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Glass transitions are characterized by large apparent activation energies and entropies. However, at sufficiently high temperatures, the activation entropy decreases to zero. This is a characteristic of noncooperative internal motions. The frequency-temperature relationship can be fitted by an equation which combines the Vogel-Fulcher and Eyring formulations with only two adjustable parameters

A theoretical analysis is made of the free energy and entropy barriers in the exchange and bridge diffusion paths for adatom selfdiffusion on Cu(100). The effective medium theory is used to calculate the energetics. A Monte Carlo method is used to find the free energy and entropy differences between the atom located in the stable fourfold hollow site and in the transition state. The activation entropy is found to be similar for the two paths. The resulting prefactor for the diffusion process is in excellent agreement with experiment. The variation of the activation free entropy with temperature is quite different from the outcome of a harmonic model but in qualitative agreement with the results using the quasi-harmonic approximation.

Thermomechanical analysis (TMA) was used to study the aging of rapidly quenched Fe85−xCoxB15 glasses. The three-point flexure technique was principally suitable for this study. The stress relaxation measurements permitted a characterization of the structural relaxation for various compositions. A theoretical approach of the aging of this type of glass was built up within the framework of the non-linear relaxation distribution (DNLR) model based on irreversible thermodynamics. For several isothermal experiments, the reduced property determined from the sample deflection was calculated. Then, the three characteristic parameters of the DNLR model were deduced: the activation enthalpy, the activation entropy, and a parameter of non-linearity. As samples in a relatively wide concentration range were used, the effect of alloying on the model parameters was examined.

Phosphine exchange at square-planar trans-Pt(SnPh3)2(PMe2Ph)2 has been studied by 31P NMR spectroscopy as a function of ligand concentration and temperature in C6D6 using magnetisation transfer techniques. Exchange of a specific phosphine takes place according to a second order rate law with kex298 = 33.1 ± 3 dm3 mol−1 s−1, ΔH‡ = 24.4 ± 7 kJ mol−1 and ΔS‡ = −131 ± 23 J K−1 mol−1. The highly negative activation entropy and the rate law indicate an associative mode of activation. In spite of the overall large steric requirements of the four tertiary ligands the activation parameters indicate a fairly closed transition state, possibly due to an electronic stabilisation of the transition state by the π-acidic stannyl ligands.

The successive formation of peroxomolybdenum(VI) complexes is discussed and it is shown that proton-consuming steps are followed by a proton-producing one; the pH maximum (=10) appears at a [H 2 O 2 ]/[MoO 4 ] 2− ratio of 2:1. The approximated formation constants are utilized to calculate the concentrations of peroxo complexes as a function of the ligand concentration. The mono-, di- and tetraperoxo complexes are formed in only negligible quantities, whereas the oxodiperoxohydroperoxo (simply, triperoxo) complex proves to be the predominant species. The triperoxo complex may react directly if a sufficiently reactive partner is present; if not, it oxidizes H 2 O 2 to 1 O 2 , which reacts further. The activation parameters of formation of O 2 are determined as a function of pH: the enthalpy minimum (50.45 kJ mol −1 ) is found at pH 10, where the negative activation entropy reaches its lowest value (−149.5 J K −1 mol −1 ). The possible functions of the peroxo ligands are considered.

International audience; The rate constants k(d) of the homolysis of the C-ON bond in styryl dyads TEMPO-based alkoxyamines have recently been published (Li et al. Macromolecules 2006, 39, 9201). The diastereoisomers exhibited different values which were higher than for the unimer TEMPO-styryl alkoxyamine 1. At a first glance, the localization of the steric strain was not obvious. To decipher this problem, diastereoisomer models 2 (RR/SS) and 3 (RS/SR), as well as the released alkyl radicals, were calculated at the \\B3LYP/6-31G(d) level. It was revealed that the increase in k(d) from 1 to 3 was due to the compression (buttressing effect) of the reactive center by the second styryl moiety. The difference in k(d) for the diastereoisomer was clearly an activation entropy effect Delta S-not equal. because the alkyl fragment of the RS/SR diastereoismer exhibited the same conformation as the released radical whereas the conformation for the RR/SS diastereoisomer was quite different and thus required the rotation of several bonds to reach the correct TS, which cost Delta S-not equal, and thus lowers k(d).

Results of a kinetic study on the alkaline hydrolysis of the aryl (methylsulfonyl)methylphenylphosphinates 3a–i show that a dissociative mechanism is not operating in this reaction. Activation entropy data (ΔS‡=–-59.5 J mol–1K–1 for the 4-nitrophenyl ester) and good Hammett σ relationship (ρ= 1.45 ± 0.07) strongly suggest that an associative bimolecular mechanism occurs, in which negligible negative charge is generated on the leaving group in the rate-determining step.

