First solvation sphere

Methods for evaluating the effect of a solvent may broadly be divided into two types those describing the individual solvent molecules, as discussed in Section 16.1, and those which treat the solvent as a continuous medium. Combinations are also possible, for example by explicitly considering the first solvation sphere and treating the rest by a continuum model. Each of these may be subdivided according to whether they use a classical or quantum mechanical description. 

For solvent models where the cavity/dispersion interaction is parameterized by fitting to experimental solvation energies, the use of a few explicit solvent molecules for the first solvation sphere is not recommended, as the parameterization represents a best fit to experimental data without any explicit solvent present. 

The mixed solvent models, where the first solvation sphere is accounted for by including a number of solvent molecules, implicitly include the solute-solvent cavity/ dispersion terms, although the corresponding tenns between the solvent molecules and the continuum are usually neglected. Once discrete solvent molecules are included, however, the problem of configuration sampling arises. Nevertheless, in many cases the first solvation shell is by far the most important, and mixed models may yield substantially better results than pure continuum models, at the price of an increase in computational cost. 

The complexes with n=5 and n=6 exhibit marked differences from the corresponding K+ systems with the fifth and sixth molecules located in the second coordination shell forming bifurcated H-bonds with the first solvation sphere (as shown in Figs. 7 and 8). 

Model h). This model implies a distinct first solvation sphere, built up of Ttgoiv solvent molecules, but random orientation of their dipoles. For this situation the t/-factor is given by ... 

Model (c). The assumption is made in this model that the solvent dipoles in the first solvation sphere are radially oriented ... 

In the case of some ion-transfer reactions the chemical desolvation step controls the rate of the overall process and the currents observed are lower than those expected for the process limited solely by the mass transport rate. The formation of such less-hydrated species was attributed [210] in the case of the electroreduction of nick-el(II) in water to a slow exchange of water molecules from the first solvation sphere of Ni(II) under the influence of the crystal field stabilization. A similar mechanism was found for Ni(II) and Co(II) in methanol [211]. 

For more labile ions which exchange solvent molecules fast from the first solvation sphere, spectrometric methods (such as NMR) which monitor the change in the immediate neighborhood of the ion are especially useful for the study of ion solvation in mixed solvents. 

The authors [224] explained these kinetic changes qualitatively by considering the energy of activation at different mole fractions of the organic component in the mixture. The minimum on the rate constant (exchange current) - mixed solvent composition dependence occurs at the largest difference in composition between the surface layer and the first solvation sphere of zinc(II). 

For the charged atoms (Fig. 50), all of the distribution functions show features typical of charged-group solvation.112,113 There are four to five solvent molecules within the first solvation sphere, defined as extending to the first minimum ing(r) that is at - 3.5 A. This is indicative of tightly bound solvent around the charged group. 

Solubility data have been used to determine solvation enthalpies, U [defined as in equation (6)] for 0, H , and D in Li and for H in Na and K. The values of 17, are collected in Table 1. Those for H and D in Li are lower than those for 0 and N by factors of ca. 2 and 3, respectively, corresponding to increasing (7, with increasing charge of solute. Those for H" in Li, Na, and K are very similar, that in Li being the greatest. Solvation enthalpies have been derived in at initio M.O. calculations of solvation clusters in Li and Na. By comparison with experimental data, the best model was deduced to be that of a tetrahedral solvation sphere of cations supplemented by a further metal tetrahedron positioned on the three-fold axes of the first solvation sphere. Other incidental results to emerge from the calculations are the effective radii for Li (0.1675 nm), Na (0.1715 nm), and H (0.0525 nm in Li and... 

It has been suggested that the relaxation of solvent in the bulk environment (Tsa) is made up of two contributions one the normal pure-solvent relaxation (rg ) the other arising from the interaction of paramagnetic ions and the solvent molecules beyond the first solvation sphere (7 J... 

In addition these results indicate the existence of a first solvation sphere with a structure determined by the geometry of the complex, and which... 

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