Formation of long, multicenter pi-[TCNE](2)(2-) dimers in solution: solvation and stability assessed through molecular dynamics simulations

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 pi-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 pi-[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 inter-planar separation of approximately 3.0 angstrom. The pi-[TCNE](2)(2-) dimers form dissociated ion pairs with the NBu4+ counterions, and their first solvation shell comprises approximately 20 CH2Cl2 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 center dot center dot center dot N contacts. The calculated free energy of dimerization of TCNE center dot- 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

Structure of a uranyl peroxo complex in aqueous solution from first-principles molecular dynamics simulations

N.S. thanks the UJF and the CNRS for financial support and the CECIC (project: liqsim) for computer resources. NWChem calculations made use of the facilities of HECToR, the U.K. national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc., and NAG Ltd and funded by the Office of Science and Technology through EPSRC's High End Computing Programme.Static ab initio and density-functional computations, as well as Car–Parrinello molecular dynamics simulations in aqueous solution are reported for [UO2(OH)(κ2-O2)(H2O)2]− and [UO2(OH)2(κ1-O2H)(H2O)]−. Whereas the κ1-hydroperoxo isomer is found to be more stable than the κ2-peroxo form in the gas phase, the order of stability is reversed in explicit bulk solution. Based on free energies from thermodynamic integration (BLYP functional), the peroxo form is favoured by ca. 32 kJ mol−1 in water. This stabilisation is discussed in terms of the hydration shells about the individual ligands and dipole moments of the complexes in water, and highlights the importance of explicit solute–solvent interactions and bulk solvation for the speciation of uranyl(VI) compounds.PostprintPeer reviewe

Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

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