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 , a heat of hydrogenation (1 equiv.) of 323.5 kJ mol , 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.