Solvational change, pressure

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... 

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. ... 

The volumes of activation were AV (k ) = — 6.2 cmVmol and AV (kr) = —11.4 cm3/mol and are very close in value to those obtained for oxidation of promazine, and clearly show that a combination of intrinsic and solvation changes are very closely matched for the two similar reactions. An identical value to the reaction volume determined kinetically, +5.2 cm3/mol, was found for the reaction volume determined from a pressure dependence study of the system at equilibrium. The volume profile is shown in Figure 7.11. 

For the acid-independent and acid-dependent aquation pathways of [Co(NH3)sS04], the respective volumes of activation A Vo (at zero pressure) and AVh (pressure averaged over 100 MPa) are -18.3 and -3.5cm mor at 35°C and -19.7 and -3.9cm mor at 55° and / = 1.0 The temperature dependence of A Vo can be accounted for in terms of solvational change indicated by its pressure dependence. The work also questions the common supposition that the molar volumes V of the penta- and hexacoordinate ammine complexes of the same metal ion are equal and suggests that there is a difference of 17-20 cm moV for the cobalt(III) case. 

There is one important caveat to consider before one starts to interpret activation volumes in temis of changes of structure and solvation during the reaction the pressure dependence of the rate coefficient may also be caused by transport or dynamic effects, as solvent viscosity, diffiision coefficients and relaxation times may also change with pressure [2]. Examples will be given in subsequent sections. 

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... 

Now, we should ask ourselves about the properties of water in this continuum of behavior mapped with temperature and pressure coordinates. First, let us look at temperature influence. The viscosity of the liquid water and its dielectric constant both drop when the temperature is raised (19). The balance between hydrogen bonding and other interactions changes. The diffusion rates increase with temperature. These dependencies on temperature provide uS with an opportunity to tune the solvation properties of the liquid and change the relative solubilities of dissolved solutes without invoking a chemical composition change on the water. 

Kujawa and Winnik [209] reported recently that other volumetric properties of dilute PNIPAM solutions can be derived easily from pressure perturbation calorimetry (PPC), a technique that measures the heat absorbed or released by a solution owing to a sudden pressure change at constant temperature. This heat can be used to calculate the coefficient of thermal expansion of the solute and its temperature dependence. These data can be exploited to obtain the changes in the volume of the solvation layer around a polymer chain before and after a phase transition [210], as discussed in more detail in the case of PVCL in Sect. 3.2.2. 

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