Solvation changes

Fig. 4.11. Solvation changes during ionization of f-butyl chloride.

Reaction of [Pd(pica)(H20)2]2+ (pica = 2-picolylamine) with cbdc, cyclobutane-1,1-dicarboxylate, to give [Pd(pica)(cbdc)(H20)], containing monodentate cbdc, is characterized by an activation volume close to zero, indicating a balance between a negative contribution from associative activation and a positive contribution from solvational changes associated with transition state formation (256). 

An attempt is made to account quantitatively for the volumes of activation, AV, of ligand substitution processes. Causes of the pressure-dependence of AV include solvational change, for which a versatile analysis is developed. The pressure-independent AV values of solvent exchange reactions are good measures of the non-sol-vational components of AV for related net reactions. For water exchange, one can predict... 

Figure 1. Constancy of AV for solvent exchange reactions where AV = —6.5 cm6/mol for Cr(DMF)63+-DMF solvent exchange at 338 K (a) and pressure dependence of AV for reactions involving charge development and, hence, solvational change where AV0 = —18.5 err /mol for Co(NH3)sSO/ direct aquation at 298 K (b).

The key assumption to be made in interpreting the pressure data in terms of solvational change is that neither the ligands in the first coordination sphere of the complex nor the solvating solvent molecules are significantly compressible relative to bulk solvent (volume Vs), the compression of which is described by the modified Tait equation... 

Nuclear reorganization or the rehybridization of the carbon is a main factor in the retardation of proton transfer involving carbon acids, and solvation changes have much less impact (7). 

Poklar, N., N. Petrovcic, M. Oblak, and G. Vesnaver. 1999. Thermodynamic stability of ribonuclease A in alkylurea solutions and preferential solvation changes accompanying its thermal denaturation a calorimetric and spectroscopic study. Protein Sci 8 832-840. 

Sec. 4.2.1). The interpretation of reactions involving charges is more difficult, because of the importance of ion solvation changes. 464 types Qf reactions have been studied and will be encountered in Part II. Representative examples are shown in Table 2.4. 5-i69 jt jg worth reemphasizing the value of determining both AK and AF for a reaction system. The construction then of volume profiles enables a detailed description of the mechanism. The reaction steps for interaction of CO2 with Co(NH3)50H2 are considered to be... 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

The trends in the entropies are associated with the different solvational changes related to the reactions of species carrying equal or opposite charges. The reactions of the positively charged complexes... 

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