Against intuitive expectation, conrotatory opening of cyclobutenes 1–4 is slightly promoted by pressure with activation volumes around − 2 to − 3 cm3.mol−1. The volume profile shows a minimum for the transition state TS. This result together with low activation entropy values demonstrates full concertedness and suggest a cyclic TS with a preferential orbital topology.

11 páginas, 6 figuras, 5 tablas, 4 esquemas., Several bis(hydrosulfido)-bridged dinuclear rhodium(I) compounds, [{Rh(μ-SH)(L)2}2], have been prepared from rhodium(I) acetylacetonato complexes, [Rh(acac)(L)2], and H2S(g). Reaction of [Rh(acac){P(OPh)3}2] with H2S(g) affords the dinuclear bis(hydrosulfido)-bridged compound [{Rh(μ-SH){P(OPh)3}2}2] (1). However, reaction of complexes [Rh(acac)(CO)(PR3)] with H2S(g) gives the dinuclear compound [{Rh(μ-SH)(CO)(PR3)}2] (R=Cy, 2; R=Ph, 4) and the trinuclear cluster [Rh3(μ-H)(μ3-S)2(CO)3(PR3)3] (R=Cy, 3; R=Ph, 5). The selective synthesis of both type of compounds has been carried out by control of the H2S(g) concentration in the reaction media. The trinuclear hydrido–sulfido cluster [Rh3(μ-H)(μ3-S)2(CO)3(PPh3)3] (5) has been also obtained by reaction of [{Rh(μ-SH)(CO)(PPh3)}2] (4) with [Rh(acac)(CO)(PPh3)], proceeding through the trinuclear hydrosulfido–sulfido intermediate [Rh3(μ3-SH)(μ3-S)(CO)3(PPh3)3]. The molecular structures of complexes 1 and 3 have been determined by X-ray diffraction methods. Compound 1 is stable in solution, but complexes 2 and 4 slowly transform in solution into the trinuclear hydrido–sulfido clusters 3 and 5, respectively, with the release of H2S(g) in a reversible way. 1H NMR kinetic experiments for the transformation of 4 into 5 have revealed that this transformation follows second-order-type kinetic. The following activation parameters, ΔH≠=24±3 kJ mol−1 and of ΔS≠=−223±8 J K−1 mol−1, have been calculated from the determination of the second-order rate constants in the temperature range 30–45 °C. The large negative value of the activation entropy is consistent with an associative character of the rate-determining step. A plausible multistep mechanism based on the chemical behaviour of hydrosulfido–metal complexes and compatible with the kinetic behaviour has been proposed., The financial support from Ministerio de Educación y Ciencia (MEC/ FEDER) is gratefully acknowledged (Project CTQ2006-03973/BQU).

Meso and racemo Additions in Propagation for Radical Polymerization of Dialkyl Fumarates I. Stereoregularity of Poly(dialkyl fumarate)s

Dielectric measurements were made on a series of polymers containing tetrafluoroethylene at temperatures from 10 to 300 K and frequencies from 10 to 10 5 Hz. In all cases, the local-mode γ-relaxation was observed. In FEP and PFA resins which contain CF 3 and n-C 3 F 7 O branches, respectively, there is also a lower temperature relaxation for which the activation entropy is close to zero. In Teflon AF, an amorphous copolymer containing a large number of dioxole rings, four relaxations were observed including the glass transition near 200°C

The fundamental origin of the compensation law observed by P. K. David (1987, 1988) and G. C. Montanari (1988) in several aging mechanisms of dielectrics is discussed. It is shown that this is due to a linear relation between activation entropy and enthalpy of a given process. An aging model, partially based on the rate theory, is presented. One of its advantages is that it characterizes dielectric aging by a free energy change, rather than by an apparent activation energy. Some aging data are analyzed with the proposed model, and it is shown that under high stresses the failure time varies exponentially with stress. Some critical comments are made regarding the use of arbitrary end points as a method of selection for a failure criterion. >

Starting from an exact quantum-statistical description, the influence of the shape of the energy landscape on the magnetic viscosity is investigated. Magnetic phase-space analysis based on Kramers’ escape-rate theory of chemical reaction kinetics theory shows that the activation entropy associated with thermally activated hopping modifies the magnetic viscosity by reducing the attempt-frequency prefactor compared to an earlier prediction by Brown [W. F. Brown, Phys. Rev. 130, 1677 (1963)]. Energetic contributions are analyzed in terms of a model applicable to a range of coherent and noncoherent magnetization processes, and in the long-time limit deviations from the linear logarithmic magnetic-viscosity law are found.

