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

Abstract

Abstract:The structure and stability of the -TCNE 2 2À dimers in K 2 TCNE 2 aggregates is revisited trying to find if the origin of their two-electron/four-centers CÀ ÀC bond are the electrostatic K þ -TCNE À 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 6-31þG(d) basis set. Our results show that the only minima of this aggregate that preserves the -TCNE 2 2À structure has the two K þ atoms placed in equatorial positions in between the two TCNE À planes. When the K þ 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 closedshell 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.

Study of the Diradicaloid Character in a Prototypical Pancake‐Bonded Dimer: The Stacked Tetracyanoethylene (TCNE) Anion Dimer and the Neutral K2TCNE2 Complex

“…However, such pancake bonds cannot be formed in the isolated TCNE 2 2À system due to the strong Coulomb repulsion between the two anions. [17] We used two options to obtain a description and interpretation of the bonding between the two anions: 1) two K + counterions were included to make the whole system neutral [11,18] and 2) the Coulomb repulsion was estimated and subsequently subtracted from the interaction energy to reveal the intrinsic radical-radical binding energy. [19] The location of the counterions has a significant effect on the dimer.…”

“…Two configurations are shown in Figure 2 following earlier work on the problem. [11,18] Note that comparisons with experiments on crystals should be done with care due to the long-range nature of Coulomb interactions that results in the Madelung energy, which we do not attempt to include herein. To date, numerous TCNE 2 2À p dimers in crystals of charge-transfer salts have been characterized by spectroscopic and theoretical studies.…”

“…DFT [18] and multireference second-order Møller-Plesset perturbation theory (MRMP2) methods have been reported. [11] Up-todate, high-level calculations are still lacking in the area of pancake bonding.…”

Exploring the Limits of Density Functional Approximations for Interaction Energies of Molecular Precursors to Organic Electronics

“…Doubly charged π-dimers (e.g., tetracyanoethylene, (TCNE) 2 2-) 29 present yet another issue that is the static correlation error. [31][32][33] In addition, doubly charged dimers tend to be unstable in gas-phase due to Coulomb repulsion and are therefore excluded from this benchmark study. Such dimers are difficult to describe even around equilibrium due to an important degree of multi-reference character.…”

Intermolecular π‐dimer of oxoverdazyl radicals with long‐distance multicenter (2e/8c) bonding via nitrogen atoms

“…In fact, moderate π ‐bonding of radicals facilitates π ‐stacks formation, electron‐transfer processes, and spin interactions which determine the conductance and magnetism of molecular materials, while strong π ‐bonding between radicals leads to the formation of solid‐state diamagnetic dyads (and impinge on material science applications) 37–39. Thus, development of supramolecular bonding ideas40–45 and applications in material science require further experimental and theoretical study of π ‐interactions of diverse open‐shell species and factors controlling the formation of their π ‐dimers. The latter, for example, are usually deterred by steric hindrance, coulombic repulsion, delocalization, or spiroconjugation in the radical species 46–48.…”