Activation volume base species

Jenner [275] has presented a thorough description of several possible contributions to both the intrinsic and the environmental parts of the activation volumes, based on accurate experimental observation of pressure effect on reactions in solutions. The intrinsic contribution to the activation volume essentially derives from the differences in structure between the transition state and the reacting species, so it is directly related to the partial cleavage and formation of chemical bonds in the transition state. In cases where the environmental contribution is negligible, the activation volume variation gives a direct insight in the molecular mechanism [275, 280]. In this case in fact, considering... 

In two earlier studies (106, 107), the oxidation of two Schiff base complexes were studied at room temperature, but in these cases only activation parameters for the overall process could be obtained since it was not possible to detect the formation of an intermediate species which could be attributed to a peroxo species. Nevertheless, the kinetic measurements provided indirect evidence for the existence of this intermediate. In both studies negative values for the activation entropies and the activation volumes were obtained. The oxidation of [Cu2(H-BPB-H)(CH3CN)2](PF6)2 (H-BPB-H = l,3-bis[iV-(2-pyridylethyl)-formidoyl]benzene) is accompanied by an activation entropy of -53 11 J K-1 mol-1 and an activation volume of -9.5 0.5 cm3 mol-1. In... 

Tab. 4.2. Rate constants and activation volumes for water exchange on trivaient hexa-aqua transition metal ions and their conjugate base species. 

One of the first systems investigated concerned the redox reaction between Co(terpy)2 and Co(bpy)3 in different solvents, where terpy = 2,2 6, 2"-terpyridine, and bpy = 2,2 -bipyridine (20). Because of the similar charge on these species, K for ion-pair formation in terms of an outer-sphere mechanism is very small, and the observed AV is a composite quantity. The reported activation volumes for the investigated solvents are -9.4 (H2O), -13.8 (HCONH2), and -5.1 (CH3CN) cm mol . Theoretical calculations based on the Marcus-Hush relationships resulted in a A value of -7.3 cm mol" for the reaction in water, which is indeed close to the experimental value (20). A series of reactions were studied where it was possible to resolve K and /cet, that is, A V(K) and A V (fcEx)- In this case, oppositely charged reaction partners were selected, as indicated in the following scheme (21-23) ... 

An associative condensation mechanism involving a penta- or hexa-coordinated intermediate is also consistent with the enhanced condensation kinetics observed at high pressures by Artaki et a/. [81]. (SeeFig. 19.) In order to explain these effects, Artaki et al. analyzed the activation volumes associated with a base-catalyzed condensation mechanism involving a hexa-coordinated intermediate (Eq. 39). They concluded that due to rearrangements of solvent molecules around the anionic nucleophile, SiO , and the smaller volume of the transition state compared to the volume of the reactant. species, both dissociation of the silanol species and the formation of the transition state contribute to a reduction in the activation volume. Thus, both reactions should be accelerated by pressure. This same reasoning is applicable to mechanisms involving pentacoordinate intermediates. 

It should be kept in mind, that these rate constants are defined based on the volume concentrations of the reacting species. Another standard rate constant hP can be defined with regard to the rate of the reaction at the standard electrode potential of the electrode reaction. This rate constant refers consequently to standard activities instead of concentrations. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

Centi, G. and Perathoner, S. Nature of active species in copper-based catalysts and their chemistry of transformation of nitrogen oxides. Appl Catal, A General, 1995, Volume 132, Issue 2, 179-259. 

Reactions within organic chemistry were studied as a function of hydrostatic pressure historically and attempts to classify reaction types according to the ionic nature or not of the reacting species and thus to the sign and magnitude of the volume of activation were made.54 Later investigators have tended to treat each system studied based upon individual characteristics and properties for interpretation purposes. Parallel efforts were made to examine whether satisfactory correlations could be made between the volume of activation and the entropy of activation.55... 

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