The distribution of relaxation times in viscoelastic relaxations can be expressed as the corresponding distribution of activation enthalpies, assuming that the activation entropy is constant. Thus the earlier finding that certain simple, noncooperative motions have activation entropies close to zero is confirmed. The widths of the distributions of activation enthalpy for many relaxations are quite similar. When the frequency-temperature behavior of local-mode relaxations is analyzed, it is found that the data on the low-temperature side of the loss peak correspond to an activation entropy close to zero while those on the high-temperature side give much larger activation energies and entropies

Numerous interpretations have already appeared concerning the compensation effect (CE). A recent proposal, associated with the study of polymers, suggests a linear relationship between Eyring's activation enthalpy and activation entropy as an explanation of CE. The paper shows that these conceptual quantities give no additional or newer physical information than the Arrhenius parameters. Furthermore, they stem from the unjustified assumption of equilibrium for the transition state of reactions. Despite the ‘direct connection’ to material properties claimed for the Eyring interpretation, it will not prevent the occasional occurrence of a misleading apparent CE.

(...) The positive activation entropy and the fact that desmedipham fits well into a Bronsted plot log k OH =f(pK a ) obtained for the hydrolysis of alkyl and aryl N-phenylcarbamates with a slope value β of −1.15 supported an E1cB reaction scheme for desmedipham hydrolysis

A hammerhead ribozyme that was previously reported to have a rate of chemical cleavage 10-fold faster than that of conventional hammerheads was analyzed in greater detail. Although originally found as a bimolecular hammerhead assembled through helices I and II, fast cleavage was observed in hammerheads in the more conventional helix I−helix III form, provided the sequence of helix I of the fast hammerhead was preserved. Mutations indicated that the fast rate of cleavage was due to the presence of both the U1.1-A2.1 and A1.2-U2.2 base pairs. The faster rate of cleavage was due to a small increase in the activation entropy of the reaction. In addition, we confirmed previous reports that increasing the length of helix I by greater than five base pairs inhibits cleavage slightly and have uncovered a similar effect in helix II.

Crystal structure and activation entropy data indicate that H-bond directed diaza-Cope rearrangement of chiral diimines takes place with a high degree of preorganization. CD spectroscopy and HPLC data show that there is inversion of stereochemistry for the reaction with excellent enantioselectivity.

The temperature dependence of the activation Gibbs free energy [\ensuremath{\Delta}G(T)] and entropy [\ensuremath{\Delta}S(T)] associated with electric conduction in 12 mol % yttria--stabilized-zirconia single crystals was analyzed. In order to determine the \ensuremath{\Delta}G(T) and \ensuremath{\Delta}S(T) functions exactly, it was necessary to impose temperature-behavior conditions on the activation Gibbs free energy to give \ensuremath{\Delta}S(T\ensuremath{\rightarrow}\ensuremath{\infty})=0. The temperature dependence of the entropy was similar to that of the activation enthalpy. Both reach stable values at low temperatures (extrinsic associated range). In this temperature range, \ensuremath{\Delta}G(T) shows a linear behavior. At higher temperatures, \ensuremath{\Delta}G(T) decreases asymptotically to a value approximately equal to the migration enthalpy of the oxygen vacancies. A value of 5.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ eV/K was calculated for the variation of entropy associated with the migration of the vacancies. Assuming the cation-vacancy-cation association to be a first-neighbor structure, a value of 4.85\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ eV/K was found for the association entropy of the vacancies. These results are in very good agreement with those obtained by applying the method used in alkaline halide studies.

MT3 reacts with DTNB by a biphasic process, which roots from the difference in reactions between two domains with DTNB. The β-domain reacts with DTNB faster, implying that the CPCP motif distorts the peptide and leads to the Cd3S9 cluster more accessible to solvent. This reaction is a SN2 process according to the activation entropy.

A bacterial protease, very stable at high temperature, was studied for its inactivation at relatively low temperature (40–60 °C). The mechanism of this inactivation can be explained by intermolecular autodigestion. Between 40 and 60 °C, the protease starts to unfold while not yet unfolded enzyme molecules are able to hydrolyse the unfolded ones. The kinetic parameters for this autodigestion model were derived by computer simulation. The rate constant for unfolding was highly temperature dependent (activation enthalpy 509 kJ.mole–1, activation entropy 1364 J.mole–1.K–1); the rate constant for refolding decreased with temperature, while it also depended on the way the enzyme had been denatured. The rate constant for autodigestion itself was normal for an enzymatic reaction.

