The effect of syn–anti isomerism on the lowest valence transitions of 1,1′-bicyclohexylidene. An ab initio MRDCI investigation

Conformational effects on the ground-state and excited-state properties of 1,1′-bicyclohexylidene were studied. The energy difference between the two conformers anti- (anti-1a) and syn-1,1′-bicyclohexylidene (syn-1b) was determined at the RHF/6-31G, MP2/6-31G//RHF/6-31G, RHF/6-311G**//RHF/6-31G and MP2/6-311G**//RHF/6-31G levels of theory. Syn-1b is lower in energy by 0.051 kcal/mol at the MP2/6-311G**//RHF/6-31G level. The valence transitions of syn-1b were calculated using the MRDCI method using its 6-31G geometry and molecular orbitals. In contrast to the predicted UV data of anti-1a for which two absorptions are found, only one absorption for syn-1b, a π→π* transition at ∼6.0 eV, is discernible. The next transition with appropriate oscillator strength is at ∼7.8 eV (π→σ*). Inclusion of polarization functions on the carbon atoms hardly affects the calculated transition energies, oscillator strengths and CI vectors.