STRUCTURE AND HARMONIC VIBRATIONAL FREQUENCIES OF CYCLOPENTADIENE IN THE LOWEST SINGLET-STATES

MCSCF calculations are reported for the electronic ground state and th e two lowest excited singlet states of cyclopentadiene. Molecular geom etries have been optimized for each state individually and the complet e harmonic force field calculated. Similar to the situation found for cis- and trans-butadiene the CC double bond length significantly incre ases on electron excitation and the CC single bond length within the c is-butadiene substructure decreases. The changes in the vibrational fr equencies and in the character of the vibrational modes on excitation is documented in detail. Whereas the electronic ground state and the 1 B-1(2) state have C2v symmetry (with the carbon ring system and the C H bonds in one plane) the 2 1A1 state is predicted to have a lower sym metry with a very shallow minimum for the respective torsional modes.