The Franck–Condon and Dushinsky effects in the lowest energy 1 2B 3u 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-consistent field (CASSCF) methods

Abstract

The absorption and resonance Raman excitation profiles of ν 4=1389 and ν 5=1195 cm −1 fundamentals are studied in the excitation region corresponding to the 1 2 B 2 g →1 2 B 3 u transition in the tetracyanoquinodimethane (TCNQ) monoanion. Both Franck–Condon (FC) and mode-mixing (Dushinsky) effects in the 1 2 B 3 u 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 1 2 B 2 g →1 2 B 3 u excitation region are in excellent agreement with those available from the experiment. The computations confirm D 2h symmetry of the TCNQ − molecule in its lowest energy 1 2 B 3 u doublet. On the other hand in at least one of two states 2 2 B 3 u or 1 2 A u , the computations suggest that the symmetry of TCNQ monoanion is unstable being reduced from the D 2h to the C 2h symmetry.