Isomerization, rates

This interpretation of the experimentally detennined pressure dependence of the isomerization rate rests on... 

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

Garrity D K and Skinner J L 1983 Effect of potential shape on isomerization rate constants for the BGK model Chem. Phys. Lett. 95 46-51... 

R. J. Loncharich, B. R. Brooks, and R. W. Pastor. Langevin dynamics of peptides The frictional dependence of isomerization rates of N-acetylalanyl-N -methylamide. Biopolymers, 32 523-535, 1992. 

The temperature of esterification has a significant influence on isomerization rate, which does not proceed above 50% at reaction temperatures below 150°C. In resins produced rapidly by using propylene oxide and mixed phthaUc and maleic anhydrides at 150°C, the polyester polymers, which can be formed almost exclusively in the maleate conformation, show low cross-linking reaction rates with styrene. 

When selecting a suitable feed symp, the main criteria are optimization of enzyme productivity and minimization of the formation of by-products. Typical feed symp specifications are shown in Table 5. Higher symp concentration and higher viscosity results in a reduced isomerization rate due to diffusion resistance in the pores of the immobilized enzyme. A deaeration step is desirable to remove dissolved oxygen that would otherwise iacrease the formation of by-products. The pH is adjusted to the optimum level for the productivity of the enzyme. 

From a general point of view, the tautomeric studies can be divided into 12 areas (Figure 20) depending on the migrating entity (proton or other groups, alkyl, acyl, metals. ..), the physical state of the study (solid, solution or gas phase) and the thermodynamic (equilibrium constants) or the kinetic (isomerization rates) approach. 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

A reaction mechanism in which the ( )-diazoate is formed by attack of the diazonium ion by a hydroxide ion in such a way that the ( )-diazoate is the primary intermediate (i. e., reaction sequence 6 - 3 in Scheme 5-14) is not consistent with the observation that the isomerization rate constant is independent of the hydroxide ion concentration. 

When Wiberg and Pracht (1972b) synthesized 3,3-di-(trimethylsilyl)-l-phenyltri-azene by reacting benzenediazonium chloride with sodium di-(trimethylsilyl)amide they found a faintly yellow compound if the reaction was carried out at -78 °C and an orange form at — 20 °C. NMR spectra were consistent with (Z)/( )-stereoiso-merism. Measurement of the isomerization rates at various temperatures in ether and in pentane indicates that the mechanism involves an inversion transition state (13.5) and not a rotation, because the free reaction enthalpies are independent of the polarity of the solvent. 

The presence of a hot spot is known to generate free radicals. Thus, reactions that are initiated by the presence of free radicals can be carried out with less or no catalyst, as has been discovered in the case of isomerization of maleic acid to fumaric acid with an 3 to 16 fold increase in the isomerization rates at reduced catalyst (thiourea) concentrations (Muzumdar, 1988). 

The form of Equation (6.7) reveals an interesting aspect of slow binding inhibiton due to enzyme isomerization. A slow forward isomerization rate is insufficient to result in slow binding behavior. The reverse isomerization rate must also be slow, and in fact must be significantly slower that the forward isomerization rate. If this were not the case, there would be no accumulation of the E I conformation at equilibrium. As the value of k6 becomes k5, the denominator of Equation (6.7) approaches unity. Hence the value of Kf approaches Kit and one therefore does not observe any time-dependent behavior. 

Di-/-butyl-6-aminothiophenol gives complex (497) under strictly anaerobic conditions. In the presence of air, deep blue-violet solutions are obtained from which mixtures of solid deep blue-black cis/trans-(498) can be isolated.1345 For the square planar Ni complex, the cis-trans isomerization rate is fast. (498) is best described as a singlet diradical due to strong antiferromagnetic coupling of the benzosemiquinonate 7r-radicals.19, 134S... 

Furthermore, the mechanism shown in Figure 12.1 considers only the all-tnmv-carotcnoid form as the initial compound however, although the all-tran.v-isomer predominates, d.v-isomcrs are also commonly found in model solutions and even more frequently in food systems, since these isomers are in equilibrium in the solution. Therefore, the initial carotenoid system often contains a mixture of isomers, whose composition changes according to the carotenoid structure, solvent, and heat treatment. For example, the isomerization rate of P-carotene is higher in nonpolar solvents, e.g., petroleum ether and toluene, than in polar solvents (Zechmeister 1944). 

Additional experimental, theoretical, and computational work is needed to acquire a complete understanding of the microscopic dynamics of gas-phase SN2 nucleophilic substitution reactions. Experimental measurements of the SN2 reaction rate versus excitation of specific vibrational modes of RY (equation 1) are needed, as are experimental studies of the dissociation and isomerization rates of the X--RY complex versus specific excitations of the complex s intermolecular and intramolecular modes. Experimental studies that probe the molecular dynamics of the [X-. r - Y]- central barrier region would also be extremely useful. 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

No general trends, comparable to phosphine influence on regioselec-tivity, isomerization rate, etc. have as yet appeared from the investigations of the influence of different parameters on the optical yield in catalytic asymmetric syntheses, with the exception of the favorable influence of decreasing the reaction temperature. The search for enantio-selective catalysts is likely to remain a matter of empiricism for some considerable length of time. 

As a result, nitroalkene (42a) is almost completely isomerized into N-oxide (47a), which is stable up to 100°C (167). Isomerization proceeds smoothly both in solutions and the solid state. The isomerization rate is approximated by the first-order equation and depends on the polarity of the solvent (isomerization in hexane occurs 70 times more slowly than that in ethanol). 

This argument, however, is unlikely on theoretical grounds. Both disproportionation and isomerization rates should depend linearly on the number of acid sites. Experimental findings have confirmed this. Also, the data in Table I show no correlation of the selectivity with Si02/Al203 ratio. 

All solvents for these solution thermolysis reactions were freshly distilled and all reactions were done in sealed glass tubes heated in a thermostatted oven. Over a wide range of solvents (DMF, naphthalene, diphenylmethane, benzene, toluene, and decalin) there was no significant variation in either isomerization rate or product composition. Reactions were done at 125°C, 155°C and 195°C and the only limitation was that DMF could not be used as the solvent in reactions at 195°C it led to substantial substrate destruction (polymer forming reactions of substrate with DMF ). Isomer compositions were ascertained both by HPLC and by NMR. 

With increasing concentration of methylated /1-cyclodextrin the selectivity to n-nonanal increases from 64% to 72%, while the conversion of the olefin is constantly as high as 97%. Obviously the addition of the methylated /i-cyclodextrin has only a moderate influence on the isomerizing hydroformylation of trans-4-octene to n-nonanal. The addition of only 0.2 mol.-% of methylated /3-cyclodextrin lowers the isomerization rate which results in the formation of slightly more branched aldehydes. In pharmacy j6-cyclodextrins are established as solvation mediators between polar and less polar solvents. This is one possible explanation for the rise in selectivity to n-nonanal with an increasing j6-cyclodextrin concentration. At higher con-... 

According to this sequence, formation of cis- and trani -stilbenes is preceded by formation of a magnetosensitive ion-radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and trani -stilbene, and the isomerization rate must depend on the intensity of the magnetic held. These predictions were conhrmed experimentally (Lyoshina et al. 1980). Hence, the ion-radical route for trans —f cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account single-electron transfer. The electron transfer takes place in reality and makes the... 

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