Isomerization and electron transfer

As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

Figure 9 A single-pole electron transfer switch. Light-induced isomerization and electron transfer processes in triad 7.

Chibisov, A.K., Shvedov, S.V., and Corner, H., Photosensitized processes in dicarbocyanine dyes induced by energy transfer delayed fluorescence, transacts isomerization and electron transfer, /. Photochem. Photobiol. A Ghem., 141, 39,2001. 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

CaY zeolite has been found either to act as a reagent to reduce stilbenes to 1,2-diarylethanes or to act as a catalyst and isomerize (Z)-stilbenes to the more stable E-form, depending on the number of Bronsted acid sites present in the zeolite. It has also been suggested that Z-to-E one-way isomerization of (Z)-stilbene through proton addition-elimination and electron-transfer processes occurs in acidic zeolite cavities. The isomerization reaction of (E)- and (Z)-a-phenylcinnamic acid molecules has been studied " at the level of semiempirical quantum chemical methods. The calculations revealed that the (Z)-a-phenylcinnamic acid is slightly more stable than the -isomer. Kinetic studies have been made on the thermal Z-E isomerization of C(40)-carotenoids, and the rotational barriers for Z-E isomerization of different proline analogues have been investigated by dynamic H NMR spectroscopy. The effects of... 

The first four reactions all contribute to chain propagation, although under different conditions. Disproportionation leads to branching and a shift in kinetics, and p-scission mediates isomerization, as was described in the previous section. Recombination (f) and electron transfer (g) terminate radical chains. Electron transfer is an active antioxidant mechanism that occurs particularly in the presence... 

In many cases, particularly for the study of fast reactions (ky> key in Scheme 164), the opening of the cyclopropylmethyl radical to the isomeric allyl-carbinyl radical, one thousand times faster than the cyclization of the 5-hexenyl radical, is a useful complementary tool. It has been used mainly by Kochi for the study of cage reactions of alkyl radicals in solution as well for the study of ligand and electron transfer oxidations of alkyl radicals. Furthermore, in the last three studies the knowledge of the fate of the isomeric cyclobutyl intermediate was very useful in distinguishing between a homolytic (no rearrangement of the cyclobutyl radical) and a heterolytic pathway (fast... 

When irradiated with visible Hght > 350 nm), polymeric systems 96 containing N and carbazole pendants (S) undergo the valence N—isomerization. The electron transfer mechanism S-N (96)... 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

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