ELECTRONIC INTERACTIONS IN SATURATED AND UNSATURATED [1.1.1]TRIBLATTANE, [2.1.1]TRIBLATTANE, [2.2.1]TRIBLATTANE, AND [2.2.2]TRIBLATTANE - PE-SPECTROSCOPIC INVESTIGATIONS

The He(I)-PE spectra of the parent [1.1.1]-, [2.2.1]-, and [2.2.2]trib lattanes 1-4, of their unsaturated or ketonic congeners 5-13 and benzo derivatives 14-20 have been recorded. For 1-4, the energy difference between the highest occupied orbitals (e'' and a' mainly localized at the central bicyclo[2.2.2]octane unit) and the subsequent MOs increase s with increasing number of ethano bridges. This observation is ration alized by using a first-order perturbation-theoretical approach. The a nalysis of the PE spectra of the exo-methylene (5-7), ketone (8-10), a nd endo-olefinic derivatives (11-13) unveils a strong pi/sigma mixing. The splitting of the pi-bands in 7 (0.95 eV) and 13 (1.0 eV), respect ively, is traced back to a stronger pi/sigma mixing of the e MOs as co mpared to the a2 MOs. An analogous interaction causes the splitting of the first bands of the ketones 8-10 (0.45-1.49 eV). For the benzo der ivatives 14-20 the split of the pi-bands is relatively small (0.35-0.7 ).