THROUGH-BOND AND THROUGH-SPACE INTERACTIONS IN A SERIES OF CYCLIC POLYENES

Electron transmission spectroscopy is used to determine the electron affinities of tetracyclo[8.2.2.22,5.26,9]-1,5,9-octadecatriene and pentacyclo[12.2.2.22,5.26,9.210,13]-1,5,9,13-tetracosatetraene. The electron transmission measurements indicate that the splittings between the π⋆ anion states of these compounds are at most a few tenths of an electronvolt. It is shown, with the assistance of ab initio molecular orbital calculations, that both through-bond (TB) and through-space (TS) interactions between the π⋆ (and π) orbitals of the above compounds and of tricyclo[4.2.2.2,5]-1,5-dodecadiene are sizable but that these two interactions oppose one another causing the net splittings in the π⋆ EA's and πIP's to be small. A simple perturbation molecular orbital model is presented which accounts for the trends in the IP's and EA's. Molecular orbital calculations are carried out which show that if the ethano bridges separating the ethylenic groups are replaced by trimethylene bridges, then the balance between the TB and TS effects is altered, and the splittings in the π⋆ and π manifolds are increased. © 1990, American Chemical Society. All rights reserved.