The Electronic Structures of Unstable Intermediates. Benzyne and Hetarynes

A semi-empirical SCF MO method for valence electron systems, a modification of our previous method made by introducing the zero-differential overlap approximation except for the one-center exchange integrals, is presented. The results calculated for benzene, pyridine, pyrimidine and thiophene, the parent molecules of benzyne and hetarynes, using this modified method are compared with those obtained by the other calculations and experiments. The unstable intermediates treated in the present paper are benzyne, pyridynes, pyrimidyne and thiophynes. The unknown bond lengths of benzyne and hetarynes are estimated by a semi-empirical relation given in the present treatment. The nature of the carbon-carbon triplet bonds of benzyne and hetarynes is studied on the basis of the bond lengths and the total π bond orders for these bonds. The charge distributions and excitation energies of benzyne and hetarynes, and the chemical reactivity of some hetarynes are also discussed. The lowest triplet states of these intermediates are shown to be close to the singlet ground states.