Pressure effects activation Volume profile

A unique aspect of the catalytic activity of CA is the fact that the hydroxo form of the enzyme catalyzes the hydration of CO2 through the direct binding of CO2 to the hydroxo ligand, whereas the aqua form of the enzyme catalyzes the dehydration of hydrogen carbonate through a ligand substitution process. This difference in mechanism is nicely demonstrated by the overall volume profile shown in Figure 23, which was constructed on the basis of the effect of pressure on the catalytic hydration and dehydration processes. Both these catalytic processes show characteristic pH dependencies that center around the pXa value of the coordinated water molecule. Many model Zn(II) and... 

Reactions of the type shown in Eq. (1.11), where L = isonicotinamide, 4-ethyl-pyridine, 3,5-lutidine, or pyridine, all exhibited volumes of activation for the forward reaction of between +3 and +7 cm mol compared to overall reaction volumes of between +19 and +26 cm mol . This indicates that electron transfer from Fe to Ru is characterized by an early transition state in terms of volume changes along the reaction coordinate (see Fig. 1.21). The overall volume changes could be accoimted for in terms of electrostriction effects centered around the ammine ligands on the ruthenium center. A number of possible explanations in terms of the effect of pressure on electronic and nuclear factors were offered to account for the asymmetrical nature of the volume profile 67]. 

Pressure is a fundamental physical property that affects various thermodynamic and kinetic parameters. Pressure dependence studies of a process reveal information about the volume profile of a process in much the same way as temperature dependence studies illuminate the energetics of the process (83). Since chemical transformations in SCF media require relatively high operating pressures, pressure effects on chemical equilibria and rates of reactions must be considered in evaluating SCF reaction processes (83-85). The most pronounced effect of pressure on reactions in the SCF region has been attributed to the thermodynamic pressure effect on the reaction rate constant (86), and control of this pressure dependency has been cited as one means of selecting between parallel reaction pathways (87). This pressure effect can be conveniently evaluated within the thermodynamic framework provided by transition state theory, which has often been applied to reactions in solutions (31,84,88-90). This theory assumes a true chemical equilibrium between the reactants and an activated transition... 

This form assumes that the effect of pressure on the molar volume of the solvent, which accelerates reactions of order > 1 by increasing the concentrations when they are expressed on the molar scale, has been allowed for. This effect is usually small, ignored but in the most precise work. Equation (7-41) shows that In k will vary linearly with pressure. We shall refer to this graph as the pressure profile. The value of A V is easily calculated from its slope. The values of A V may be nearly zero, positive, or negative. In the first case, the reaction rate shows little if any pressure dependence in the second and third, the applied hydrostatic pressure will cause k to decrease or increase, respectively. A positive value of the volume of activation means that the molar volume of the transition state is larger than the combined molar volume of the reactant(s), and vice versa. 

Another contribution to AjF was observed to arise in some reactions involving high steric hindrance (see references cited in Ref. 275) this is indicated as A V, the steric volume of activation. According to observation, high pressure promotes the most hindered process. This effect has been ascribed to a displacement of the most hindered transition states toward the more compact products in the reaction profiles [285]. 

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