Entropy negative value

Solutes in Aqueous Solution. As mentioned in See. 88, when we say that we expect to find a correlation between the /1-coefficients of viscosity of various species of ions, and their entropy of solution, this refers only to the unitary part of the entropy, the part associated with the ionic co-sphere. We are inclined to adopt the view that a negative //-coefficient for a pair of ions should be accompanied by a positive increment in entropy, while a positive //-coefficient should be accompanied by a decrease in entropy. The values of AS0, the conventional entropy of solution, to be found in the literature, do not, give a direct answer to this question, since they contain the cratic term, which in water at room temperature amounts to 16 e.u. This must be subtracted. 

The activation entropies were considerably different from the large negative values expected for a second-order reaction and this was attributed to the effect of the internal return mechanism. 

Table 3 shows that the small activation enthalpies of the reactions (3) and (4) are clearly affected by the zero point energy corrections. But the relative order of the activation enthalpies remains the same with or without the corrections. The activation entropies have great negative values, which is of mechanistic interest (see part 4.3.1). However, because of their similarity, when comparing the three reactions to one another they have only small importance, e.g. for estimation of copolymerization parameters (see part 4.3.2). 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

From these it will be seen that a more negative value of or a more positive value of will in each case lead to an increase in Ke. Furthermore, there is a hypothesis that if the degree of disorder in a system increases it will almost certainly be accompanied by an increase in entropy... 

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

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... 

An important factor is the temperature. Firstly, more of the salt is extracted into the organic layer at lower temperature (Sousa et al., 1978). This indicates that for these salts the entropy of complexation has a substantial negative value (Kyba et al., 1978). Secondly, the temperature has an effect on the value of the EDC lower temperatures give rise to higher EDC and d(dG°) values (Table 73). The available data on the effect of temperature hardly allow a quantitative... 

Negative values for redox couple entropy have also been obtained for the Cu(II)/Cu(I) reduction, in aqueous medium, of the blue copper proteins stellacyanin, plastocyanin and azurin.14 The decrease in molecular disorder has been attributed in this case to the fact that the charge neutralization of the redox site (from + 1 to 0) favours the formation of hydrogen bonds between the solvent (water) and the copper centre.17... 

Analysis of the transition state in terms of energy is certainly a key aspect of the S 2-ET problem. Entropy considerations may, however, bring about additional information, possibly helping us to conceive better the transition between the two mechanisms. It was observed in this connection that, whereas the entropy of activation of both the anthracene anion radical and of the ETIOPFe(O) porphyrin (pp. 99, 100) (which have about the same standard potential) is close to zero in their reaction with s- and t-butyl bromides a definitely negative value, ca. — 20 eu is obtained for the reaction of the porphyrin with n-butyl bromide (Lexa et al., 1988). The same was found for the reaction of two other iron porphyrins, TPPFe(o) and OEP-Fe(i). These activation entropies were estimated from (153), where Z is... 

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