The origin of the two-electron/four-centers CC bond in π-TCNE₂²⁻ dimers: Electrostatic or dispersion?

Abstract

The structure and stability of the π-TCNE₂²⁻ dimers in K₂TCNE₂ aggregates is revisited trying to find if the origin of their two-electron/four-centers CC 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 π-TCNE₂²⁻ structure has the two K⁺ atoms placed in equatorial positions in between the two TCNE⁻ planes. When the K⁺ atoms are placed along the D_2h 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. © 2006 Wiley Periodicals, Inc. J Comput Chem 2007