Nature 419, 743–747 (2002). The upper limit on the polypeptide reconfiguration time (τ0) was inadvertently calculated using (σapp - σ0)2, instead of (σapp2 - σ02), as given in the formula in the text (page 745, right column). The correct upper limit is therefore 0.2 ms. This results in a lower limiton the free energy barrier (Δ) of 2kBT, corresponding to an activation entropy of +3kB (page 746, right column), and an upper limit on the pre-exponential factor (2πτ0) of 1 ms.

By means of the Eyring-Frenkel equation and the assumption that A0 = 10−3 poise the influence of shear rate, molecular weight and concentration on some structural changes of acrylic solutions during non-Newtonian flow was investigated. It was established that the shear rate and the concentration influenced the activation heat and the activation entropy of viscous flow. The structural changes of acrylic solutions depending on molecular weight possessed an entropy character. Mit Hilfe der Eyring-Frenkel-Gleichung und unter der Annahme, das A0 = 10−3 Poise ist, wurde der Einflus der Scherrate, des Molekulargewichts und der Konzentration auf einige strukturelle Anderungen von Acryl-Losungen wahrend des nicht-Newtonschen Fliesens untersucht. Es wurde gefunden, das die Scherrate und die Konzentration die Aktivierungswarme und die Aktivierungsentropie des viskosen Fliesens beeinflust. Die strukturellen Anderungen der Acryl-Losungen, die vom Molekulargewicht abhangen, haben einen entropischen Charakter.

The kinetics of reactions of some more basic P-donor ligands with Ru 3 (CO) 10 (μ-dppm) have been studied and shown to include an associative term. Electronic and steric effects due to different attacking nucleophiles can be separated. The sensitivity of Ru 3 (CO) 10 (μ-dppm) to the basicity of the nucleophiles is slightly greater than that of Ru 3 (CO) 12 but the former is considerably more susceptible to steric effects. The intrinsic susceptibility of Ru 3 (CO) 10 ,(μ-dppm) to nucleophilic attack is defined and shown to be similar to that of Ru 3 (CO) 12 . Activation parameters show, however, that this is due to the combination of a much more favourable activation enthalpy balanced by a much more unfavourable activation entropy. The implications of these results are discussed.

Consistency requirements for the thermal activation analysis of low temperature deformation data are presented. The cases of elastic (ΔS ≠ 0) and inelastic (ΔS = 0) obstacles are considered, as are the alternatives of constant and of stress-dependent pre-exponentials. The requirements are expressed in terms of the allowable curvatures of the free energy-temperature and enthalpy–temperature diagrams constructed at constant strain rate. Low-temperature deformation data for five hexagonal metals are examined in the light of the above analysis, which indicates that the rate controlling obstacles are likely to be elastic in nature. It is observed for these metals that the activation entropy decreases and the pre-exponential increases with effective stress. It is concluded that the most probable rate controlling obstacles are the forest dislocations in Cd. Zn. and Mg, while in α-Zr and α-Ti, plastic flow is controlled either by interstitial-dependent lattice friction or by the cooperative overcoming of solute atoms. Es werden die Konsistenzforderungen fur die thermische Aktivierungsanalyse der Tieftemperatur-Deformationswerte mitgeteilt. Die Falle von elastischen (ΔS ≠ 0) und unelastischen (ΔS = 0) Hindernissen werden berucksichtigt, die alternativ einem konstanten und einem spannungsabhangigen Vorfaktor vor der Exponentialfunktionen entsprechen. Die Forderungen werden mit den zulassigen Krummungen der Freie-Energie–Temperaturund Enthalpie–Temperatur-Diagramme ausgedruckt, die fur konstante Deformationsgeschwindigkeit konstruiert werden. Die Tieftemperatur-Deformationswerte fur funf hexagonale Metalle werden im Hinblick auf die obige Analyse untersucht, die zeigt, das die geschwindigkeitssteuernden Hindernisse wahrscheinlich von elastischer Art sind. Es wird fur diese Metalle beobachtet, das die Aktivierungsentropie abnimmt, und das der Vorfaktor mit der effektiven Spannung zunimmt. Es wird geschlossen, das die wahrscheinlichsten, geschwindigkeitskontrollierenden Hindernisse Waldversetzungen in Cd, Zn und Mg sind, wahrend in α-Zr und α-Ti das plastische Fliesen entweder durch Zwischengitterplatzabhangige Gitterreibung oder durch die kooperative Uberwindung der gelosten Atome geregelt wird.

