The vibronic effects in the 11B2 and 21A1 states of 1,3-dicyanomethylene croconate dianion: The resonance Raman study in terms of CASSCF and DFT methods

Franck–Condon (FC) and vibronic coupling effects in the excited 11B2 and 21A1 states of the 1,3-dicyanomethylene croconate (LD) dianion are studied using CASSCF and B3LYP methods on the aug-cc-pVDZ basis set level. The study concerns with the resonance Raman spectra measured for the laser excitation energies corresponding to the low-energy 11A1 → 11B2 and 11A1 → 21A1 transitions in the LD dianion absorption. It is shown that the main features of the empirical Raman spectra are clearly due to FC effect in the 11B2 and 21A1 states. We argue that certain features, like these observed at 495 cm−1 and 760 cm−1 may represent fundamentals of b2-symmetry modes enhanced because of vibronic coupling between 11B2 and 21A1.