Structure and harmonic vibrational frequencies of cyclopentadiene in the lowest singlet states

MCSCF calculations are reported for the electronic ground state and the two lowest excited singlet states of cyclopentadiene. Molecular geometries have been optimized for each state individually and the complete harmonic force field calculated. Similar to the situation found for cis- and (trans-butadiene the CC double bond length significantly increases on electron excitation and the CC single bond length within the cis-butadiene substructure decreases. The changes in the vibrational frequencies and in the character of the vibrational modes on excitation is documented in detail. Whereas the electronic ground state and the 1 ¹B₂ state have C_2v symmetry (with the carbon ring system and the CH bonds in one plane) the 2 ¹A₁ state is predicted to have a lower symmetry with a very shallow minimum for the respective torsional modes.