Methodologies for computational studies of quininoidal diiminediyls: Biradical vs dinitrene behavior

Density functional and post Hartree-Fock ab initio computations were carrie d out on the lowest singlet, triplet, and quintet states of 1,4-phenylenedi nitrene, biphenyl-4,4'-dinitrene, (E)-stilbene-4,4'-dinitrene, and (E,E)-1, 3-bis(4-nitrenophenyl)-1,3-butadiene, and (E,E,E)-1,6-bis(4-nitrenophenyl)- 1,3,5-hexatriene. Near-degenerate singlet and triplet quinonoidal ground st ates were found for all systems using CASSCF methodology, with a slight fav oring of the singlet, in accord with experimental results. The aromatic qui ntet dinitrene states lie much higher in energy. Restricted B3LYP hybrid de nsity functional theory (DFT) methods give artifactually high biradical sin glet state energies relative to the triplet biradical states, but unrestric ted (mixed-state) B3LYP methods correctly give singlet energies that lie so mewhat below the triplet state energies, as well as giving geometric result s that compare well to the best CASSCF results we could achieve for these b iradical states. Appropriate guidelines for selecting CASSCF versus DFT pro cedures in such cases are suggested in light of comparisons of computed to experimental results.