The effect of molecular weight in a narrow range on some rheological properties of acrylic solutions during Newtonian flow at different temperatures and concentrations was investigated. The results showed that the change in viscosity with temperature and concentration was dependent on the molecular weight. It was found that the molecular weight in a closer range influenced the activation entropy and the free energy of activation during Newtonian flow. Der Einflus des Molekulargewichts in einem schmalen Bereich auf einige rheologische Eigenschaften von Acrylatlosungen wahrend des Newtonschen Fliesens bei verschiedenen Temperaturen und Konzentrationen wurde untersucht. Die Ergebnisse haben gezeigt, das eine anderung der Viskositat mit der Temperatur und der Konzentration vom Molekulargewicht abhangt. Es wurde gefunden, das das Molekulargewicht in einem engeren Bereich die Aktivierungsentropie und die freie Aktivierungsenergie wahrend des Newtonschen Fliesens beeinflust.

The rate constant, k , activation energy, E a , and activation entropy, Δ S ≠ , were determined by the NMR line broadening technique for the ligand exchange reactions, (CH 3 ) 3 SbL[ZY] (X, Y = Cl. Br, NO 3 , OC 6 H 4 NO 2 - p , O 2 CCH 3 , O 2 CCH 2 Cl, O 2 CCHCl 2 , and O 2 CCCl 3 ; L = oxinato and 2-methyl-8-hydroxyquinolinato). Most of the ligand exchange reactions in these systems were found to proceed through a bridging intermediate. The rate constants, k , of these reactions are mainly decided by the activation entropy, Δ S ≠ , rather than the activation energy, E a . The liability of the ligands in these reactions increases in the following order: O 2 CCH 3 ≲ OC 6 H 4 NO 2 - p 2 CCH 2 Cl 2 CCHCl 2 2 CCCl 3

The pentacarbonyl(thioaldehyde), -(selenoaldehyde), and -(selenoketone)complexes (CO) 5 M[XC(Ph)R] (I) (M = Cr, X = Se, R = Ph; M = W, X = S, R = H; M = W, X = Se, R = H, Ph, C 6 H 4 Br- p , C 6 H 4 Me- p , C 6 H 4 NMe 2 - p react with dienes such as cyclopentadiene, trans -1-methyl-1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 1,3-cyclohexadiene via [4 + 2]-cycloaddition to give metal-coordinated thia- and selenacycles. The reaction follows second-order kinetics: -d[I]/d t = k 2 [I] · [diene]. The activation enthalpy Δ H ≠ is small (46 to 28.7 kJ/mol) and decreases along the series selenoketone, thioaldehyde, selenoaldehyde complex. The activation entropy Δ S ≠ is strongly negative (−144 to −147 J/(mol K) and is independent of the type of complex. The rate constant k 2 for the reaction with cyclopentadiene decreases clearly with increasing donor capacity of Y in (CO) 5 W[Se = C(Ph)C 6 H 4 Y- p ]. A good positive correlation is observed between k 2 and the Hammett constants ϱ p of Y (ϱ = 2.5). Exchange of R = Ph in (CO) 5 W[SeC(Ph)R] for R = H accelerates the reaction with cyclopentadiene by a factor of 73500. In contrast the influences of X ( k 2 (Se) = 19.0· k 2 (S)), M ( k 2 (W) = 1.87 · k 2 (Cr)), and the solvent are small. The rate constant for the reaction of dienes with (CO) 5 W[SeC(Ph)H] increases significantly along the series 1,3-cyclohexadiene ≈ trans -1-methyl-1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, cyclopentadiene. A concerted biradicaloid mechanism is proposed.

In the equilibrium between the two diastereomeric (Z, Z, Z) and (E, Z, Z) forms of aetiobiliverdine-IV-γ the former is preferred by about 20kJ/mol. For the thermal isomerization a barrier of 108kJ/mol has to be surmounted—as measured for the latter isomer. A value of 12.4 for the logarithm of the preexponential factor leads to a polar singlet mechanism, which was deduced previously for pyrromethenones and arylmethylenepyrrolinones, but a much more positive activation entropy for the verdinoid system indicates a pronounced delocalisation of charge over the entire conjugated system in the corresponding transition state.

In order to examine the influence of the polarity of the reaction medium on the polyesterification mechanism, the kinetics of the reaction of 1-octadecanol with octadecanoic acid in octadecanyl octadecanoate is studied with and without p-toluenesulfonic acid catalyst. Without added catalyst the overall order is 2.5 and the orders in acid and in alcohol are 1.5 and 1, respectively. The mechanism consists of an autocatalysis by the acid and involves a nonnegligible dissociation of ion pairs. This differs from the observation made in more polar media. With added p-toluenesulfonic acid, the reaction is first order in alcohol, acid, and catalyst. Activation entropy is the same both for catalyzed and noncatalyzed reactions. The catalytic effect is due to a large decrease of activation enthalpy.

