Conclusions
Obtaining the rate constants in the absolute values affords many advantages compared with the relative rates. It had been believed that the absolute values obtained by the flash photolysis method were not precise enough to examine substituent effects such as the Hammett equation. In this review, however, one can recognize that we can examine substituent effects and solvent effects with the absolute rate constants obtained by the flash photolysis. Especially, comparison among the reactivities of different attacking radicals becomes possible with measurements of the absolute rate constants.
For the reversible reactions, we found that the selective radical trapping method is effective to determine the rate constants and relative equilibrium constants. The both quantities are very useful for examining the LFER of the free radical reactions. The potential energy diagrams are easily illustrated with the use of the rate and equilibrium constants. The deviation of LFER affords information about the transition state of the reaction.
The theoretical approach for the reactivity of the free radicals may need more absolute rate constants and precise equilibrium constants for various reaction systems. For this purpose, photochemical methods for the analysis of fast reactions are useful.
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Ito, O. Flash photolysis study on reversible addition reactions of thiyl radicals. Res. Chem. Intermed. 21, 69–93 (1995). https://doi.org/10.1163/156856795X00080
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DOI: https://doi.org/10.1163/156856795X00080