title: Theoretical study of the electronic structure of [TCNQ]2(2-) (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) dimers and their intradimer, long, multicenter bond in solution and the solid state.
abstract: 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](*-)...[TCNQ](*-) 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](*-)...cation(+) electrostatic interactions, whose sum exceeds the sum of the [TCNQ](*-)...[TCNQ](*-) and cation(+)...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...N and 6 C...C). In solution, their stability originates from the [TCNQ](*-)solvent attractive interactions, as calculated for {[TCNQ](2)(S)(4)}(2-) (S = CH(2)Cl(2), H(2)O), 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 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 ground-state electronic structure is well described from B3LYP calculations.
authors.affiliation: Departament de Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain.
authors.forename: Iñigo
authors.initials: I
authors.lastname: Garcia-Yoldi
authors.forename: Joel S
authors.initials: JS
authors.lastname: Miller
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Novoa
mesh_terms: D003198:Computer Simulation; D019281:Dimerization; D004583:Electrons; D008956:Models, Chemical; D009570:Nitriles / Q000737:chemistry*; D012996:Solutions
chemical_list: D009570:Nitriles; D012996:Solutions; C013703:tetracyanoquinodimethane