The electronic structure of the radical cations of butadiene, vinylacetylene and diacetylene: similarities and differences

Abstract

The electronic absorption (EA) spectra of the radical cations of vinylacetylene (VA) and diacetylene (DA) in argon matrices are reported and analyzed for the first time. A comparison with the EA spectrum of butadiene (BD) radical cation shows that the first excitation energy remains virtually constant throughout the series BD + −VA + −DA + as expected from qualitative considerations. In accord with a simple MO picture of polyene radical-cation electronic structure, both BD + and VA + show an intense second band around 300 nm. In contrast, DA + has only weak features in this spectral region. This discrepancy is explained in terms of a semi-quantitative model including the effects of configuration interaction on the different excited states of BD + and DA + . Ab initio CI calculations serve to substantiate this model and bring it into quantative agreement with experiment.