Dissociation, photochemical

Of the materials which dissociate photochemically, nitrogen-containing o-quinone diazides have received by far the most study278 -278... 

Heterolytic dissociation of a cyano derivative. Triphenylcyanomethane dissociates photochemically to form cyanide ions CN , the reaction being reversible in the dark. 

When a diatomic molecule dissociates photochemically it yields atoms either in their ground or in excited states. When the optical transition leading to dissociation is not forbidden, and does not arise from predissociation, one of the two atoms formed will not be in its ground state. 

While the main focus of the present article is on ozone, we note briefly in passing that several mechanisms have been postulated for the MIF in the CAIs, in particular an on-dust chemical mechanism [27] that is a surface reaction analog of the ozone formation mechanism discussed here, depicted schematically in Figure 2.6, and spectral self-shielding mechanisms [28-30]. In the latter, CO is dissociated photochemically in the early solar system, so producing a domain that is largely physically separated from that of the less abundant isotopes... 

Pollutant oxides of nitrogen, particularly NO2, are the key species involved in air pollution and the formation of photochemical smog. Eor example, NO2 is readily dissociated photochemically to NO and reactive atomic oxygen ... 

Ozone, O3, is found in trace quantities in the upper atmosphere where it is believed to be formed by the photochemical dissociation of oxygen molecules by the intense ultra-violet light from the sun ... 

Vanadium (IV) Chloride. Vanadium(IV) chloride (vanadium tetrachloride, VCy is a red-brown hquid, is readily hydrolyzed, forms addition compounds with donor solvents such as pyridine, and is reduced by such molecules to trivalent vanadium compounds. Vanadium tetrachloride dissociates slowly at room temperature and rapidly at higher temperatures, yielding VCl and CI2. Decomposition also is induced catalyticahy and photochemically. This instabihty reflects the difficulty in storing and transporting it for industrial use. 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

CN > NO2 > NH3 > H2O, F > Cl . Exceptions do occur. Photochemical Ligand dissociation is useful in the synthesis of multinuclear metal complexes such as diiron nonacarbonyl [15321-51 from iron pentacarbonyl [13463-40-6]... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

In the case of 1,3-diphenylisoindole (29), Diels-Alder addition with maleic anhydride is readily reversible, and the position of equilibrium is found to be markedly dependent on the solvent. In ether, for example, the expected adduet (117) is formed in 72% yield, whereas in aeetonitrile solution the adduet is almost completely dissociated to its components. Similarly, the addition product (118) of maleic anhydride and l,3-diphenyl-2-methjdi.soindole is found to be completely dissociated on warming in methanol. The Diels-Alder products (119 and 120) formed by the addition of dimethyl acetylene-dicarboxylate and benzyne respectively to 1,3-diphcnylisoindole, show no tendency to revert to starting materials. An attempt to extrude carbethoxynitrene by thermal and photochemical methods from (121), prepared from the adduct (120) by treatment with butyl-lithium followed by ethyl chloroform ate, was unsuccessful. 

Photochemical dissociation, 202 Photolyses, 200 Pine s wave function, 306 Plasma frequency, 306 Plasma model, 207, 304, 306, 318, 319, 323... 

Visser, A. J. W. G., et al. (1997). Time-dissolved fluorescence study of the dissociation of FMN from the yellow fluorescence protein from Vibrio fischeri. Photochem. Photobiol. 65 570-575. 

The evidence is that the thermolytic route does not involve radicals but the photochemical one does. A dissociative mechanism for the thermolytic route is indicated by its inhibition by added phosphine it is likely that once a phosphine group has dissociated, a metal-hydrogen bond is formed, with generation of a coordinated alkene (Figure 3.58). 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

E. Rabinowitch and W. C. Wood, Properties of illuminated iodine solutions. 1. Photochemical dissociation of iodine molecules in solution. Trans. Faraday Soc. 32, 547-555 (1936). 

Alkene-metal complexes are usually prepared by a process by which some other ligand is dissociated from the metal. Both thermal and photochemical reactions are used. 

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