Energetics, electronic structures and geometries of naphthalene, quinoline and isoquinoline analogues of 1,2-didehydrobenzene

B3LYP/cc-pVTZ electronic structure calculations employed in conjunction with additive corrections derived from experimental data for 1,2-didehydrobenzene predict the standard enthalpies of formation of 1,2- and 2,3-didehydronaphthalenes to be equal to 121.0 and 123.7 kcal mol(-1), respectively. The corresponding singlet-triplet splittings amount to 40.1 and 35.4 kcal mol(-1). The positional dependence of both of these quantities is preserved in those didehydroquinolines and didehydroisoquinolines in which the didehydrogenation sites are separated by at least one carbon from the heteroatoms. The effect of the adjacent heteroatoms on the singlet-triplet splittings is significantly more pronounced than that on the standard enthalpies of formation. Test G3 calculations on 2,3-didehydronaphthalene confirm the reliability of the additive correction scheme in the prediction of properties of annelated analogues of 1,2-didehydrobenzene. Such a scheme opens an avenue to facile electronic structure calculations on didehydrogenation reactions of polycondensed heterocyclic compounds with six-membered rings.