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Quasichemical and structural analysis of polarizable anion hydration
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Image of FIG. 1.
FIG. 1.

Schematic representation of the quasichemical approach to solvation.

Image of FIG. 2.
FIG. 2.

Free energy cost for creating an oxygen unoccupied cavity of size , , in NPT simulations of AMOEBA or SPC/E water models.

Image of FIG. 3.
FIG. 3.

Ion-water(hydrogen) and ion-water(oxygen) radial distribution functions for the , , , and ions modeled with the AMOEBA force field.

Image of FIG. 4.
FIG. 4.

Effect of polarizability on the energy of collapsing the inner-shell partition. For ions of all sizes (first index: , , ), the curve shifts downward with increasing polarizability (second index: , , , ) indicating that increased polarization causes water to pack more closely.

Image of FIG. 5.
FIG. 5.

Contributions to the OS-LR conditional free energy average at . As explained in the text, the components are averages over the coupled and uncoupled cases. The corresponding contributions from a nonpolarizable system are shown as single points. In the upper left-hand figure, the upper curves for each ion are the total hydration free energies obtained by adding the inner-shell and outer-shell packing contributions.

Image of FIG. 6.
FIG. 6.

Electrostatic potential at the center of a test solute in NPT simulations of AMOEBA or SPC/E water models: (lower section) hard spheres of size , (upper section) and ions (left and right of plot, respectively).

Image of FIG. 7.
FIG. 7.

Effect of size and polarizability on the total electrostatic contribution to the OS-LR free energy component. Decreases in size and, to a lesser extent, increases in polarizability lead to more favorable interactions. Interestingly, variations in the ion sizes produce about a 5 kcal/mol separation when compared at equal nominal radii, contradicting the Born model prediction. Labels are as in Fig. 4.

Image of FIG. 8.
FIG. 8.

Effect of size and polarizability on the solvation shell organization. Plotting the projection of the average center of mass of the closest n solvent waters onto a vector in the direction of the first three shows how quickly subsequent layers of solvent relax to an isotropic distribution. Labels are by polarizability parameter with the exception of the lower right panel, which shows uncharged, nonpolarizable solutes with the same vdW parameters as the corresponding ions.


Generic image for table
Table I.

Summary of the systems modeled in the present study. The labels are defined in the text.

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Table II.

Partial molar hydration quantities for whole salts at infinite dilution, in kcal/mol and . Numbers in parentheses are one standard deviation errors.

Generic image for table
Table III.

Comparison of local solvation environment indicators: , the number of waters required for Fig. 8 to reach zero, average distance to the closest water oxygen (Å), average ion dipole moment (Debye), average first-shell water dipole moment (Debye), and single-ion total hydration free energy (kcal/mol). Numbers in parentheses indicate numerical uncertainty in the last digit. The label indicates an uncharged solute.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Quasichemical and structural analysis of polarizable anion hydration