than cyclic hydrocarbons in the ISM. The search for already known isomers was conducted here with SciFinder R and those acyclics found are shown in Fig. while cyclic species are shown in Fig. . The objectives of this work are two-fold: (1) to provide quality data for a series of species, some of which lie outside the normal range of molecules encountered in most chemical databases, which can then be used by machine-learning artificial intelligence procedures as standards or learning sets, and (2) to test whether any of the isomers would be easily detectable in emissions from the ISM. Note that the GDB-17 database which contains upwards of 166 billion organic small molecules, with the number of 'heavy' atoms ≤ 17, and is comprised of the elements C, § None of these unfortunately were available for abstraction by Chemical Abstracts Services.