ASSIGNMENT OF THE 2 LOWEST VALENCE TRANSITIONS OF 1,1'-BICYCLOHEXYLIDENE - AN AB-INITIO MRD-CI INVESTIGATION

In contrast to the vapor phase UV spectrum of tetramethylethene (2, br oad band centered at 7.0 eV, f 0.3), 1,1'-bicyclohexylidene (1) posses ses two absorption bands (5.95 eV, f 0.19 and 6.82 eV, f 0.26). For 1 MRD-CI (6-31G) calculations reveal that they have to be assigned to a pi --> pi (9.28 eV, HOMO --> LUMO, b(u), f 0.849) and a pi --> sigma* (10.49 eV, HOMO --> LUMO f 1, b(u), f 0.13) valence transition. Both transitions are perfectly polarized along the carbon-carbon double bon d. In the case of 2 similar calculations reveal that the 7.0 eV band c onsists of two transitions (pi --> phi, 9.75 eV, HOMO --> LUMO, b(2), f 0.900 and pi --> sigma, 10.16 eV, HOMO --> LUMO + 1, b(1), f 0.045 ). Whereas the pi --> pi transition is perfectly polarized along the double bond, the pi --> sigma transition is polarized perpendicular t o this bond.