title: The origin of the two-electron/four-centers C--C bond in pi-TCNE(2)2- dimers: electrostatic or dispersion?
abstract: The structure and stability of the pi-TCNE(2)2- dimers in K2TCNE2 aggregates is revisited trying to find if the origin of their two-electron/four-centers C--C bond are the electrostatic K+-TCNE- interactions or the dispersion interactions between the anions. The study is done at the HF, B3LYP, CASSCF (2,2), and MCQDPT/CASSCF (2,2) levels using the 6-31+G(d) basis set. Our results show that the only minima of this aggregate that preserves the pi-TCNE(2)2- structure has the two K+ atoms placed in equatorial positions in between the two TCNE- planes. When the K+ atoms are placed along the D2h axis of the anions the structure is not a minimum. The main energetic component responsible for the stability of these aggregates comes from the cation-anion interactions. However, a proper accounting of the dispersion component (as done in the MCQDPT/CASSCF (2,2) calculations) is needed to make the closed-shell singlet more stable than the open-shell singlet. Thus, the bond results from the combination of the electrostatic and dispersion components, being the first the dominant one. The optimum geometry of the closed-shell singlet is very similar to the experimental one found in crystals.
authors.affiliation: Departament de Qúmica Física, Facultat de Qúmica, and CERQT, Parc Científic, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain.
authors.forename: Iñigo
authors.initials: I
authors.lastname: García-Yoldi
authors.forename: Fernando
authors.initials: F
authors.lastname: Mota
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Novoa