Heat-inactivation studies were carried out on the two primary erythrocyte carbonic anhydrase isoenzymes, CA I and CA II, and the secondary isoenzyme of CA I, CA I (+1). In addition, two genetic variants of human isoenzyme CA I, CA Id Michigan (100 Thr-->Lys) and CA If London (102 Glu-->Lys), and one variant of isoenzyme CA II, CA IIh (251 Asn-->Asp), were similarly analysed. The first-order rate constants and Arrhenius plots for these six enzyme forms showed that (1) isoenzyme CA II is more heat-stable than CA I, (2) isoenzyme CA I (+1) is less heat-stable than CA I, (3) the variants CA IIh and CA If London are less heat-stable than the normal enzymes, and (4) isoenzyme CA Id Michigan is more heat-stable than normal CA I. From the values of the slopes of the Arrhenius plots, the energy of activation (E(a)) for each isoenzyme and isoenzyme variant was determined, and the following thermodynamic activation parameters were calculated at 55 degrees C: the free energy of activation (DeltaG(double dagger)), the activation enthalpy (DeltaH(double dagger)) and the activation entropy (DeltaS(double dagger)). The DeltaG(double dagger) for the enzymes shows a relative constancy with compensating variation in DeltaH(double dagger) and DeltaS(double dagger). When the values for DeltaH(double dagger) are plotted against DeltaS(double dagger), an increase in DeltaH(double dagger) involves a concomitant increase in DeltaS(double dagger).

The flow activation quantities were experimentally determined at 25.0°C with mineral lubricating oils blended with various concentrations of VI-improver. While polylauryl-methacrylate increases activation volume substantially and decreases activation entropy considerably, it does not affect activation enthalpy. Polyisobutylene changes the quantities only slightly. Molecular interpretation of these observations has been attempted.

“Tailor-made” dicarboxylic acids and dihydroxynaphthalene-triethylamine complexes catalyse the isomerisation of Δ-5-cholestenone. The reaction is faster and of lower kinetic order than with simple bifunctional catalysts; the activation entropy is more favourable, but the activation enthalpy is higher, due possibly to steric and electronic interference, between the catalysing functions, which are absent in the corresponding isomerases.

The substitution reactions of some Ni(II)–L complexes (L: ethylenediamine-N,N′-diacetate, edda2−·, nitrilo-triacetate, nta3−·, iminodiacetate, ida2−) with 4-(2-pyridylazo)resorcinol (H2par) have been studied spectrophoto-metrically at I=0.10. Rate constant k (mol dm−3, s) at 25°C, activation enthalpy ΔH\eweq/kJ mol−1 and activation entropy ΔS\eweq/J K−1 mol−1 are respectively: 1.7×103, 58.5 and 12.5 for k[Ni2+][Hpar−]; 4.0×102, 51.8 and −22 for k[Ni(nta)−][Hpar−]; 1.0×103, 41.8 and −46 for k[(par)Ni(edda)][Hpar−]; 2.7×10−4, 109 and 30 for k[(par)Ni(edda)2−]; 5.1×103, 88 and 17 for k[(par)Ni(Hedda)−]; 1.7×103, 122 and 226 for k[Ni(ida)][Hpar−]. In terms of activation parameters some discussions are made on the effect of coordinated ligands on the lability of leaving ligands and pre-equilibrium involving partial dissociation of leaving ligands.

Ketoximes react rapidly with X2PCl compounds were X = Ph, Me2N, EtO and X2 = OCH2CH2O at low temperatures (−60 to −80°C) in the presence of triethylamine to give a PIII intermediate 2, which rearranges by a unimolecular process to the corresponding N-phosphinylated imine 3. Free radicals, formed by capture of the initially produced phosphonyl radical, are detected by ESR spectroscopy, and evidence for a radical cage process is obtained from 31P CIDNP studies. Where X2 = OCH2CH2O, the PIII intermediate (2d, e) can be isolated, and the structure established from the 13C NMR spectra. Kinetic measurements show compound 2e to rearrange (30–60°C) ca. 10 times more slowly than the open chain compound (X = OEt), and the negative activation entropy suggests that this particular system (2e) rearranges, in part, by a cyclic transition state. Reaktion zwischen Oximen und dreiwertigen Phosphorverbindungen: Radikalische Umlagerung bei niedriger Temperatur Ketoxime reagieren mit X2PCl-Verbindungen X = Ph, Me2N, EtO, X2 = OCH2CH2O) bei niedrigen Temperaturen (−60 bis −80°C) in Gegenwart von Triethylamin zu einem PIII-Zwischenprodukt 2, das unimolekular zum entsprechenden N-Phosphinylimin 3 umlagert. Freie Radikale, die durch Abfangen des im Anfangsschritt gebildeten Phosphonyl-Radikals entstanden sind, werden durch ESR-Spektroskopie nachgewiesen, und Hinweise auf einen Radikalkafig-Prozes werden von 31P-CIDNP-Untersuchungen erhalten. Bei X2 = OCH2CH2O kann die PIII-Zwischenstufe (2d, e) isoliert werden, deren Konstitutionen anhand der 13C-NMR-Spektren zugeordnet werden. Nach kinetischen Messungen lagert 2e bei 30–60°C ca. 10mal langsamer um als die offenkettige Verbindung 2 mit X = OEt. Die negative Aktivierungs-Entropie zeigt, das 2e teilweise uber einen cyclischen Ubergangszustand umlagert.

