Anion water radial distribution

Figure 14. Calculated anion-water radial distribution function vs. center of mass separation R for the dilute aqueous solution of fluoride at T = 25°C...

We have recently carried out Monte Carlo computer simulation of dilute aqueous solutions of the monatomic cations Li, Na and K and the monatomic anions F and Cl using the KPC-HF functions for the ion-water interaction and the MCY-CI potential for the water-water interaction. The temperature of the systems was taken to be 25° and the density chosen to be commensurate with the partial molar volumes as reported by Millero. - The calculated average quantities are based on from 600- 900K configurations after equilibration of the systems. The calculated ion-water radial distribution functions are given for the dilute aqueous solutions of Li", K" ", Na" ", F and Cl" in Figures 11-15, respectively. 

For a solution, the radial distribution function will typically have a structure as shown in Figure 14.6 for a simulation of a benzene radical anion in water. ... 

In contrast to the behavior of noble gases, the radial distribution functions of water around the infinitely dilute anion (C/ ) and cations (Na and Lt) exhibit a rather strong re-structuring, Le,y relative to unperturbed water there is a substantial increase in the local water density around the ions due to strong ion-dipole interactions (Figure 13). The presence of an ion induces an increase in the solvent... 

Figure 13. Radial distribution functions for the water-cation and water-anion at infinite dilution at 7 = 1.0 and = 1.5.

Using the hybrid ADMP/ONIOM technique, Rega et al. [104] have published the result of a molecular dynamics simulation at the B3LYP/6-31+G(d,p) level and with AMBER/TIP3P water model of a chloride anion embedded in a cluster of 256 water molecules. The time step was 0.25 fs and they have performed a 3 ps simulation after thermalization, allowing them to report the atom-atom radial distribution functions. 

Fig. 2.14 Cl-O radial distribution function g(r) for a chlorine anion solvated in water in a QM/MM (BLYP/fTIP3P) simulation. The Cl anion is the active center. Each panel shows reference full QM dotted) and MM (dashed) results, as compared with an adaptive QM/MM simulation (solid). The top panel shows results of a Abmpt simulation, while the bottom panel shows the results of a BE simulation. The unshaded parts of the plots represent the QM regions, with the gradient in shading corresponding to the hysteresis range, a refinement used by the authors to reduce the fluctuations in the set of molecules that constimte the active and transition region. Reproduced from Ref. [32] with permission...

Analysis of the radial pair distribution function for the electron centroid and solvent center-of-mass computed at different densities reveals some very interesting features. At high densities, the essentially localized electron is surrounded by the solvent resembling the solvation of a classical anion such as Cr or Br. At low densities, however, the electron is sufficiently extended (delocalized) such that its wavefunction tunnels through several neighboring water or ammonia molecules (Figure 16-9). 

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