Are the hydrophobic AsPh4+ and BPh4- ions equally solvated? A theoretical investigation in aqueous and nonaqueous solutions using different charge distributions

We present a molecular dynamics study of the solvation properties of the te trahedral AsPh4+ and BPh4- ions in water and chloroform solutions. Accordin g to the "extrathermodynamic" TATE (telraphenylarsonium tetraphenylborate) hypothesis, these nearly isosterical ions have identical free energies of s olvation in any solvent, as the latter are generally assumed to display lit tle dependence on the details of the charge repartition, provided that the total +/- charge is delocalized and that the ion's periphery is relatively inert. We compare eight different sets of charges obtained consistently for both ions and find that the anion is always better hydrated than the catio n, as evidenced by ion-solvent interaction energies and changes in free ene rgies of ion charging. This is explained by specific OH-Jr bridging interac tions in the anion and the positive electrostatic potential at the center o f the fictitious AsPh40 and BPh40 all-neutral species. With all models, the cation is also predicted to be more easily transferred from water to dry c hloroform. The conclusions obtained with standard solvent models (TIP3P wat er and OPLS chloroform) are validated by tests with the polarizable Wallqvi st and Berne water model and the Chang et al. chloroform model, and with co mputer simulations on a "wet chloroform" solution. The recently developed T IP5P water model yields, however, much closer hydration energies of AsPh4and BPh4-. The importance of "long-rangre" electrostatic interactions on th e charge discrimination by solvent is demonstrated by the comparison of sta ndard vs corrected methods to calculate the Coulombic interactions. These r esults are important in the context of the "TATB hypothesis" and for our un derstanding of solvation of large hydrophobic ions in pure liquids or in he terogeneous liquid environments.