1. Kinetic studies of the liquid-phase, reaction of the C4H6O2·GeCl2 complex with benzyl chloride suggest that this reaction proceeds through preliminary dissociation of the complex, followed by penetration of the free dichlorogermylene into the C-Cl bond of the chloride. 2. Rate constants and activation parameters for this reaction have been determined at temperatures ranging from 52 to 94°C. The transition state for the penetration step is characterized by a high, negative value of the activation entropy.

Self-diffusion studies have shown that V and Nb are characterized by a non-linear Arrhenius behaviour. This behaviour is usually explained within the frame of the existence of two or three diffusion mechanisms and the relevant analysis requires the determination of a number of adjustable parameters. It is shown that the non-linearity disappears when one plots In D vs. (BΩ)/(kT) instead of the usual In D vs. 1/T. Such an analysis does not need the introduction of any adjustable parameter and allows the determination of the activation entropy, enthalpy, and attempt frequency.

The volume diffusion of 51 Cr in ZrSn alloys was measured in coarse-grained polycrystals, using the “thin film” method. The Sn content in the alloys ranged from 0.39 to 6.7 at%. In the β-phase. diffusion temperatures were between 982°C and 1240°C. Sn was found to reduce the 51 Cr diffusion in the β-phase, the effect being small but well defined. The linear dehancement coefficient b and its temperature dependence were calculated, assuming the formation of Sn-Cr dimers. For the more dilute alloys Q and D 0 were determined and upon application of the Zener theory for D 0 , a negative contribution to the activation entropy was found. In the α-phase where three experiments were done at different temperatures, 51 Cr was shown to be a very fast-diffusing solute. No definite correlation with the Sn content was found probably owing to the anisotropy of the α-phase.

The electrostatic effect of Cu(II)-partially-quaternized po1y(vinylpyridine)(QPVP)catalyst on the oxidative polymerization of 2, 6-xylenol was discussed in terms of the thermodynamic parameters of the catalysis and the shape of the QPVP complex. The intermediate complex is stable in the QPVP system from an ehthalpy point of view (Table 2), and the Cu(1)complex is considered to be relatively stabilized by the strain of the QPVP-chain due to the electrostatic repulsion (Fig.1). Activation entropy of the QPVP catalyst is large, and the larger entropy was observed in the QPVP complex having the more compact structure (Table 2, 3). ln the QPVP-catalyzed oxidation of p-phenylenediamine, the chelate tstructure of the QPVP complex was destroyed by the coordination of the substrate

Aus den mit einer Elektronenstrahl-Mikrosonde aufgenommenen Konzentrationsprofilen fur Se und Te wird der Interdiffusionskoeffizient der Chalkogene in der Halbleiterlegierung Hg(SexTe1-x) als Funktion des Massenbruchs cHgSe und der Temperatur bestimmt. Die Messungen werden an undotierten Kristallen und an Hg- und Chalkogen-gesattigten Kristallen durchgefuhrt. Die Abhangigkeit des Interdiffusionskoeffizienten von der Eigendotierung zeigt, das die Diffusion uber Chalkogenzwischengitterdefekte verlauft. Die Diskussion dieser Ergebnisse im Vergleich mit der neueren Literatur uber die Hg-Chalkogenide fuhrt zu dem Schlus, das es sich hier um neutrale Zwischengitterdefekte handelt. Die Beweglichkeit dieser Zwischengitterdefekte last im HgTe unerwartet hohe Abhangigkeit von der Eigendotierung erkennen. Die Aktivierungsentropie der Diffusion der Chalkogene hangt linear vom Quotienten aus Aktivierungsenthalpie und Solidustemperatur ab. From the Se- and Te-concentration profiles recorded by an electron microprobe the chalcogen interdiffusion coefficient of the semiconductor alloy Hg(SexTe1 - x) is determined as a function of mass fraction cHgSe and temperature. Undoped crystals and Hg- or chalcogen-saturated crystals are studied. The dependence of the interdiffusion coefficient on the deviation from stoichiometry shows, that the diffusion proceeds via chalcogen interstitial defects. A discussion of this result in comparison with recent papers on Hg-chalcogenides shows that these defects are electrically neutral. The mobility of the chalcogen interstitials in HgTe yields an unexpectedly high dependence on deviation from stoichiometry. The activation entropy of diffusion of the chalcogenes depends linear on the ratio of activation enthalpy to solidus temperature.

