Studying Free Radical Reactions Laser Flash Photolysis

Methods for Studying Free Radical Reactions Laser Flash Photolysis 

The light is incident on the sample and measures the transient absorbance after each laser pulse at a wavelength selected by the monochromator. All systems are controlled by a suitably programmed personal computer. 

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Photoinitiation can be switched on and off extremely rapidly. For example, the time of laser flash can be as short as 1 psec (10-12 s) and shorter. The practical absence of time inertia of photoinitiation lies in the timescales of the experimental techniques for studying fast free radical reactions (flash photolysis, rotating sector technique, photo after-effect [109]). 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Such reactions are, of course, unimolecular decompositions, not free radical chain processes. This fact made perfluorodiacyl peroxides ideal precursors for the laser flash photolysis studies which will be described in Sect. 4. Kinetics for the thermal decomposition of a number of perfluorodiacyl peroxides have been measured and their AH+ values were approximately 24 kcal/mol, about 5 kcal/mol... 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

Other applications of laser flash photolysis have included (a) a study of the bimolecular rate constant for the reaction between singlet oxygen and several lipid-soluble substances,(b) an investigation of quenching, solvent, and temperature effects on the photolysis of indoles, (c) the time dependence of the quenching of aromatic hydrocarbons by tetramethylpiperidine TV-oxide, and d) the kinetics of the geminate recombination of aromatic free radicals. Finally, flash photolysis has been utilized in order to examine the feasibility of a tunable IF laser (479—498 nm) and an ICl laser (430 nm). ... 

A large range of free radicals and other reactive oxygen species (ROS) can be produced biologically and in vivo and a variety of antioxidant species quench these ROS. Pulse radiolysis and laser flash photolysis are useful techniques for producing these radicals and ROS and for studying their reaction mechanisms. 

Laser induced fluorescence is particularly well suited to combustion chemistry, as a sensitive "in-situ" probe for free radicals in flames or under more controlled conditions in laboratory flash photolysis, discharge flow tube, or shock tube experiments. Using laser-saturation fluorescence previous studies from this laboratory (J ) have shown that C2(a3n ) is present in high concentrations in the hot region of an oxy-acetylene flame. C2(a-,n and X1 ) reacts with 0 .(2,3 4) One of the products of this reaction (and/or the reaction of C2H+02) is CC0.(2) In the present study, we report C20()rn.-f, i 7 fluorescence excitation spectra, A"3 , lifetimes and quenching rate constants, and... 

Those combustion reactions described here are best studied by the flash photolysis-laser resonance absorption technique. Other well-established techniques (such as chemiluminescence) are less useful for these cases because free-radical species cannot be cleanly produced by microwave and electrical discharge methods. 

Combustion processes are driven by energy-releasing chemical reactions. Detailed knowledge of the chemical kinetics of these individual reactive steps is required input to combustion models. For more than a decade, elementary gas-phase reaction kinetics has been successfully studied with the flash photolysis/resonance fluorescence technique (1-8). Typically, following broadband photolysis of a molecular precursor, reactant decays have been measured under pseudo-first-order kinetic conditions with cw resonance lamp excitation of free radical fluorescence. Increased utilization of laser probes in kinetic studies is exemplified by the recent pulsed-laser photolysis/pulsed-laser-induced fluorescence experiments of McDonald, Lin and coworkers (9-13). 

One important photochemical technique is flash photolysis, in which an intense flash of radiation of very short duration initiates a reaction by producing excited molecules, atoms, and free radicals. The method, first used in 1950 by G. Porter and R. G. W. Norrish (1887-1978), has been applied extensively to the study of fast reactions, both in the gas phase and in solution. Extremely rapid processes can be studied by the use of pulsed lasers, which can have a duration of less than 1 psec (10 sec). This is sufficiently short to allow the study of the fastest of chemical reactions, and even molecular relaxation process in which molecules are changing their vibrational and rotational states. 

The study of bimolecular gas reaction rate coefficients has been one of the primary subjects of kinetics investigations over the last 20 years. Largely as a result of improved reaction systems (static flash photolysis systems, flow reactors, and shock tubes) and sensitive detection methods for atoms and free radicals (atomic and molecular resonance spectrometry, electron paramagnetic resonance and mass spectrometry, laser-induced fluorescence, and laser magnetic resonance), improvements in both the quality and the quantity of kinetic data have been made. Summarizing accounts of our present knowledge of the rate coefficients for reactions important in combustion chemistry are given in Chapters 5 and 6. 

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