Assignment of the two lowest valence transitions of 1,1′-bicyclohexylidene: an ab initio MRD-CI investigation

In contrast to the vapor phase UV spectrum of tetramethylethene (2, broad and centered at 7.0 eV, ƒ 0.3), 1,1′-bicyclohexylidene (1) possesses two absorption bands (5.95 eV, ƒ 0.19 and 6.82 eV, ƒ 0.26). For 1 MRD-CI (6-31G) calculations reveal that they have to be assigned to a π → π∗ (9.28 eV, HOMO → LUMO, bu, ƒ 0.849) and a π → σ∗ (10.49 eV, HOMO → LUMO + 1, bu, ƒ 0.13) valence transition. Both transitions are perfectly polarized along the carbon-carbon double bond. In the case of 2 similar calculations reveal that the 7.0 eV band consists of two transitions (π → π∗, 9.75 eV, HOMO → LUMO, b2, ƒ 0.900 and π → σ∗, 10.16 eV, HOMO → LUMO + 1, ƒ 0.045). Whereas the π → π∗ transition is perfectly polarized along the the double bond, the π → σ∗ transition is polarized perpendicula bond.