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

Correlated electronic structure calculations at single and multireference l evels of theory have been carried out for several neutral a nd radical ani on electronic states of 2,3-didehydro-1,4-benzo quinone, a quinone analog o f o-benzyne. The molecule is predicted to be a ground-stale singlet ((1)A(1 )) with a 298 K heat of formation of 200.6 kJ mol(-1), a heat of hydrogenat ion (1 equiv.) of -323.5 kJ mol(-1), and an electron affinity of 1.95 eV; t he latter quantity is in good agreement with an experimental value of 1.86 eV. The lowest energy triplet (1(3)B(2)), derived from an in-plane pi-->pi* excitation, is predicted to be 2.23 eV higher in energy than the singlet, The singlet and triplet states have biradical stabilization energies of 2.0 1 and -0.22 eV, respectively. Other triplet states derived from excitations involving the out-of-plane pi system are also examined. A recent photoelec tron spectrum of the radical anion is interpreted, and a poorly resolved fe ature is proposed to correspond to the singlet ((1)A(1)) ground state of 2, 6-didehydro-1,4-benzoquinone. Technical aspects governing the suitability o f various levels of theory are also discussed.