Theoretical Study of the Electronic Structure of [TCNQ]22- (TCNQ=7,7,8,8-Tetracyano-p-quinodimethane) Dimers and Their Intradimer, Long, Multicenter Bond in Solution and the Solid State

The long, multicenter bonding in 7,7,8,8-tetracyano-p-quinodimethane anion radical dimers, [TCNQ](2)(2-), observed in both the solid state and in solution, was computationally investigated via B3LYP, CASSCF(2,2), and MCQDPT/CASSCF(2,2) methods. Isolated [TCNQ](center dot-)center dot center dot center dot[TCNQ](center dot-) interactions are repulsive by all three methods, although a metastable minimum is found at the MCQDPT/CASSCF(2,2) level. In the solid state, the stability of [TCNQ](2)(2-) dimers primarily originates from [TCNQ](center dot-)center dot center dot center dot cation(+) electrostatic interactions, whose sum exceeds the sum of the [TCNQ](center dot-)center dot center dot center dot[TCNQ](center dot-) and cation(+)center dot center dot center dot anion(+) repulsive interactions. In the solid state, as observed for K-2[TCNQ](2) aggregates, [TCNQ](2)(2-) has a long, two-electron bond with contributions from 20 centers (4 N center dot center dot center dot N and 6 C center dot center dot center dot C). In solution, their stability originates from the [TCNQ](center dot-) solvent attractive interactions, as calculated for {[TCNQ](2)(S)(4)}(2-) (S = CH2Cl2, H2O), and these have contributions from 16 C centers as the N atoms do not contribute because the nitriles bend away from the nominal plane of the fragment. MCQDPT/CASSCF(2,2) calculations indicate that the electronic ground 2 - state of these [TCNQ](2)(2-) dimers is a closed-shell singlet with a non-negligible contribution from the open-shell singlet, as is experimentally observed. This -round-state electronic structure is well described from B3LYP calculations.