Kinetic of cis-trans isomerization

Leon-Camacho, M., Ruiz-Mendez, M.V., Graciani-Constante, M., Graciani-Costante, E. (2001) Kinetics of cis-trans isomerization of linoleic acid in the deodorization and/or physical refining of edible fats. Eur. J. Lipid Sci. Technol., 103, 85-92. 

R. B. CuNDALL, The kinetics of cis-trans isomerization. Chapter 4 of Progress in Reaction Kinetics, Vol. 2, Pergamon, Oxford, 1964. 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Studies of the kinetics of cis-trans interconversion in disilene 33 indicate that it takes place by dissociation to the silylene and recombination, rather than by Si=Si bond rotation as is the case for other disilenes187. This isomerization occurs even at 50 °C. 

In the first part of this article, we present a kinetic study on the isomerization of l-phenylazo-2-hydrozynaphthalene (1PA2N) measured by picosecond spectroscopy. The selection of this compound was based on its known photochemical properties of cis-trans isomerization and tautomerization by proton transfer. The phototautomerization of 1PA2N is evidenced by the fact that in solution this compound is a mixture of the azo tautomer A and the hydrazo tautomer as shown in Figure 2.1. It is thought that the inter-... 

A database search yields more than 20 000 references that contain Z-E isomerization, cis-trans isomerization, or geometric isomerization as keywords, and the general tendency is an increase in the number of papers devoted to the kinetic aspects of cis-trans isomerization (CTI) in all fields. The main isomerization pathways have probably been discovered, though many remain the object of intense theoretical (see Chapter 7) and experimental research (see Chapters 4—6, 8-10, 13, and 14). In the present chapter, general CTI mechanisms will be divided into homolytic and heterolytic cleavage of the 7r-bond which allows isomerization, though some molecular motifs such as amides are able to switch from cis to trans via both processes. An overview of CTI in metal complex (mainly thermal, photochemical, and oxidative isomerizations) will be the purpose of Chapter 14 and will not be detailed here. 

The kinetics of the photoisomerization of bilirubin has been studied because of the relevance to phototherapy. The fluorescence of bilirubin increases on binding to human serum albumin. This and other primary photoprocesses have been investigated by picosecond spectroscopy. Karvaly has put forward a new photochemical mechanism for energy conversion in bacteriorhodopsin. An extensive review of the photophysics of light transduction in rhodopsin and bacteriorhodopsin has been made by Birge. The dynamics of cis-trans isomerization in rhodopsin has been analysed by INDO-CISD molecular orbital theory. Similar calculations on polyenes and cyanine dyes have also been reported. A new picosecond resonance Raman technique shows that a distorted... 

Kinetic studies of racemization of optically active allylic alcohols, of isotope exchange of 0-labeled allylic alcohols, and of cis-trans isomerization of suitably substitued allylic alcohols have contributed to knowledge of the detailed mechanism and stereochemistry of allylic alcohol isomerization. 

Here At and A -j are the absorbance at time t and infinite time. k and 2 are the rate constants of the fast and the slow processes. a and U2 represent the relative contributions of the fast and slow processes to the kinetics of the cis trans isomerization. The rate constants of the fast (ki) and the slow (k2) proeesses at 20°C are 1.7 X 10 " s and 1.7 x 10 s respectively. The rate constant of the thermal cis- trans isomerization of azobenzene derivatives in solution is usually on the order of 10 s Compared with this value, the rate of cis trans isomerization of azobenzene ligands incorporated into the nanocomposite films is faster for ki and is in the same order for k2, which is good for applications requiring fast switching between the two isomerization states. 

According to Fig. 20, the energy to twist the N state of ethylene by 90° is 65 kcal, breaking the pi bond. This barrier has been observed experimentally from studies of the kinetics for cis-trans isomerization dideuterated ethylene. 

