Chlorine flash photolysis

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

Klaning, U. K., K. Sehested, and T. Wolff, Ozone Formation in Laser Flash Photolysis of Oxoacids and Oxoanions of Chlorine... 

It has now been shown that a population inversion is not achieved between the aPi/2 and 2P% states of the chlorine atom in the vacuum ultraviolet flash photolysis of CF3C1,29 and thus the construction of a chlorine atom photodissociation laser appears remote at present. 

Such a process would have an intensity exponent of unity as observed. A further fact in support of the disproportionation mechanism is that the yield of CF2CI2 is largely independent of the ketone pressure at room temperature an abstraction mechanism would require a first-order dependence upon ketone pressure. While the dimer of CF2, tetrafluoro-ethylene, has never been observed in the reaction mixture, a white solid collected in the cell which was probably a polymer of CF2. While the experimental conditions are not strictly comparable, it is significant that the absorption spectrum of CF2 has been observed in the flash photolysis of 1,3-dichlorotetrafluoroacetone.39 When the temperature is raised, however, the yield of CF2C12 in normal photolysis, increases rapidly suggesting an energy of activation and this process can only be chlorine abstraction. The rate function ... 

The flash photolysis of Br2-02 mixtures was examined by Burns and Nor-rish , who also observed the spectrum of BrO as an intermediate. When a Pyrex filter was used to exclude radiation below 3000 A, no BrO was produced. Since, under these conditions, Br atoms are produced, it was concluded that, contrary to the chlorine case, BrO does not result from the reactions of Br with O2 as postulated by Durie and Ramsay The principal mode of formation of BrO was through absorption of radiation near 1800 A by O2 to form either 0 or O atoms, both of which react further with O2 to produce O3. The Br atoms which are then present can react with O3 via reaction (81) to produce BrO. Up to 90% of the BrO is formed in this way. The remaining BrO results from the reaction of highly electronically excited Bt2 with O2, viz. 

The formation and decay of OH was observed following flash photolysis of mixtures containing 0.1-0.4 Torr NO2, 2.5 Torr H2 and 20-300 Torr He. A typical oscilloscope trace is shown in fig. 2. In this system OH radicals are produced as a result of processes (1) to (3). The rate coefficients for reactions (2) and (3) are /t2 = 2x lO" cm molecule" s and 3 = 5x 10 cm molecule s With the NO2 and H2 concentrations used in these experiments, the half-life of H atoms was 6 s, that of 0( D) atoms much shorter, and OH radicals were essentially formed completely within the life-time of the photolytic flash. The chlorine filter almost completely prevented long wavelength photolysis of NO2 and even if 0( P) atoms were produced, either by photolysis or by quenching of 0( /)) atoms, they would react quite rapidly with NO2... 

Reaction was produced by flash photolysis of chlorine-formic acid mixtures and the rate was followed by rapid-scan infrared spectroscopy. 

Direct calculation of extinction coefficients (and hence oscillator strengths) requires a knowledge of both intensity of absorption and concentration of absorbing species. For the transition described above, concentrations of OH were calculated from the known fractional dissociation of water at elevated temperatures. In a few other instances, it may be possible to estimate the concentration of the intermediate from chemical considerations. Thus, Lipscomb et alP were able to calculate the extinction coefficient at A = 2577 A of the radical CIO from flash photolysis studies of chlorine dioidde. It was shown that the disappearance of CIO obeyed a second order rate law, so that... 

Photoreductiom in Aqueous Solution. Solvated electrons form on the irradiation of natural water samples or of aqueous solutions of natural organic matter isolated from surface waters (12, 45, 46). The solvated electron is a powerful reductant that reacts rapidly with electronegative substances such as chlorinated, brominated, and iodinated compounds. The interaction of pho-toejected electrons and a halocarbon is demonstrated by the laser flash photolysis data shown in Figure 3. 

We conclude our description of experiments involving the photon excitation by mentioning a single contribution by British researchers who combined GED with flash-photolysis [109]. These authors conducted diffraction studies on the decomposition of chlorine dioxide and biacetyl using electron pulses. 

Flash photolysis of chlorine dioxide, OOO, is also a good source of CIO radicals, " and Edgecombe, Norrish, and Thrush additionally showed that these radicals can be formed by the flash photolysis of CI2O. Several full kinetic studies of OO radical reactions using these methods have berai reported, and these early lesulte have been reviewed in detail." It has been shown that... 

The TT-complex of a chlorine atom with an aromatic molecule can approach the selectivity of a bromine atom. The observed selectivity of a chlorine atom in the presence of a complexing agent also depends upon the substrate concentration , because the rate of the reaction of Cl with the substrate and the complexing agent are of the same magnitude Scheme 4 presents the absolute rate constants that have been deduced by a study of the chlorination of DMB by the laser flash photolysis technique ... 

Figure 11.7. Characteristic decay of the triplet state of protoIX, chlorin e-6 and SnTPPC in aqueous phase monitored by flash photolysis. The composition of the electrolyte is identical to the one in photocurrent measurement at the water/DCE interface. Photoinduced heterogeneous electron transfer is effectively in competition with their relaxation process.

Fig. 6.2 Photographic plates showing a sequence of spectra of CIO after flash photolysis of a chlorine-oxygen mixture and the bimolecular decay of this species reprinted by permission from [16]...

Porter G, Wright FJ (1953) Studies of free radical reactivity by the methods of flash photolysis. The photochemical reaction between chlorine and oxygen. Discuss Faraday Soc 14 23-34... 

All l and aryl azides. The high photoreactivity of azides has long made them favourite chromophores for a variety of applications. Beginning from small inorganic molecules, one may mention the photochemistry of chlorine azide by laser flash photolysis in the femtosecond regime. 

Sander, S.P., Friedl, R.R. Kinetics and product studies of the reaction chlorine monoxide + bromine monoxide using flash photolysis-ultraviolet abstsption. J. Phys. Chem. 

Simple Compounds. The kinetics of chlorination of ammonia by MegCCOCl, producing successively NHgCl, NHCI2, and NCI3, have been investigated in cyclohexane solution containing 0.2M-t-butyl alcohol. Possible mechanisms are discussed. Flash photolysis and pulse radiolysis of aqueous solutions of hydroxylamine involve... 

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