title: 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]22-(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]22-dimers form dissociated ion pairs with the NBu4+counterions, and their first solvation shell comprises approximately 20 CH2Cl2molecules. 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.
authors.affiliation: Departament de Ciència de Materials i Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.|Laboratoire MSM, UMR CNRS 7140, Institut de Chimie, 4 rue B. Pascal, 67000, Strasbourg, France.|Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007, Tarragona, Spain.
authors.forename: Marçal
authors.initials: M
authors.lastname: Capdevila-Cortada
authors.affiliation: Departament de Ciència de Materials i Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.
authors.forename: Jordi
authors.initials: J
authors.lastname: Ribas-Arino
authors.affiliation: Laboratoire MSM, UMR CNRS 7140, Institut de Chimie, 4 rue B. Pascal, 67000, Strasbourg, France.
authors.forename: Alain
authors.initials: A
authors.lastname: Chaumont
authors.affiliation: Laboratoire MSM, UMR CNRS 7140, Institut de Chimie, 4 rue B. Pascal, 67000, Strasbourg, France.
authors.forename: Georges
authors.initials: G
authors.lastname: Wipff
authors.affiliation: Departament de Ciència de Materials i Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Novoa
keywords: dimers; long multicenter bonds; molecular dynamics simulations; radical ions; tetracyanoethylene