Absorption, resonance, and near-resonance Raman studies of the tetracyanoquinodimethane neutral and its monoanion in terms of density functional theory and complete active space self-consistent field methods

The electronic structure of the 11B1u and 12B3u excited electronic states of the tetracyanoquinodimethane (TCNQ) neutral and its charged derivative are studied within the framework of complete active space self-consistent field (CASSCF) and Becke's three-parameter hybrid method with Lee–Yang–Parr correlation functional (B3LYP) methods applied to the level aug-cc-p-VDZ basis set. Both CASSCF/aug-cc-p-VDZ and B3LYP/aug-cc-p-VDZ treatments provide the ground-state and the excited state geometries; these are then used to assess the Franck–Condon (FC) parameters in the 11B1u state of the neutral TCNQ and in the 12B3u state of the TCNQ monoanion. The quality of numerical results is then tested on the base of available experimental near-resonance and resonance Raman data. The studies are performed in terms of the vibronic model, which takes both FC and mode-mixing (Dushinsky) effects into account. This somewhat simplified vibronic model leads to very good agreement between the theory and the Raman experiments concerning both neutral TCNQ and its monoanion. In particular, the calculated excitation profiles of the ν2 = 2215 cm−1, ν4 = 1389 cm−1, ν5 = 1195 cm−1, and ν9 = 336 cm−1 fundamentals are shown to be in excellent agreement with those for the TCNQ monoanion. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006