Controversy over the interpretation of the kinetics for cis-trans isomerizations of square-planar complexes continues. The topic is important for the understanding of substitution mechanisms in general. Associative and dissociative mechanisms have been proposed, as well as intramolecular rearrangements via tetrahedral intermediates. ... 

Cobalt(iii) Complexes.—Two kinetic studies of cis-trans isomerization of cobalt(iii) complexes of simple ligands have been described, for the complexes c/j -[Co(en)2(glycinenitrile)Cl] + and [Co(en)2(OH2)(N3)] +. Possible mechanisms of isomerization and racemization for complexes containing flexible quadridentate ligands have been discussed in general terms, for amino-acid complexes [Co(trien)(LL)] +, and for the specific case of the complex [Co(trien)(N-methyl-S -alaninato)] +. The rearrangement of the a -R- to the aj8-5-form of [Co(tetren)(NCS)] + follows the rate law... 

The only photoreaction which the 4-styrylpyridine (4-stilbazole) complexes [Ru(bipy)(cw-4-stilbazole)] + and [Ru(bipy)(/ ra 5-4-stilbazole)] + undergo is that of cis trans isomerization of the stilbazole ligands there is no significant photodissociation. The anion [Pt(NH3)4(NH2)(N02)] " photoisomerizes to [Pt(NH3)4(NHa)(ONO)]. The quantum yield has been determined for this process, but no kinetic results are available. ... 

Kinetic studies by H n.m.r. spectroscopy of cis-trans isomerization of the formally seven-co-ordinate molybdenum compounds (6), with L = a phosphine or phosphite and R = H, D, Me, PhCHg, Cl, Br, or I, show that the kinetic parameters of this intramolecular process vary to a small extent as L varies, but vary greatly as R varies. Thus the activation energies for the compounds (6) with R = H or D are between 12 and 14 kcal mol , but for compounds with R = halide, activation energies are in the range 22—26 kcal mol. The most probable transition state for these isomer-izations is described as approximating to a trigonal bipyramid with the... 

Sui Q, Borchardt D, Rabenstein DL (2007) Kinetics and equilibria of cis/trans isomerization of backbone amide bonds in peptoids. J Am Chem Soc 129 12042-12048... 

The above studies are consistent with the hypothesis that the metathesis reaction itself brings about cis-trans isomerization (46). This hypothesis is further supported by the results of a kinetic study of the reactions of the three linear butenes on the metathesis catalyst Mo(CO)6-A1203 by Davie et al. (107), who concluded that cis-trans isomerization for their system is a bimolecular reaction. 

One may conclude that the rate-determining step of the renaturation is at least partly influenced by the cis-trans isomerization of the peptide bond the secondary nitrogen atom of which arises from proline. Otherwise, only the entropy-controlled slow nuclea-tion should be observed kinetically. The covalent bridging through Lys-Lys, therefore, gives rise not only to thermodynamic stabilization of the triple helix but also to kinetic properties which have hitherto been observed in the case of type III procollagen146) and its aminoterminal fragment Col 1-3144). 

Information on the E-Z isomerization of the disilenes sheds light on the nature of the chemical bonding of the silicon-silicon double bond. Kinetic parameters, which have been reported for cis-trans isomerization of five disilenes, are listed in Table IV. 

Double bond cis-trans isomerization occurs during hydrogenation with a relative rate dependent on structure. The less stable double bond isomerizes to the more stable one, but, of course, kinetics and thermodynamics control the extent of isomerization. In a linear carbon chain, one can expect the cis alkene to isomerize to trans and vice versa if the thermodynamics are favorable. However, in a strained cyclic system, trans will isomerize to cis (Fig. 2.13).117... 

Kistiakowsky and Smith [J. Am. Chem. Soc., 56 (638), 1934] have studied the kinetics of the cis-trans isomerization of isostilbene. The reaction is believed to be pseudo first-order in both the forward and reverse directions under the experimental conditions used by these investigators. 

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