Primary photochemical reaction

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Photochemical Reactions. The photochemistry of chlorine dioxide is complex and has been extensively studied (29—32). In the gas phase, the primary photochemical reaction is the homolytic fission of the chlorine—oxygen bond to form CIO and O. These products then generate secondary products such as chlorine peroxide, ClOO, chlorine, CI2, oxygen, O2, chlorine trioxide [17496-59-2] CI2O2, chlorine hexoxide [12442-63-6] and... 

Atmospheric pollutants released by combustion of fossil fuels fall into two main categories those emitted direcdy into the atmosphere as a result of combustion and the secondary pollutants that arise from the chemical and photochemical reactions of the primary pollutants (see Airpollution). 

TABLE 7.5 Primary Photochemical Reactions of an Excited Molecule A—B—C ... 

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

The primary photochemical reaction is an electron transfer reaction which occurs within a highly structured reaction-center protein which spans the thylakoid membrane. 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

In the mechanism of a photochemical reaction, at least one step involves photons. The most important such step is a reaction in which the absorption of light (ultraviolet or visible) provides a reactive intermediate by activating a molecule or atom. The mechanism is usually divided into primary photochemical steps and secondary processes that are initiated by the primary steps. 

The N-diazeniumdiolates are quite photosensitive. Studies of various 02-substi-tuted compounds, both alkyl and aryl, revealed a primary photochemical reaction involving cleavage of the N=N bond to yield a nitrosoamine and an O-substituted nitrene which rearranges to a C-nitroso compound (Scheme 3.31), the latter is often isolated as the oxime [224]. 

The primary sources of PCDDs and PCDFs in the environment can be divided into four categories chemical reactions, thermal reactions, photochemical reactions and enzymatic reactions4. 

The results and discussion section is divided into two parts. The first part deals with direct laser flash photolysis of the MDI-PUE polymer and appropriate small molecule models. The transient spectra generated by direct excitation of the polyurethane are interpreted by consideration of the primary photochemical reactions of the carbamate moiety. The second part describes results obtained by production of a radical transient species which is capable of abstracting labile hydrogens from the polyurethane. This latter procedure represents an alternative method for production of the transient species which were obtained by direct excitation. 

It is the same rate as given in thermal reaction given by equation (3.31). Thus, both in thermal and photochemical reactions, the primary process is the dissociation of Br2 molecules. In spite of the chain mechanism, the quantum yield of H2 and Br2 reaction is very small, i.e. about 0.01 at ordinary temperature, although it increases as the temperature is raised. The reason is that the reaction immediately after initiation following the primary state, i.e. 

Describe the primary process in photochemical reactions with help of suitable examples. 

The importance of tertiary amines in the photochemically induced electron transfer reactions has also been addressed5. Direct irradiation of aromatic or aliphatic amines often leads to the scission of C—N, N—H or C—H bonds that lead to the subsequent chemical reactions by radical pathways6. In this section, photochemical reactions of amines reported since 1978 will be considered with emphasis on photoinduced electron transfer. Photochemical reactions of inorganic and organometallic compounds will not be included unless photochemistry of amine moieties is the primary interest. 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

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