Polarization and charge transfer in the hydration of chloride ions

104

The absolute intrinsic hydration free energy G • H + ,wat of the proton, the surface electric potential jump χ • wat upon entering bulk water, and the absolute redox potential V • H + ,wat of the reference hydrogen electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na + ) and potassium (K + ) ions. The calculations rely on thermodynamic integration based on quantum-mechanical molecular-mechanical (QM/MM) molecular dynamics (MD) simulations involving the ion and 2000 water molecules. The ion and its first hydration shell are described using a correlated ab initio method, namely resolution-of-identity second-order Møller-Plesset perturbation (RIMP2). The next hydration shells are described using the extended simple point charge water model (SPC/E). The hydration free energy is first calculated at the MM level and subsequently increased by a quantization term accounting for the transformation to a QM/MM description. It is also corrected for finite-size, approximate-electrostatics, and potentialsummation errors, as well as standard-state definition. These computationally intensive simulations provide accurate first-principle estimates for G • H + ,wat , χ • wat , and V • H + ,wat , reported with statistical errors based on a confidence interval of 99%. The values obtained from the independent Na + and K + simulations are in excellent agreement. In particular, the difference between the two hydration free energies, which is not an elusive quantity, is 73.9 ± 5.4 kJ mol 1 (K + minus Na + ), to be compared with the experimental value of 71.7 ± 2.8 kJ mol 1 . The calculated values of G • H + ,wat , χ • wat , and V • H + ,wat ( 1096.7 ± 6.1 kJ mol 1 , 0.10 ± 0.10 V, and 4.32 ± 0.06 V, respectively, averaging over the two ions) are also in remarkable agreement with the values recommended by Reif and Hünenberger based on a thorough analysis of the experimental literature ( 1100 ± 5 kJ mol 1 , 0.13 ± 0.10 V, and 4.28 ± 0.13 V, respectively). The QM/MM MD simulations are also shown to provide an accurate description of the hydration structure, dynamics, and energetics. Published by AIP Publishing.

Section: B Hydration Structure and Dynamicsmentioning

confidence: 63%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Absolute proton hydration free energy, surface potential of water, and redox potential of the hydrogen electrode from first principles: QM/MM MD free-energy simulations of sodium and potassium hydration

Hofer

Hünenberger

2018

104

“…21,22 This results in ions that have charges of around ±0.7 e to ±0.9 e. 14,21,22 Treating the charges as variational parameters leads to charges of ±0.80 e. 18 These values are close to those that result from charge transfer to the solvent. [23][24][25][26][27] However, in the charge transfer models, unlike in ECC, all ions are not necessarily reduced in charge by the same amount, the charge of an ion fluctuates in time, and the water acquires transient charges. That these methods tend to agree on the charge values and the ion effects on water diffusion leads to questions about physical origins of the reduce charges.…”

Section: Introductionmentioning

confidence: 99%

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

Nguyen

Rick

2018

A relationship between the effect of uni-univalent electrolytes on the structure of water and on its volatility The Journal of Chemical Physics 148, 222807 (2018) The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions The diffusion rates for water molecules in salt solutions depend on the identity of the ions, as well as their concentration. Among the alkali metal ions, cesium and potassium increase and sodium strongly decreases the diffusion constant of water. The origin of the difference can be understood by examining the simulation results using different potential models. In this work, aqueous solutions of salts are simulated with a variety of models. Commonly used non-polarizable models, which otherwise reproduce many experimental properties, do not capture the trend in the diffusion constant, while models which include polarization and/or charge transfer interactions do. For the non-polarizable models, the diffusion constant decreases too strongly with salt concentration. The changes in the water diffusion constant with increasing salt concentration match the diffusion constant of the ion. The ion diffusion constant is dependent on the residence time for water in the ion solvation shell. The non-polarizable models over-estimate the residence time, relative to the translational diffusion constant and so tend to under-estimate the ion and water diffusion constants. Published by AIP Publishing.

“…Recently, there has been considerable interest in developing empirical potentials to describe such an extreme polarization. [18][19][20] However, developing an empirical potential to describe such an extreme polarization is still very difficult. 13,18 In the simulations reported here, the forces between atoms are calculated directly from the electronic Schrödinger equation ͑first principles or AIMD simulations͒.…”

Section: Introductionmentioning

confidence: 99%

Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

Cauët

Bogatko

Weare

et al. 2010

Results of ab initio molecular dynamics ͑AIMD͒ simulations ͑density functional theory+ PBE96͒ of the dynamics of waters in the hydration shells surrounding the Zn 2+ ion ͑T Ϸ 300 K, Ϸ 1 gm/ cm 3 ͒ are compared to simulations using a combined quantum and classical molecular dynamics ͓AIMD/molecular mechanical ͑MM͔͒ approach. Both classes of simulations were performed with 64 solvating water molecules ͑ϳ15 ps͒ and used the same methods in the electronic structure calculation ͑plane-wave basis set, time steps, effective mass, etc.͒. In the AIMD/MM calculation, only six waters of hydration were included in the quantum mechanical ͑QM͒ region. The remaining 58 waters were treated with a published flexible water-water interaction potential. No reparametrization of the water-water potential was attempted. Additional AIMD/MM simulations were performed with 256 water molecules. The hydration structures predicted from the AIMD and AIMD/MM simulations are found to agree in detail with each other and with the structural results from x-ray data despite the very limited QM region in the AIMD/MM simulation. To further evaluate the agreement of these parameter-free simulations, predicted extended x-ray absorption fine structure ͑EXAFS͒ spectra were compared directly to the recently obtained EXAFS data and they agree in remarkable detail with the experimental observations. The first hydration shell contains six water molecules in a highly symmetric octahedral structure is ͑maximally located at 2.13-2.15 Å versus 2.072 Å EXAFS experiment͒. The widths of the peak of the simulated EXAFS spectra agree well with the data ͑8.4 Å 2 versus 8.9 Å 2 in experiment͒. Analysis of the H-bond structure of the hydration region shows that the second hydration shell is trigonally bound to the first shell water with a high degree of agreement between the AIMD and AIMD/MM calculations. Beyond the second shell, the bonding pattern returns to the tetrahedral structure of bulk water. The AIMD/MM results emphasize the importance of a quantum description of the first hydration shell to correctly describe the hydration region. In these calculations the full d 10 electronic structure of the valence shell of the Zn 2+ ion is retained. The simulations show substantial and complex charge relocation on both the Zn 2+ ion and the first hydration shell. The dipole moment of the waters in the first hydration shell is 3.4 D ͑3.3 D AIMD/MM͒ versus 2.73 D bulk. Little polarization is found for the waters in the second hydration shell ͑2.8 D͒. No exchanges were seen between the first and the second hydrations shells; however, many water transfers between the second hydration shell and the bulk were observed. For 64 waters, the AIMD and AIMD/MM simulations give nearly identical results for exchange dynamics. However, in the larger particle simulations ͑256 waters͒ there is a significant reduction in the second shell to bulk exchanges.