P-Phenylenebis(methylene): a CASSCF study of σ2p2, σ3p1 and σ4 electronic states

Abstract

The 1A g, 3B u and 5A g ( σ 2 p 2), 3A′′ ( σ 3 p 1), and 1A g ( σ 4) electronic states of p-phenylenebis(methylene) are studied by means of ten-electron ten-orbital CASSCF calculations. The σ 2 p 2( 1A g) state is found to be the electronic ground state and it can be described as a 1(σ,σ) diradical. The associated 3(σ,σ) triplet ( 3B u) and the quintet 55A g states are located 1.9 kcal mol −1 and 25.2 kcal mol −1, respectively, above the ground state. Both the 1A g and the 3B u electronic states exhibit a marked quinonoidal distortion. The electronic states that do not involve a local 3σp configuration on each carbene unit ( σ 3p 1 ( 3A′′) and σ 4 ( 1A g) ) are found to be higher in energy (37.0 kcal mol −1 and 59.3 kcal mol −1, respectively). The former ( 3A′′) can be described as resulting from the coupling of a σ 2 and a 3σp carbene. Finally, the CASSCF wave functions of the three nearly diradical states ( 1A g, 3B u and 3A′′) are analyzed in the light of Salem's classification, i.e. homosymmetric and heterosymmetric diradicals.