Absorption, resonance, the preresonance Raman study of the 1,3-dicyanomethylene croconate dianion using complete active space self-consistent field and density functional theory methods

Abstract

The electronic structures of the 11B2 and 21A1 excited electronic states of the 1,3-dicyanomethylene croconate dianion are studied in the framework of complete active space self-consistent field (CASSCF) and Becke’s three-parameter hybrid method with a Lee–Yang–Parr correlation functional methods applied on the level aug-cc-pVDZ basis set. The CASSCF/aug-cc-pVDZ treatment provides the ground (11A1) and the excited 11B2 and 21A1 states geometries, which are then used to evaluate the Franck–Condon parameters in the 11B2 and 21A1 states. The quality of the numerical results is verified on the bases of experimental near-resonance and resonance Raman data available in the vis-UV excitation region. The analysis is done in terms of the vibronic model, which treats the totally symmetric vibrations as displaced harmonic oscillators. Under the resonance with the 11A1→11B2 electronic transition, that somewhat simplified vibronic model leads to excellent agreement between the theoretical and empirical excitation profiles for the ν2=2234 cm−1, ν9=911 cm−1, ν13=376 cm−1, and ν14=318 cm−1 fundamentals. At preresonance with the 11A1→11B2 electronic excitation the agreement with the experimental Raman spectrum is reasonable but some discrepancies are noticed for the ν4=1620 cm−1 and ν5=1546 cm−1 fundamental lines. We argue that the observed discrepancies can be removed when mode-mixing (Duszynski) effects between ν4 and ν5 vibrations in the 11B2 state are taken into account.