Propensities in the solvation of M+–Benzene systems (M = Na, K, Rb) investigated by cluster dynamics

Abstract

The study of size-specific interactions of alkali ions (M+) with aromatic compounds is crucial to understand the mechanisms governing the selectivity in protein channels. In particular, the investigation of the aqueous solvent effect on M+–π systems is of fundamental importance. The related processes are typically governed by several intermolecular interaction contributions as hydrogen bonds, dispersion, induction and electrostatics, which are often weak and difficult to evaluate in detail. In the present paper, the behavior of the M+–Benzene (M=Na, K, Rb, Benzene=Bz) aggregates surrounded by water molecules is analyzed performing molecular dynamics (MD) simulations. As the accuracy of such simulations depends on the reliability of the used intermolecular potential energy formulation we adopt a potential model based on a combination of electrostatic and non electrostatic components, whose reliability has been previously tested on some prototype systems by comparing predictions of the model with both accurate ab initio calculations and/or high level experimental data, has been used. The non electrostatic component has been described as sum of improved Lennard Jones (ILJ) functions, whose parameters have been derived from polarizabilities of atoms, groups of atoms and/or molecules. The electrostatic contribution has been calculated as a sum of Coulombic potentials arising from the interaction between permanent ion charge and/or permanent molecular charge distributions, which, at long range, for M+–Bz, H2O–Bz and M+–H2O reproduces the ion-quadrupole, dipole-quadrupole and the ion-dipole interactions, respectively. Energetics and structure of the clusters are found to depend on the competition between ion and benzene solvations. MD results show that while the solvation of the ion originates a diminution of the M+–Bz interaction, that of the aromatic compound enhances its interaction with the ion. Moreover, it has been found that such behavior is size-specific dependent.