Isomerization redistribution

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

According to the theory of constitutional chemistry, a chemical reaction is interpreted as a redistribution of the valence electrons i.e., as the transformation of an EM into an isomeric EM (in which both the atomic cores and the valence electrons are preserved). The difference between the final E (End) and the initial B (Begining) BE-matrices is called the R-matrix (Reaction matrix) ... 

Isomerization and transalkylation reactions to redistribute methyl groups on aromatic molecules are important processes in the production of benzene, toluene and xylenes (BTX). In particular, the production of para-xylene is preferred. The interconversion of C8 aromatics is covered in much greater depth in Section 14.3. 

Example IV The Thermal and Oxidative Isomerization of Tetraalkyl Substituted Tetrahedrane Clusters to Cyclobutadiene Derivatives. Other remarkable structural changes during redox reactions, i.e. charge redistributions enforced by the respective energy differences, are observed for cluster compounds (1). On... 

Internal rearrangements, isomerizations, and eliminations Another common type of cellular reaction is an intramolecular rearrangement, in which redistribution of... 

The isomerizations of n-butenes and n-pentenes over a purified Na-Y-zeolite are first-order reactions in conversion as well as time. Arrhenius plots for the absolute values of the rate constants are linear (Figure 2). Similar plots for the ratio of rate constants (Figure 1), however, are linear at low temperatures but in all cases except one became curved at higher temperatures. This problem has been investigated before (4), and it was concluded that there were no diffusion limitations involved. The curvature could be the result of redistribution of the Ca2+ ions between the Si and Sn positions, or it could be caused by an increase in the number of de-cationated sites by hydrolysis (6). In any case the process appears to be reversible, and it is affected by the nature of the olefin involved. In view of this, the following discussion concerning the mechanism is limited to the low temperature region where the behavior is completely consistent with the Arrhenius law. 

Base-catalyzed rearrangement of ethylene oxides is a topic that baa, until now, received only limited attention in the literature, chiefly because epoxides undergo simple nudeophilio attack rather than isomerisation with most bases. Strictly speaking, a base-catalysed epoxide isomerization is one in which the initial event is direct proton abstraction from the oxide ring. This may bo followed by redistribution of bonding electrons in any of several possible ways, to give ultimately one or more carbonyl compounds. For the general case the course i>f such a reaction may be depicted a in Eq. (480),... 

This chapter is concerned with the influence of mechanical stress upon the chemical processes in solids. The most important properties to consider are elasticity and plasticity. We wish, for example, to understand how reaction kinetics and transport in crystalline systems respond to homogeneous or inhomogeneous elastic and plastic deformations [A.P. Chupakhin, et al. (1987)]. An example of such a process influenced by stress is the photoisomerization of a [Co(NH3)5N02]C12 crystal set under a (uniaxial) chemical load [E.V. Boldyreva, A. A. Sidelnikov (1987)]. The kinetics of the isomerization of the N02 group is noticeably different when the crystal is not stressed. An example of the influence of an inhomogeneous stress field on transport is the redistribution of solute atoms or point defects around dislocations created by plastic deformation. 

Mixture B K[SiPh(3-fcat)2 and K[SiPh(dbcat)2] (3-fcat 2,3-dihydroxybenzaldehyde, dbcat 3,5-di-f-butylcatechol) contained two complexes with asymmetric catechols. Each complex showed the presence of two resonances due to the isomerism described above. The equilibrated mixtures showed the presence of two further species (Figure 9). These are attributed to isomers of the [SiPh(3-fcat) (dbcat)]- anion. Equilibrium was not established even after 8 weeks, whereupon decomposition prevented a more quantitative kinetic analysis. Flowever, it is apparent from the two experiments described that the kinetics of redistribution of ligands between complexes varies dramatically according to the cate-cholate involved. It is reasonable to conclude that the rate of redistribution decreases as the strength of the catecholate derivative increases. The nonstatistical distribution of complexes in a mixture indicates a thermodynamic stability of the complexes in Me2SO. The likely explanation lies in the electronic rather than the steric effects in the complex, since the live-coordination imposes little steric constraint. 

Above 100° C, thermal redistributions were observed in the trialkylborane systems involving also isomerization of primary alkyl groups to secondary alkyl groups and subsequent equilibration of the primary and secondary alkyl groups on the boron atoms (115,141,171). 

It represents the conversion of an ensemble of educt molecules EM(B) into an isomeric ensemble of product molecules EM(E) by a chemical reaction. The ensembles of molecules (EM) at the Beginning and the End of the reaction are described by their BE-matrices B and E. The so-called reaction matrix R corresponds to a pattern of valence electron redistribution, or also a scheme of bond breaking/making during the reaction (ref. 4). 

These recent experimental and theoretical results make it clear that control of selectivity depends on the kinetics rather than thermodynamics. It is interesting to speculate how one might improve the [4 + 2] addition selectivity of a Si(100)-(2 x 1) surface toward the diene systems. By replacing hydrogens with other appropriate groups, one may be able to alter the barrier for either the [2 + 2] reaction or the isomerization reaction. The former may control the initial distribution of surface products, while the latter may change the selectivity of the surface by thermal redistribution. 

Redistribution of halogens in chloroform and bromoform is taking place in presence of aluminum trichloride226 and also in presence of sodium hydroxide under PTC227. Bromotrichloromethane isomerizes to a mixture of all the possible five isomers in presence of traces of chloroform and Bu4NF (TBAF) catalyst (equation 28)228 ... 

Excitation to inner-sphere charge transfer (ISCT) transitions induces radial charge shift within the coordination entity, which may result in redox reactions including the central atom and ligands. The charge redistribution increases the complex susceptibility towards protonation, isomerization, redox processes, or nucleophilic or electrophilic attack. 

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