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

Abstract

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, b u, ƒ 0.849) and a π → σ ∗ (10.49 eV, HOMO → LUMO + 1, b u, 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, b 2, 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.