Volume of activation complexes

AV activation volume (difference between partial molar volumes of activated complex and... 

The observation that in the activated complex the reaction centre has lost its hydrophobic character, can have important consequences. The retro Diels-Alder reaction, for instance, will also benefit from the breakdown of the hydrophobic hydration shell during the activation process. The initial state of this reaction has a nonpolar character. Due to the principle of microscopic reversibility, the activated complex of the retro Diels-Alder reaction is identical to that of the bimoleciilar Diels-Alder reaction which means this complex has a negligible nonpolar character near the reaction centre. O nsequently, also in the activation process of the retro Diels-Alder reaction a significant breakdown of hydrophobic hydration takes placed Note that for this process the volume of activation is small, which implies that the number of water molecules involved in hydration of the reacting system does not change significantly in the activation process. 

Differentiation between inner- and outer-sphere complexes may be possible on the basis of determination of activation volumes of dediazoniations catalyzed by various metal complexes, similar to the differentiation between heterolytic and homolytic dediazoniations in DMSO made by Kuokkanen, 1989 (see Sec. 8.7). If outer-sphere complexes are involved in a dediazoniation, larger (positive) volumes of activation are expected than those for the comparable reactions with inner-sphere complexes. Such investigations have not been made, however, so far as we are aware. 

Reductions of various Co(ni) complexes by Fe(II) have been studied under high pressures . The motivation for performing such experiments resides in the possibility that the volume of activation (AF ), like the entropy of activation, might be a criterion for distinguishing between inner- and outer-sphere reactions. For reactions of the type... 

The assessment of k is of some importance since it relates to the question as to how much if any of the free energy of activation barrier is due to the spin-forbidden character of the transition. From the experimental point of view, Eq. (49) shows that the transmission coefficient k and the activation entropy AS appear in the temperature-independent part of the rate constant and thus cannot be separated without additional assumptions. Possible approaches to the partition of — TAS have been discussed in Sect. 4 for spin transition complexes of iron(II) and iron(III). If the assumption is made that the entropy of activation is completely due to k, minimum values between 10 and 10 are obtained for iron(II) and values between 10 and 10 for iron(III). There is an increase of entropy for the transition LS -+ HS and thus the above assumption implies that the transition state resembles the HS state. On the other hand, volumes of activation indicate that the transition state should be about midway between the LS and HS state. This appears indeed more reasonable and has the... 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

The effect of external pressure on the rates of liquid phase reactions is normally quite small and, unless one goes to pressures of several hundred atmospheres, the effect is difficult to observe. In terms of the transition state approach to reactions in solution, the equilibrium existing between reactants and activated complexes may be analyzed in terms of Le Chatelier s principle or other theorems of moderation. The concentration of activated complex species (and hence the reaction rate) will be increased by an increase in hydrostatic pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules. The rate of reaction will be decreased by an increase in external pressure if the volume of the activated complex molecules is greater than the sum of the volumes of the reactant molecules. For a decrease in external pressure, the opposite would be true. In most cases the rates of liquid phase reactions are enhanced by increased pressure, but there are also many cases where the converse situation prevails. 

Negative volumes of activation for the forward (—23.6 cm3 mol-1) and reverse (—12.6 cm3 mol-1) reactions were observed for the formation of the initial complex between 29 and a-CD. The overall volume change for this complex formation was... 

The important influence of the LFAE is also nicely demonstrated on the labilities of V2+ andV3+.The smaller LFAE of t g [V(H20)6]3+ makes it about six times more labile than t2g [V(H20)6]2+ despite the smaller charge of the 2+ complex (Fig. 5). Both exchange processes are a-acti-vated with negative volumes of activation of —8.9 (V3+) (113) and —4.1 cm3 mol-1 (V2+) (68). 

A number of systems have been studied where a formal ligand displacement reaction is followed by an isomerization process. One recently described example involves the cleavage of a Pt-C(alkyl) cr-bond in complexes of the type cis-[Pt(R)(R )(PEt3)2] to produce cis-[Pt(R)(PEt3)2(MeOH)]+ and R H, which subsequently isomerizes to nms-[Pt(R)(PEt3)2(MeOH)]+ (90). The first protonolysis reaction is characterized by significantly negative volumes of activation between... 

The activation entropies determined for both these systems are large and positive, which is in general agreement with a d-activation (see Table V). Furthermore, the determination of the volume of activation of +10.6 cm3 moL1 for the anation of the [WO(OH2)(CN)4]2 complex by N3 ions (74) provides strong evidence in favor of a dissociative mode of activation for the substitution reactions in the aqua oxo complexes. 

For regular solutions, the influence of the solvent is determined by molar volumes and internal pressure terms. Since the molar volumes do not vary greatly, the internal pressure factor is more important. If the internal pressures of solvent, reactants and activated complex are similar, the solvent will have little effect on the rate of reaction as compared to a solvent in which reaction behaves ideally. If the internal pressure of the solvent is close to that of reactants but appreciably different from that of the activated complex, the rate of reaction in this solvent will be low. On the other hand, if solvent has an internal pressure similar to that of activated complex, but different from one or both the reactants, rate of reaction in this solvent will be high. Since the activated complex has properties which approach the properties of the product, it may be concluded, in general, that the reaction in which the products are of higher internal pressure than the reactants, it is accelerated by solvent of high internal pressure. 

In the literature there are only few studies on the water-exchange processes of the manganese(II) species in general (33,38- 1), and the only seven-coordinate Mn(II) complexes studied are [Mn(EDTA) (H20)] and its derivatives (38,39,42,43). Such studies are essential for understanding the mechanism of the manganese-containing SOD mimetics. The volume of activation for the water-exchange reaction... 

In the one-step symmetry-allowed mechanism, with little charge separation in the activated complex, the Hammett p-values for p-XCgH4- substituted substrates are very small. As anticipated, the reaction has a negative volume of activation. 

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