Formation of long, multicenter π-[TCNE]22- dimers in solution: solvation and stability assessed through molecular dynamics simulations
Formation of long, multicenter π-[TCNE]22- dimers in solution: solvation and stability assessed through molecular dynamics simulations
M. Capdevila-Cortada, J. Ribas-Ariño, A. Chaumont, G. Wipff, J. J. Novoa.
Chem. Eur. J., 22 (2016) 17037.
Organic radical-ions dimerize in solution at low temperature forming long, multicenter bonds, despite the metastability of the isolated dimers. In this article, we presented 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]22- dimers in dichloromethane was evaluated. While 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 ca. 3.0 Å. The π-[TCNE]22- dimers form dissociated ion pairs with the NBu4+ counterions, and their first solvation shell comprises ca. 20 CH2Cl2 molecules. Among them, the 8 molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer via 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.