Laser flash photolysis atomic transfer kinetics

Shorthand notations such as ET (electron transfer), HAT (hydrogen atom transfer), BDE (bond dissociation energy), NHE (normal hydrogen electrode), CV (cyclic voltammetry), LFP (laser flash photolysis), EPR (electron paramagnetic resonance) and KIE (kinetic isotope effect) will be used throughout the chapter. In addition, recurring chemical compounds such as TEMPO (2,2,6,6-tetramethylpiperidine-Ai-oxyl), HBT (1-hydroxyben-zotriazole), BTNO (benzotriazole-A-oxyl), HPI (iV-hydroxyphthalimide), PINO (phthal-imide-iV-oxyl), NHA (A-hydroxyacetanilide) and a few others will be referred to by means of the capital-letter acronym. 

The final product ArCH ONO is formed in further oxidation of ArCH/ to ArCH/ by CAN and the subsequent reaction with NOj . For toluene derivatives with electron-donating substituents such as the methoxy group, the electron transfer reaction (Equation 4.73) was confirmed by the laser flash photolysis method [44]. For toluene, there is a probability for direct H-atom abstraction (Equation 4.72) with a highly polar transition state. Furthermore, for toluene derivatives with electron-withdrawing substituents, the addition ability of NO3 to phenyl 7t-bonds can be considered on the basis of data for reactions with phenols [41] and furan [45]. To clarify the interchanges in the reaction paths by the substituent in toluene, reaction rate constants for various toluene derivatives were evaluated by flash photolysis [44]. The substituent effect of the rate constants for toluene derivatives was correlated with ionisation energies (lEs) of these substances. The reaction rate for anisole is too fast to obtain accurate rate constants, and only lower limits of the rate constants are obtained (anisole) >310 M -s h For p-nitrotoluene, the rate constant is 2.3T0 M -s IE = 9.5 eV. A deuterium kinetic isotope effect of 1.6 was observed for the reaction of NO3 with toluene and toluene - dg. This indicates that NO3 predominantly abstracts the H atom from methyl groups. In the case of p-xylene, the deuterium isotope effect was not observed [43]. The rate constant forp-xylene (> 2 x 10 M/s) is close to the diffusion-controlled limit in acetonitrile, and consequently selectivity becomes low. 

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