The electronic and vibrational structures of the vis–UV transitions in the 1,3-dicyanomethylene croconate anion in terms of CASSCF and DFT theory. The Franck–Condon analysis of 12B1→12A2 and 12B1→22A2 transitions

The visible and near UV absorption spectra of the 1,3-dicyanomethylene croconate anion radical are discussed in terms of density functional theory (DFT) and the complete active space (CASSCF) method employed with augmented cc-pVDZ basis set. It is shown that the three transitions 12B1→22A2, 12B1→22B1 and 12B1→32B1 shape the absorption in the near UV region. The FC activity in the most intense 12B1→22A2 near UV transition was shown to be quite modest. On the other hand the near visible absorption of the croconate derivative is shown to be due to 12B1→12A2 transition accompanied by a short progression of v5(a1)=1567 cm−1 mode.