Substrates short-lived transient species

Using these characteristics of lasers, photochemical reactions that cannot be conducted with conventional Hght sources can be brought about, and a multiphoton reaction is one of these processes. Multiphoton reactions can be classified in two categories (1) stepwise reactions through short-lived transient species (Path 1 in Scheme 1) and (2) reactions by concerted multiphoton absorptions (Path 2 in Scheme 1). In many cases. Path 1 predominates when the substrates are complex organic compounds. The transient species can be either long-lived excited states, such as triplet states, or chemical species, such as radicals, carbenes, nitrenes, and thermally unstable molecules. 

The Fenton-type chemistry between Fe q and several rhodium hydro-peroxides yields Rh(IV) species believed to have the general formula L(H20)Rh0, Eq. (11) (72 although this was not established experimentally for these short-lived transients. Just like the other LMO species generated by this route (58 the compounds L(H20) RhO (L = (NH3)4, L, and L ) react rapidly with the Fe + present in solution. The addition of substrates for L(H20)Rh0 resulted in a competition, as shown for methanol in Eqs. (12) and (13). 

To determine the chemical nature, concentration, and kinetics of reactive intermediates, time-resolved techniques are used. To detect short-lived species, an inert matrix at extremely low temperature [7], an extremely high-intensity light source, extremely sensitive detection method, or combination of these methods is used. The method using an intensive light source, called flash photolysis, is a technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce sufficient concentration of a transient species for spectroscopic observation. The method can be applied to follow concentrations of substrates, intermediates, and products as a function of time after the flash, which enables in the elucidation of photochemical reaction mechanisms (kinetic spectroscopy) [8,9],... 

One electron transfer from the highest filled MO of a neutral substrate 170 (Eq. (236) ) to the anode yields a radical cation 171 as product. This may be either a transient intermediate or a stable, long-lived species depending on its substituents and the nucleophilicity of the solvent. The reaction paths of radical cations have been expertly and comprehensively reviewed by Adams 2 5 2 9 so that a short summary seems sufficient at this place. Deprotonation and 1 e-oxidation of 171 with a subsequent Sj l reaction of the resulting cation yields side-chain substitution products 172 (path a), see 9.1. Solvolysis of 171 followed by le-oxidation... 

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