The Franck–Condon and Dushinsky effects in the lowest energy 12B3u doublet of the tetracyanoquinodimethane (TCNQ) monoanion: the absorption and resonance Raman studies in terms of the density functional theory (DFT) and complete active space self-consiste

The absorption and resonance Raman excitation profiles of ν4=1389 and ν5=1195 cm−1 fundamentals are studied in the excitation region corresponding to the 12B2g→12B3u transition in the tetracyanoquinodimethane (TCNQ) monoanion. Both Franck–Condon (FC) and mode-mixing (Dushinsky) effects in the 12B3u electronic state are considered. These effects are discussed in terms of B3LYP and CASSCF methods. It is shown that both absorption and excitation profiles discussed in the 12B2g→12B3u excitation region are in excellent agreement with those available from the experiment. The computations confirm D2h symmetry of the TCNQ− molecule in its lowest energy 12B3u doublet. On the other hand in at least one of two states 22B3u or 12Au, the computations suggest that the symmetry of TCNQ monoanion is unstable being reduced from the D2h to the C2h symmetry.