Formation of Long, Multicenter π‐[TCNE]₂²⁻ Dimers in Solution: Solvation and Stability Assessed through Molecular Dynamics Simulations

Abstract

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of π-[TCNE] (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π-[TCNE] dimers form dissociated ion pairs with the NBu counterions, and their first solvation shell comprises approximately 20 CH Cl molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C-H⋅⋅⋅N contacts. The calculated free energy of dimerization of TCNE in solution at 175 K is -5.5 kcal mol . These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.

Reversible Solution π‐Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical

“…[33] Note that Capdevila-Cortadaa nd co-workersr eported the importance of explicit modeling of solvents in tetracyanoethylene (TCNE) radicald imers by adopting molecular dynamics (MD) simulations. [36] The optimized structure of the solution dimer is shown in Figure 4( top) and is similart ot he solid- state dimer structure, supporting the formation of a p-dimer in solution.T he binding energy calculated at the level of M05-2X/ 6-31G(d,p) with solvation effects using PCM and counterpoise correction is 15.2 kcal mol À1 ,w hich is slightly overestimated from the experimentally determined DH value. In the calculated solution structure, the pair are slightly separated compared with the geometry in the crystal;t he calculated intermolecular distance C4ÀO1 is 3.012 andt he calculatedd istance between the centralr ing centroids is 3.427 .W ea lso performed the geometry optimizations of s-dimers of 1.However,positive interaction energies were calculated at the obtained geometries and therefore the formation of s-dimers is an unfeasible process (see Supporting Information for details, FigureS9a nd Ta bles S10-S13).…”

“…Also, it has been reported that M05‐2X with D3 dispersion correction is over‐parameterized, not affording results as good as in a phenalenyl radical dimer . Note that Capdevila‐Cortada and co‐workers reported the importance of explicit modeling of solvents in tetracyanoethylene (TCNE) radical dimers by adopting molecular dynamics (MD) simulations . The optimized structure of the solution dimer is shown in Figure (top) and is similar to the solid‐state dimer structure, supporting the formation of a π‐dimer in solution.…”

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions

“…Despite the positive charges in both units, solvent molecules can be mediators of the p-dimerization, as extensively reported experimentally 13,14 and rationalized computationally. 44,45 In this class of compounds, screening and dispersion effects act hand-in-hand to generate unusual regimes.…”

Cation Dependence of the Dimerization Enthalpy for A2[tetracyanoethylene]2 (A=NMe4, Mepy, NEt4) Possessing a Long, Multicenter Bond

“…This is attributed to an overall decrease in the computationally determined A + ⋅⋅⋅[TCNE] − attractive interactions with increasing cation size as previously reported . This is in contrast with predictions based off of simulations of [N( n Bu) 4 ] 2 [TCNE] 2 in CH 2 Cl 2 ; however, the solvent clearly plays a role in these transformations; thus, the interactions in MeTHF may differ from CH 2 Cl 2 …”