Solvation values

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

An important lesson from this is that the idea of nucleophilicity in the real world of organic reactions is not easy to pigeonhole. Polarizability is important, but basicity is also very important and can be influenced by solvation. Values of the pKa of a given compound vary as a function of solvent, and so does basicity. You can make a species, anions in particular, more reactive by putting them in solvents that don t solvate them very well. Dipolar aprotic solvents interact nicely with cations, but not so well with anions. Polar protic solvents (e. g., water, alcohols) can hydrogen bond to anions, diminishing their basicity and literally blocking them sterically. 

The entropy of solvation values reflect solvational structure near an ion. The following discussion of models that are more in agreement with the experimental values of solvational entropies follows the seminal treatment due to Bockris and Saluja in 1982. Models for the region near an ion are shown in Fig. 2.37. The entropy of hydration from the model with no SB region was 170 to 250 J mol" lower (more negative) than the experimental values, and was therefore not pursued further. 

The change in free energy of solvation calculation for the reaction is the largest source of error in pKa calculations. To determine the most accurate method we must look both at the type of solvation model used, implicit, explicit, or cluster continuum method (likewise described as implicit-explicit), and the specific level of theory. As previously mentioned, ionic species, in particular, are extremely difficult to calculate because of their strong electrostatic effects and large free energy of solvation values [8,14,23,25]. 

The addition of components to this set of 92, the change of a few parameter values for existing components, or the inclusion of additional UNIQUAC binary interaction parameters, as they may become available, is best accomplished by adding or changing cards in the input deck containing the parameters. The formats of these cards are discussed in the subroutine PARIN description. Where many parameters, especially the binary association and solvation parameters are to be changed for an existing... 

In principle, simulation teclmiques can be used, and Monte Carlo simulations of the primitive model of electrolyte solutions have appeared since the 1960s. Results for the osmotic coefficients are given for comparison in table A2.4.4 together with results from the MSA, PY and HNC approaches. The primitive model is clearly deficient for values of r. close to the closest distance of approach of the ions. Many years ago, Gurney [H] noted that when two ions are close enough together for their solvation sheaths to overlap, some solvent molecules become freed from ionic attraction and are effectively returned to the bulk [12]. 

This fomuila does not include the charge-dipole interaction between reactants A and B. The correlation between measured rate constants in different solvents and their dielectric parameters in general is of a similar quality as illustrated for neutral reactants. This is not, however, due to the approximate nature of the Bom model itself which, in spite of its simplicity, leads to remarkably accurate values of ion solvation energies, if the ionic radii can be reliably estimated [15],... 

The analysis of recent measurements of the density dependence of has shown, however, that considering only the variation of solvent structure in the vicinity of the atom pair as a fiinction of density is entirely sufficient to understand tire observed changes in with pressure and also with size of the solvent molecules [38]. Assuming that iodine atoms colliding with a solvent molecule of the first solvation shell under an angle a less than (the value of is solvent dependent and has to be found by simulations) are reflected back onto each other in the solvent cage, is given by... 

Solvation increases solubility above predicted values. When the components of a solution possess an abnormally large attraction for each other, solvates are formed. Thus certain oxygen-containing compounds have a great tendency to form hydrates, thus contributing to increased water solubility hydrogen bondir also plays an important role. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

Since rather than

[Pg.595]

The second term allows for solvation, which effectively increases the volume fraction of the particles to a larger value than that calculated on the basis of dry solute. Equation (9.18) shows how this can be quantified. 

It is a frustrating aspect of Eq. (9.20) that the observed intrinsic viscosities contain the effects of ellipticity and solvation such that the two cannot be resolved by viscosity experiments alone. That is, for any value of [77], there is a whole array of solvation-ellipticity values which are consistent with the observed intrinsic viscosity. 

This state of affairs is summarized in Fig. 9.4a, which plots contours for different values of [77] in terms of compatible combinations of mj /nij and a/b. For the aqueous serum albumin described in Example 9.1 as an illustration, any solvation-ellipticity combination which corresponds roughly to [77] = 5 is possible for this system. Data from some other source are needed to pin down a more specific characterization. 

The dependence of f/fo on solvation and eilipticity has been worked out in detail. Since the situation parallels the way [77] exceeds its value for nonsolvated spheres, we shall not elaborate on the details, but merely summarize the conclusions ... 

The quantitative analysis of this problem results in a set of contours in terms of the axial ratio a/b and the solvation m /mj for constant values... 

Figure 9.4b shows a theoretical f/fQ contour for a value of this ratio equal to 1.45. As noted in the discussion of this figure in Sec. 9.3, the intersection of the f/fQ and [77] contours permits the state of solvation and ellipticity of such a protein molecule to be characterized uniquely. 

Note that this method of standardizing D values makes no allowance for the possibility that a molecule may change size, shape, or solvation with changes in temperature. In the next section we shall survey the behavior of polymeric materials in an ultracentrifuge. We shall see that diffusion coefficients can be... 

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