Theoretical study of the vibronic structure of the 1 1A1→1 1B2 and 1 1A1→1 3B2 electronic transitions in cyclopentadiene

The geometry of the ground and excited singlet and triplet 1B2 electronic states of cyclopentadiene is optimized by ab initio (configuration interaction singles) and semiempirical (quantum chemical force field) methods. The optimized structures provide displacement parameters of the totally symmetric modes for the singlet→singlet and singlet→triplet transitions. The calculated singlet→singlet absorption spectrum for cyclopentadiene and its fully deuterated isotopomer agree very well with the observed spectra. It is demonstrated that the difference of these spectra from those of a linear polyene (butadiene) results from the strong Franck–Condon activity of the lowest frequency a1 mode which describes CCC bending. It is shown that the properties of the triplet and singlet 1B2 states differ greatly as a result of their different nature; the former being mostly covalent, whereas the latter is mostly ionic. This leads to different Franck–Condon structures of the 1 1A1→1 1B2 and 1 1A1→1 3B2 transitions. The ionic nature of the singlet 1B2 state leads to its inherent instability with respect to the out-of-plane b1 distortion.