Understanding the scale of the single ion free energy: A critical test of the tetra-phenyl arsonium and tetra-phenyl borate assumption

Abstract

Re-examining the tetraphenyl-arsonium/tetraphenyl-borate (TATB) hypothesis for single-ion solvation free energies The Journal of Chemical Physics 148, 222830 (2018) The tetra-phenyl arsonium and tetra-phenyl borate (TATB) assumption is a commonly used extrathermodynamic assumption that allows single ion free energies to be split into cationic and anionic contributions. The assumption is that the values for the TATB salt can be divided equally. This is justified by arguing that these large hydrophobic ions will cause a symmetric response in water. Experimental and classical simulation work has raised potential flaws with this assumption, indicating that hydrogen bonding with the phenyl ring may favor the solvation of the TB anion. Here, we perform ab initio molecular dynamics simulations of these ions in bulk water demonstrating that there are significant structural differences. We quantify our findings by reproducing the experimentally observed vibrational shift for the TB anion and confirm that this is associated with hydrogen bonding with the phenyl rings. Finally, we demonstrate that this results in a substantial energetic preference of the water to solvate the anion. Our results suggest that the validity of the TATB assumption, which is still widely used today, should be reconsidered experimentally in order to properly reference single ion solvation free energy, enthalpy, and entropy. Published by AIP Publishing. https://doi

Assessing long-range contributions to the charge asymmetry of ion adsorption at the air–water interface

“…This persistent bias weighs against the basis of the tetra-phenyl arsonium/tetra-phenyl borate ('TATB') extrathermodynamic assumption, an issue that has also been raised by others. 39,[61][62][63] The highly simplied description of molecular interactions in SPC models is certainly a crude approximation to real microscopic forces. But the specic ion effects it exhibits cannot be ascribed simply to an errant surface potential.…”

Hydration Mimicry by Membrane Ion Channels

“…5. do not involve the potential of the phase (109). The examination of the TATB hypothesis (53,125,127,128,129,130), and Marcus's modeling of his tabulated values (127,131) with the QCT single-ion free energies that use the PCM here for the cluster free energy of Eq. 6.…”

Ambiguities in solvation free energies from cluster-continuum quasichemical theory: lithium cation in protic and aprotic solvents

“…Marcus employed the tetraphenyl arsonium tetraphenyl borate (TATB) assumption [8][9][10] that assigns the same solvation energies to large spherical ions TA + and TBto derive both the ∆G tr (W → S) and the absolute proton hydration free energy of -254.3 kcal/mol (TATB scale). As the validity of the TATB assumption is discussed, [11][12][13][14][15] the resulting estimates of Marcus might be insufficiently accurate.…”