Electronic interactions in aryne biradicals. Ab initio calculations of the structures, thermochemical properties, and singlet-triplet splittings of the didehydronaphthalenes

Structural and energetic properties for the lowest energy singlet and triplet states of the 10 didehydronaphthalene isomers are predicted using density functional and multireference second-order perturbation theories. These levels of theory offer excellent agreement with known heats of formation for three singlet isomers when appropriate isodesmic equations are used for prediction. Singlet-tripler splittings and biradical stabilization energies are examined to gain insight into the degree of interaction between the biradical centers. This interaction operates via three distinct mechanisms, namely, through space (overlap), through sigma-bonds, and through pi-bonds, in order of increasing distance over which quantitative impact is predicted. The first two effects are especially sensitive to the relative orientations of the biradical centers and the shape of the molecular framework that joins them. Simpler models are examined for their utility in predicting singlet-triplet splittings; proton hyperfine splittings in antecedent monoradicals are the best predictors of biradical-state energy splittings.