The vibronic effects in the 1 1B 2 and 2 1A 1 states of 1,3-dicyanomethylene croconate dianion: The resonance Raman study in terms of CASSCF and DFT methods

Abstract

Franck–Condon (FC) and vibronic coupling effects in the excited 1 1B 2 and 2 1A 1 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 1 1A 1 → 1 1B 2 and 1 1A 1 → 2 1A 1 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 1 1B 2 and 2 1A 1 states. We argue that certain features, like these observed at 495 cm −1 and 760 cm −1 may represent fundamentals of b 2-symmetry modes enhanced because of vibronic coupling between 1 1B 2 and 2 1A 1.