Motivated by development of lithium-ion batteries, we study the structure and dynamics of LiBF(4) in pure and mixed solvents with various salt concentrations. For this purpose, we have developed force field models for ethylene carbonate, propylene carbonate, dimethyl carbonate, and dimethoxyethane. We find that Li(+) is preferentially solvated by the cyclic and more polar component of the mixtures, as the electrostatic interaction overcomes possible steric hindrances. The cation coordination number decreases from 6 to 5 with increasing salt concentration due to formation of ion-pairs. The uniform decline of the diffusion coefficients of the two ions is disrupted at mixture compositions that perturb the ion-pair interaction. We show that the Stokes' model of diffusion can be applied to the very small Li(+) ion, provided that the size of the first solvation shell is properly taken into consideration. The strong coordination of the ions by the polar, cyclic components of the solvent mixtures established in our simulations suggests that the less polar linear component can be optimized in order to reduce electrolyte viscosity and to achieve high electrical conductivity.

No abstract

Molecular dynamics (MD) simulations of dimethyl sulfoxide (DMSO) solutions of Li + , Me 4 N + , BPh 4 À , big spherical ions of the same size but different charge, such as S + , S À , S 0 have been performed at 298 K in NVT ensembles by using a four-interacting-sites model of DMSO and reaction field method for Coulombic interactions. Similar simulations were also performed on neat DMSO in which one DMSO molecule acted as a solute. The microscopic structures of ion-solvation shells have been analysed by employing a concept of co-ordination centres and characteristic vectors of the solvent molecule. Results are given for the atom-atom and ion-atom radial distribution functions (RDFs), orientation of the DMSO molecules and their geometrical arrangements in the first solvation shells of the ions. For the solvophilic Li + , a highly symmetric and wellpronounced first solvation shell (FSS) with fixed co-ordination number is observed. The co-ordination number and geometry of the FSS of lithium ion is strongly defined by the short-range non-Coulombic interactions between the ion and the surrounding DMSO molecules. The results show the importance of charge distribution in the solvent molecule and consequently the sign of ionic charge in creating local order around the solvated ion. It is found that the DMSO solvates S + better than S À , which is better solvated than S 0 . The ' solvophobic ' nature of the big multiatomic ions in non-aqueous media creates the possibility of the solvent molecules penetrating into the solute that is typically observed from our simulations not only for the charged species like Me 4 N + and BPh 4 À , but also for the neutral solute represented by the DMSO molecule in neat DMSO.

Molecular dynamics simulations of complexes of Mg(2+), Ca(2+), Sr(2+), and Ba(2+) with 3-hydroxyflavone (flavonol, 3HF) and ClO₄⁻ in acetonitrile were performed. The united atoms force field model was proposed for the 3HF molecule using the results of DFT quantum chemical calculations. 3HF was interpreted as a rigid molecule with two internal degrees of freedom, i.e., rotation of the phenyl ring and of the OH group with respect to the chromone moiety. The interatomic radial distribution functions showed that interaction of the cations with flavonol occurs via the carbonyl group of 3HF and it is accompanied with substitution of one of the acetonitrile molecules in the cations' first solvation shells. Formation of the cation-3HF complexes does not have significant impact on the rotation of the phenyl ring with respect to the chromone moiety. However, the orientation of the flavonol's OH-group is more sensitive to the interaction with doubly charged cations. When complex with Mg(2+) is formed, the OH-group turns out of the plane of the chromone moiety that leads to rupture of intramolecular H-bond in the ligand molecule. Complexation of Ca(2+), Sr(2+), and BaClO₄⁺ with 3HF produces two structures with different OH-positions, as in the free flavonol with the intramolecular H-bond and as in the complex with Mg(2+) with disrupted H-bonding. It was shown that additional stabilization of the [MgClO4(3HF)](+) and [BaClO4(3HF)](+) complexes is determined by strong affinity of perchlorate anion to interact with flavonol via intracomplex hydrogen bond between an oxygen atom of the anion and the hydrogen atom of the 3-hydroxyl group. Noticeable difference in the values of the self-diffusion coefficients for Kt(2+) from one side and ClO₄⁻, 3HF, and AN in the cations' coordination shell from another side implies quite weak interaction between cation, anion, and ligands in the investigated complexes.

No abstract