2,3-Didehydro-1,4-benzoquinone. A quantum thermochemical study

Correlated electronic structure calculations at single and multireference levels of theory have been carried out for several neutral and radical anion electronic states of 2,3-didehydro-1,4-benzoquinone, a quinone analog of o-benzyne. The molecule is predicted to be a ground-state singlet (¹A₁) with a 298 K heat of formation of 200.6 kJ mol^-1, a heat of hydrogenation (1 equiv.) of -323.5 kJ mol^-1, and an electron affinity of 1.95 eV; the latter quantity is in good agreement with an experimental value of 1.86 eV. The lowest energy triplet (1³B₂), derived from an in-plane π→π* excitation, is predicted to be 2.23 eV higher in energy than the singlet. The singlet and triplet states have biradical stabilization energies of 2.01 and -0.22 eV, respectively. Other triplet states derived from excitations involving the out-of-plane π system are also examined. A recent photoelectron spectrum of the radical anion is interpreted, and a poorly resolved feature is proposed to correspond to the singlet (¹A₁) ground state of 2,6-didehydro-1,4-benzoquinone. Technical aspects governing the suitability of various levels of theory are also discussed.