THE EFFECT OF SYN-ANTI ISOMERISM ON THE LOWEST VALENCE TRANSITIONS OF1,1'-BICYCLOHEXYLIDENE - AN AB-INITIO MRDCI INVESTIGATION

Conformational effects on the ground-state and excited-state propertie s of 1,1'-bicyclohexylidene were studied. The energy difference betwee n the two conformers anti-(anti-1a) and syn-1,1'-bicyclohexylidene (sy n-1b) was determined at the RHF/6-31G, MP2/6-31G//RHF/6-31G, RHF/6-311 G*//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 leve l. The valence transitions of syn-1b were calculated using the MRDCI m ethod 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 pi --> pi transition at similar to 6.0 eV, is discernible. The next transition with appropriate oscillato r strength is at similar to 7.8 eV (pi --> sigma). Inclusion of polar ization functions on the carbon atoms hardly affects the calculated tr ansition energies, oscillator strengths and CI vectors. (C) 1998 Elsev ier Science B.V.