Reaction of monosubstituted epoxypropanes with benzylamine The reaction of the epoxides of the general formula with benzylamine at 40, 50 and 60°C has been examined. The determination of rate constants by variation of X has shown, that in the case of X = S or N no anchimeric acceleration occurred. These results have been approved by product analysis and by synthesis of the possible reaction products. Transition states of the epoxide/benzyl-amine reaction are discussed by means of the experimentally determined activation energy and activation entropy.

In this paper it is shown that the equation for the entropy of activation for dislocation motion, derived previously by the authors, is in accordance with the classical definition of that quantity. The proof is presented in three stages. First, the entropy to form a dislocation in a linear elastic solid, to which no external tractions are applied, is calculated using both approaches. It is shown that both methods yield the same result. In the second stage, the entropy associated with the motion of a dislocation from a ground to an activated state, in a solid subjected to a uniform applied stress, is formulated. It is proven that the entropy of activation in this case is given by the difference between the formation entropies of the ground and activated states. This result illustrates that the entropy of activation is determined entirely by the internal stress states, and is independent of stresses produced by the applied tractions. Finally, a more general proof of the identity of the two methods is presented.

This paper is concerned with the evaluation of old and new evidence concerning the mechanism of polymerisation of 1,3-dioxacycloalkanes (DCA). The most recent evidence on polymerisations initiated by perchloric acid shows that in strictly anhydrous systems the propagating species is a sec. oxonium ion, so that under these conditions the propagation must be by the ring-expansion mechanism. In the presence of water, tert. oxonium ions are formed. A comparison of the polymerisation behaviour of DCA and tetrahydrofuran (THF) leads to the conclusion that the propagation mechanisms must be different. By comparing the activation entropy of DCA polymerisations initiated by organic cations on the one hand with that of DCA polymerisations initiated by perchloric acid, and on the other with that of the polymerisation of THF, it appears that the polymerisations of DCA initiated by organic cations are most probably also propagated by the ring-expansion mechanism. Some mechanistic suggestions by Penczek and by Okada are scrutinized and the status of the oxy-carbenium ion as a propagating species is analysed.

In der vorliegenden Untersuchung wird der Versuch unternommen, mittels DSC-Analyse das Aushartungsverhalten verschiedener Phenol-Formaldehyd-Harze naher zu studieren. Unter Verwendung eines speziellen dynamisch-statischen Versuchsprogramms ist es gelungen, den Aushartungsgrad in Abhangigkeit von der Hartungszeit, der Hartungstemperatur und der geratebedingten Aufheizrate experimentell zu bestimmen. Weiterhin war es moglich, die fur einen bestimmten Aushartungsgrad eines vorgegebenen Harzes notwendige Hartetemperatur und die notwendige Zeit der Hartung zu bestimmen. Die Interpretation der Mesergebnisse last den Schlus zu, das die Aktivierungsentropie der Vernetzungsreaktion mit zunehmendem Vernetzungsgrad deutlich zunimmt. In this paper the attempt is made to investigate the curing behaviour of some phenol formaldehyde resins by means of DSC-analysis. With a special dynamic-static test programme the amount of curing could be determined as a function of the time of curing, the temperature of curing and the heating rate. Furthermore the temperature of curing and the required time for a certain degree of curing of a given resin could be determined. The interpretation of the results leads to the conclusion that the activation entropy of the crosslinking reaction increases significantly with increasing degree of crosslinking.

The kinetics of the reactions[Formula: see text]have been studied (M = Si, Ge, Sn). All three reactions proceed by predissociation of the iridium complex and concerted cis addition of Ph3MH to the square-planar intermediate. The relative rate of addition of Ph3MH increases in the order Si (1) 0. It is concluded that, for this particular series of oxidative addition reactions, solvation effects related to a change in polarity on passage to the transition state are more important than bond energies, or acid–base properties, in determining relative reaction energetics. The relevance of entropy barriers to the problem of paraffin activation is discussed.