Preference of [6e/4p], [6e/5p], and [6e/6p] Conjugations in Determining Relative Stabilities of Fused Five-Membered Heteroaromatic Isomers

Abstract

Abstract Ab initio molecular orbital calculations showed that fused five-membered heteroaromatics 1 (X=O, NH, S) could be slightly or almost equally stable relative to their isomers 2. The results suggest that the [6e/6p] triene conjugation is only a little in preference to (or in the balance with) the counteracting [6e/4p] and [6e/5p] conjugations. Kinetically, the isomers 2 were predicted from the qualitative orbital theory and the calculated HOMO energies to be more stable.