Isomerization reactions, rate constants

In contrast, the photoisomerization reaction of the dll-trans retinal that is fully exposed to methanol solvent occurs at three different bonds. In addition, the isomerization reaction rate constant in the protein environment is much larger than that in the solution environment. 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Temperature Dependence of the Activity and Selectivity of Xylene Isomerization over AP Catalyst. Based upon our analysis of the intracrystalline diffusional resistance in AP catalyst, we would expect that when the reaction temperature is increased, the selectivity would shift toward p-xylene since the diffusional effects are increased as the activity increases. A shift in selectivity toward p-xylene as the reaction temperature was increased was observed and is shown in Figure 6. The role of diffusion in changing the selectivity can be seen in the Arrhenius plot of Figure 7. The reaction rate constant for the o-xylene - p-xylene path, fc+3i, goes from an almost negligible value at 300°F to a substantial value at 600°F. Furthermore, the diffusional effects are also demonstrated by the changing... 

The first-order reaction rate constant for the isomerization of peroxynitrous acid to nitrate is 4.5 s 1 at 37°C therefore, at pH 7.4 and at 37°C the half-life of the peroxynitrite/peroxynitrous acid couple (let both these species be referred to as peroxynitrite for the sake of brevity) is less than 1 s. The reaction mechanism of peroxynitrite decomposition was a subject of controversy. Primarily proposed was that peroxynitrous acid decomposes by homolysis, producing two strong oxidants hydroxyl radical and nitrous dioxide (B15) ... 

The single most revealing mechanistic parameter for prolyl isomerization and amide rotation is the secondary deuterium isotope effect. In general for such studies, the hydrogens on the carbon that is bonded to the carbonyl carbon of the amide or imide (the /3-hydrogens ) are substituted with deuterium and reaction rate constants are measured for... 

More recently, the unimolecular isomerization CH3NC —CH3CN gave rise to elaborated studies by Bunker and collaborators " The pressure dependence of the thermal reaction rate constant is well explained by the RRKM theory, applying the simple concept of the geometry and vibrations of the activated molecule However, the fact that the hot-atom displacement reactions... 

The isomerization of 37 was slower than that of 8-HOABA by a factor of 10-100 under acidic conditions and by a factor of 3-10 under alkaline conditions. This indicated that the introduction of a fluorine at C-3 stabilized 8-HOABA kinetically in addition to thermodynamically. The pH-dependence of the isomerization rate of 37 was similar to that of 8 -HOABA at 25°C, the reaction rate constant at pH 10 was 50,000-fold larger than that at pH 3. At the same pH, the of the isomerization... 

The two rate constants of Equation 19, klt and k21, are, respectively, proportional to the net dehydrogenation rate of step 1 and that of step 2. Thus, if experimental conditions are chosen so that the isomerization reaction rate between 1-butene and 2-butenes is slow compared with the two dehydrogenation rates of n-butane, then the concentration distributions of 1-butene and 2-butenes in the reaction products provide information concerning the relative value of kit and k2t. To fulfill the above experimental conditions, we performed several experimental runs with a very low partial pressure of n-butane at differential conversion levels. Analyses of these experimental data indicated that the value of klt may be approximately 10 times larger than that of k2ty provided that the reaction scheme shown by Equation 23 is correct. From Table III the ratio of kit/k2t can be calculated to be 15. The separation of these reactions will be studied further. 

Figure 17 (a) Dependence of catalytic activities of aluminophosphate catalysts on the P/Al ratio (I) reaction rate constant for isobutanol dehydration, (II) reaction rate for I-butene isomerization, (b) H MAS-NMR spectra of aluminophosphate catalysts with different P/Al ratios A 1.6 B 1.4 C 1.0 D 0.5 E model compound AI(H2P04)2. Spinning sidebands are indicated by asterisks. Centerband chemical shifts are indicated in the plot. (From Ref. 14.)... 

Figure 5.17 Stern-Volmer plots for the isomerization reaction of allyl isocyanide at several laser energies. The slope [/cj//c(E)) is inversely proportional to the reaction rate constant. Adapted with permission from Reddy and Berry (1979a).

Photoreactions of [Ru(bipy)3]Cl2 and of [Ru(phen)3]Cl2 in acetonitrile proceed more slowly as pressure increases. The activation volumes increase dramatically with temperature, from +12 and +9 cm mol respectively at 288 K to +22 and +27 at 333 K. These increases are ascribed to a key role for chloride ion pairs in these dissociatively activated reactions. Rate constants for base-catalyzed isomerization of the y to the p form of [Ru(azpy)2Cl2], azpy = (22), are not linear in hydroxide concentration, prompting speculation on the detailed role of hydroxide in this conversion.Rate constants for replacement of coordinated water in cis- and tra 5-[Ru(LL)2X(OH2)]" by acetonitrile span a range of nearly 2 X 10" times. Several factors have to be invoked to explain the observed trend in the effects of ligand X on the ease of replacement of the aqua-ligand. " The... 

The presence of several different ionic particles and therefore, centres with different reactivity, should contribute to the values of the chain growth and chain termination reaction rate constants. The presence of associated, nonassociated and isomeric forms of catalyst particles, the influence of electrolytic dissociation, intramolecular and intermolecular interaction, leading to the formation of catalytic complexes is the reason for the presence of different centres in ionic catalytic systems. 

Figure 5.3 depicts potential energy surface of OH -1- NO2 reaction obtained by quantum chemical calculation (Pollack et al. 2003). Reaction rate constants calculated by RRKM calculation using the electronic structure of the transition state has been compared with the observed values (Sumathi and Peyerimhoff 1997 Chakraborty et al. 1998), and Golden et al. (2003) reported that recently calculated rate constants reproduced well the temperature and pressure dependence obtained by experiments. It has not been elucidated yet, however, if the reaction intermediate HOONO isomerizes to HONO2 or it regenerates more reactive chemical species by photolysis or reaction with other reactive species in the atmosphere, which would affect the ozone formation efficiency in the troposphere. 

As is inversely proportional to solvent viscosity, in sufficiently viscous solvents the rate constant k becomes equal to k y. This concerns, for example, reactions such as isomerizations involving significant rotation around single or double bonds, or dissociations requiring separation of fragments, altiiough it may be difficult to experimentally distinguish between effects due to local solvent structure and solvent friction. 

Nucleophilic reactivity of the sulfur atom has received most attention. When neutral or very acidic medium is used, the nucleophilic reactivity occurs through the exocyclic sulfur atom. Kinetic studies (110) measure this nucleophilicity- towards methyl iodide for various 3-methyl-A-4-thiazoline-2-thiones. Rate constants are 200 times greater for these compounds than for the isomeric 2-(methylthio)thiazole. Thus 3-(2-pyridyl)-A-4-thiazoline-2-thione reacts at sulfur with methyl iodide (111). Methyl substitution on the ring doubles the rate constant. This high reactivity at sulfur means that, even when an amino (112, 113) or imino group (114) occupies the 5-position of the ring, alkylation takes place on sulfiu. For the same reason, 2-acetonyi derivatives are sometimes observed as by-products in the heterocyclization reaction of dithiocarba-mates with a-haloketones (115, 116). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

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. 

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