Activation entropy, for

A reaction with a high activation energy tends to have a weaker interaction with the surface and hence will have enhanced mobihty that is reflected in a larger activation entropy. For this reason, the pre-exponents of surface desorption rate constants are lO — lO larger than the pre-exponents of surface reaction rates. 

II. 5 kcal.mole (activation energy), and —15 cal.deg . mole (activation entropy) for deuterated solvent the corresponding values are 9.1 l.mole sec , 13.2 kcal.mole S and —10.2 cal.deg . mole... 

It is noteworthy that the value of this substrate is smaller by one order compared to non-cyclic compounds. According to the discussions proposed above, this is considered to be due to its conformation already being fixed to the one that fits to the binding site of the enzyme. This estimation was demonstrated to be true by the examination of the effect of temperature on the kinetic parameters. Arrhenius plots of the rate constants of indane dicarboxylic acid and phenyl-malonic acid showed that the activation entropies of these substrates are —27.6 and —38.5 calmol K , respectively. The smaller activation entropy for the cyclic compound demonstrates that the 5yn-periplanar conformation of the substrate resembles the one of the transition state. 

Table 8.6 Activation Energies and Activation Entropies for the Alkene Hydrogenation on the Supported and Imprinted Rh-Dimer Catalysts...

One of the compounds from the previous study (10, R = R, = R2 = Me) was subjected to a careful band-shape study over a wide temperature range (34). The CN barriers obtained in the two rotamers with respect to the C=C bond were quite different from those obtained by the coalescence approximation. It was also found that the activation entropy for the C=C rotation was 10 to 18 e.u. more negative than that for the C—N rotation, in agreement with results to be discussed later. 

The ionized 6-yl radicals react with the nitrobenzenes also by addition, however with rate constants considerably higher (k = 1 x 10 to 1 x 10 M " s ) than those for the case of the neutral radicals. This indicates that also the transition state for the addition reaction is ionic [26]. The same conclusion can be reached from the increase of the rate constants for addition with increasing reduction potential of the nitrobenzenes and also from the very negative activation entropies for the addition reaction [15]. 

Fig. 10. Plot of the activation enthalpies versus the activation entropies for the reactions of MX5NO complexes with OH- (see Table III).

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

A positive value of (AHf/ — AHf/8) means that the formation of a heterotactic triad is easier with increasing temperature if isotactic diads are formed at syndiotactic ones and not vice-versa. More interesting are the differences of activation enthalpies and activation entropies for the other five ratios of rate constants. Because very few data for the temperature dependency of triad fractions have been reported in literature, the calculation of the corresponding activation enthalpies and en-... 

The values of the activation enthalpies and activation entropies for the forward (for) and reverse (rev) rate constants of an elementary step are constrained by two thermodynamic relationships ... 

It can be seen from Table 2 that the rate constants at 27 are higher for the a lymers than for the mixtures. Hiis order is reversed at higher temperature, due to the naUer Ea-value for the copolymers. The strong negative values of the activation entropies for the copolymers should be pointed out. Worth noting is also that the kinetic parameters vary with the photochrome content in the copolymer. 

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