ELECTRONIC AND VIBRATIONAL STRUCTURE AND SCALED DENSITY-FUNCTIONAL FORCE-FIELD OF CYCLOPENTADIENE AND ITS RADICAL-CATION

The photoelectron (PE) spectra of cyclopentadiene (CP) and its perdeut erated isotopomer (CP-d(6)) and the electronic absorption (EA) and inf rared (IR) spectra of the corresponding radical cations are presented. The electronic structure of CP.+ is discussed on the basis of CASSCF/ CASPT2 calculations which reproduce the excited-state energies as deri ved from the PE and EA spectra very well. Density functional force fie lds are scaled to fit the observed bands in the IR spectra and vibrati onal progressions in the first PE bands. The changes in valence force constants upon ionization of CP are discussed on the basis of qualitat ive expectations and compared to those found earlier for butadiene.