Kinetics of radical reactions

This review focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. In the past two decades, progress in determining the absolute kinetics of radical reactions in general has been rapid. The quantitation of kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase has been particularly noteworthy, progressing from a few absolute rate constants available before 1980 to a considerable body of data we summarize here. 

The kinetics of radical reactions can be studied by direct methods (discussed in Chapter 18 of this volume) or by indirect methods. Indirect kinetic studies require no special instrumentation, and are popular for obtaining relative or absolute rate constants for an intermediate that might be formed in a specific conversion. The radical of interest is generated in a reaction where two pathways compete, the reaction of interest and the basis reaction, for which a rate constant is known. Relative rate constants are then determined from the product mixture by spectroscopy or chromatography and used with the basis rate constant to calculate the absolute rate constant for the reaction of interest. The method is most easily applied... 

Griller, D. and Ingold, K.U. (1980) Accounts of Chemical Research, 13, 317 Newcomb, M. (2001) Kinetics of radical reactions radical clocks. In P. Renaud and M.P. Sibi (Eds) Radicals in Organic Synthesis (vols 1 and 2). Wiley-VCH, Weinheim, Germany, p. 317. 

The flash technique was designed primarily for the study of the kinetics of radical reactions but much information is also obtained from the interpretation of the spectra themselves, and two such spectra, of the radicals CIO and SH, will now be considered. 

Within the last two decades Electron Spin Resonance-(ESR) spectroscopy has become a standard experimental technique in electrochemical research. The main interest was in the field of electrochemical generation of radicals to characterize their structure by ESR spectroscopy or to prove their presence in electrode reactions. The studies have been extended to the kinetics of radical reactions and the set up of reaction mechanism, to the solvation phenomena in radical electron densities and to radical conformation and ion complex structure. The latest development is the study of the electrode materials and their surface layers in electrochemical systems by simultaneous ESR spectroscopic and electrochemical measurements, e.g., of polymer modified electrodes. 

Theoretical and experimental studies are needed to specify the kinetics of radical reactions involving H2O molecules. These reactions, involving high activation energy, are insignificant at ambient and elevated temperatures, but can contribute significantly to the chemistry in near and supercritical water. 

Mechanical fracture of polymers Kinetics of radical reactions Initiation reactions (using photons or high-energy radiation)... 

The use of such flow and/or stopped flow systems has been used quite extensively to investigate the kinetics of radical reactions. These include the use of a stopped flow EPR system to investigate the one-electron oxidation of a range of phenols by a dimeric manganese(rV/TV) triazacyclononane complex with and without hydrogen peroxide. 

Observation of the changes displayed by the EPR signal and elucidation of the nature and kinetics of radical reactions. 

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