Formation of Long, Multicenter π-[TCNE]₂^2- Dimers in Solution: Solvation and Stability Assessed through Molecular...

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]₂^2- (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]₂^2- dimers form dissociated ion pairs with the NBu₄⁺ counterions, and their first solvation shell comprises approximately 20 CH₂ Cl₂ 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^-1 . 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.