Antiaromaticity in open-shell cyclopropenyl to cycloheptatrienyl cations, anions, free radicals, and radical ions

The cationic, anionic, and free radical conjugated cyclic hydrocarbons were studied. The ESR spectrum of the cycloheptatrienyl radical 9 was also discussed both in solutions and in matrixes and it was observed that the radical shows an eight-line spectrum that was assumed to arise from a planar, symmetrical structure. The results suggest that antiaromaticity is a highly useful concept and demonstrates the powerful predictive ability of the simple Hückel-derived rule.