Light-induced dissociation

Fig. 8.13. Schematic showing the photoreaction process of photlLOV2 detected by TG (a) light-induced association of two monomers and (b) light-induced dissociation of a dimeric form...

Methylcobalamin and adenosylcobalamin are light-sensitive, with light inducing dissociation of the Co-C bond. 

Photodissociation means light-induced dissociation. The energy from a photon of iight can break chemicai bonds and therefore dissociate, or break apart, a moiecuie. 

The initiation step in the mechanism is light-induced dissociation of CI2 ... 

Alicyclic ew-dialkyldiazenes are very thermolabile when compared to the corresponding tram-isomers, often having only transient existence under typical reaction conditions. It has been proposed49 that the main light-induced reaction of the dialkyldiazenes is tram-cis isomerization. Dissociation to radicals and nitrogen is then a thermal reaction of the cis-isomer (Scheme 3.19),... 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

When subjected to shock-waves, phosphine dissociates into hydrogen and red phosphorus. The radiation thus emitted is only visible in reflection. In contrast, the shock-wave induced dissociation of phosphine diluted with argon is accompanied by emission of visible light. In this case the reaction products are hydrogen and white phosphorus... 

There are also many photoreactions, in which rapid, efficient charge transfer between electron donor and acceptor moieties occurs, yielding a radical ion pair that subsequently dissociates because the solvent is not oxidized or reduced in these light-induced reactions, these reactions belong to photo- rather than radiation chemistry. Of particular interest, in the context of radiation chemistry studies, are photoreactions in which the solvent is... 

Upon irradiation of fats, the formation of a multitude of products is possible after primary ionization and excitation, and deprotonation followed by various dimerization, disproportionation reactions, dissociations, or decarboxylation. It is generally assumed that irradiation in the presence of oxygen leads to accelerated autooxidation of lipids, and that the pathways are the same as in light-induced or metal-catalyzed autooxidation. 

Most photocleavable linkers for carboxylic acids used today are based on the photoisomerization of 2-nitrobenzyl esters and on the light-induced cleavage of phen-acyl esters (Figure 3.10). Several possible mechanisms have been proposed for the photolytic cleavage of benzoin esters. One of the most recent is the dissociation of the excited phenacyl ester into a carboxylate and a phenacyl cation ( photosolvolysis , Figure 3.10 [136]). 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

In terms of tons of chemicals per year, this is by far the most important industrial process in which a light-induced reaction plays a key role. The photochemical reaction itself is extremely simple, being the homolytic dissociation of molecular chlorine, Cl2, into Cl atoms,... 

The chlorination of saturated hydrocarbons can be induced by light, but also can be carried out at temperatures of about 300° in the dark. Under such circumstances the mechanism is similar to that of light-induced chlorination, except that the chlorine atoms are formed by thermal dissociation of chlorine molecules. Solid carbon surfaces catalyze thermal chlorination, possibly by aiding in the cleavage of the chlorine molecules. 

The primary effect of micelles on light-induced electron transfer involves the intervention of an interfacial region which can significantly influence the radical ion association and dissociation equilibria by a combination of electrostatic and hydro-phobic interactions. Diffusive encounters of reaction partners are controlled within a micelle by the diffusion of one reactant to the highly polar surface, by collision of two reactants confined within the hydrophobic region in the interior of the micelle, and by the reaction of two reactants whose motions are confined to diffusion along the micellar surface. 

The unified theory (UT) considers the bimolecular generation of exciplexes, or their light-induced generation in line with their subsequent reversible dissociation (Section V.D). Under the condition of instantaneous exciplex formation, this theory reduces to IET. 

As will be shown throughout this book, quantum control of molecular dynamics has been applied to a wide variety of processes. Within the framework of chemical applications, control over reactive scattering has dominated. In particular, the two primary chemical processes focused upon are photodissociation, in which a molecule is irradiated and dissociates into various products, and bimolecular reactions, in which two molecules collide to produce new products. In this chapter we formulate fie quantum theory of photodissociation, that is, the light-induced breaking of a chemical bond. In doing so we provide an introduction to concepts essential for the 1 remainder of this book. The quantum theory of bimolecular collisions is also briefly ydiscussed. 

The light-induced dimer should eventually dissociate to return to the monomers, because no permanent change was observed. It may be reasonable to assume that the dimer dissociates when the photoadduct state of LOV2 goes back to the ground state. We should emphasize that this TG technique for the D measurement in the time domain has been the only one technique that can detect such transient dimer formation. 

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