THE LOWEST VALENCE TRANSITIONS OF 1,1'-BICYCLOHEXYLIDENE AND 1,1'4',1'-TERCYCLOHEXYLIDENE - AN AB-INITIO MRDCI STUDY

Ab initio MRDCI calculations were performed on the oligo(cyclohexylide nes) 1,1'-bicyclohexylidene (1) and 1,1':4',1 ''-tercyclohexylidene (2 ) using RHF/6-31G geometries and molecular orbitals fur the assignment of their lowest valence transitions taking into account the four lowe st states of each symmetry. The calculations unequivocally show that t he valence transitions of 1 and 2 correspond to pi --> pi and pi --> sigma transitions. In the case of 1, the MRDCI results were verified by Direct-CI calculations. Mulliken population analyses suggest that t he pi --> sigma transitions possess charge transfer character; upon e xcitation electron density shifts from the olefinic carbon atoms towar ds hydrogen atoms positioned in the cyclohexylidene rings. The transit ion energies and oscillator strengths of the pi --> pi and pi --> sig ma transitions of 1 and 2 differ from those of the reference compound s tetramethylethene (3) and 1,4-diisopropylidene-cyclohexane (4), resp ectively. In going from 3 to 1 both the pi --> pi and the first sigma --> pi transitions of 1 are bathochromically shifted by ca. 0.24 eV. The first pi --> sigma transition of 1 is hypsochromically shifted ( 0.79 eV) with respect to that of 3. In the case of 2 both the pi --> p i and pi --> sigma* transitions are ca. 0.9 eV bathochromically shift ed with respect to those of 4. (C) 1997 Elsevier Science B.V.