THE LOWEST SINGLET AND TRIPLET-STATES OF O-BENZYNE - SPIN-COUPLED INTERPRETATION OF THE ELECTRONIC-STRUCTURE AT CAS SCF EQUILIBRIUM GEOMETRIES

The electronic structure of the ($) over tilde X (1)A(1), and ($) over tilde a(3)B(2) states of o-benzyne is analyzed by means of spin-coupl ed (SC) wave functions involving eight active orbitals: two at the deh ydro centers, in the plane of the ring, and six pi: orbitals, inherite d from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out result from separate full geom etry optimizations employing '8 in 8' complete active space self-consi stent field (CAS SCF) wave functions incorporating an analogous core-v alence partitioning. The diradical character of the ground state of o- benzyne, as approximated by the singlet SC wave function, is reflected in the value of the overlap (0.46) between the two in-plane sp(2)-lik e SC orbitals at the dehydro centers, which are just 1.274 Angstrom ap art, and in the nonnegligible extent of triplet coupling between the s pins of the electrons in these orbitals - ca. 6.4% of the total spin f unction for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an a ryne-like character, which is further confirmed by the pattern of alte rnation of the overlaps between neighboring pi valence orbitals, and b y the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin-coupling patterns corr esponding to an arynic structure.