FREQUENCY INCREASE OF THE C=C-AG STRETCH MODE OF POLYENES IN THE 21AG- STATE - ABINITIO MCSCF STUDY OF BUTADIENE, HEXATRIENE, AND OCTATETRAENE

The harmonic force fields in the ground (11Ag) and lowest energy excited singlet (21Ag-) states of trans-butadiene, -hexatriene, and -octatetraene have been computed by the ab initio MCSCF method. The effect of mode mixing was analyzed with the Wilson GF matrix formalism. Although the calculated frequencies for the C=C symmetric stretch mode are uniformly 15% too high, the relative increase in the frequency of this mode on going from the ground (11Ag) to the excited (21Ag-) state as well as the dependence of this relative increase on chain length agrees well with the experimental results. In these calculations, the frequency increase of the C=C symmetric stretch comes primarily from couplings between this mode and the C-C single-bond stretches and C-C-H bends. © 1990 American Chemical Society.