Experimental Evidence Spin Chemistry Techniques

Measurement of kinetic isotope effects (KIE) provides valuable insight into the mechanism of the PFR. Thus, the absence of any detectable KIE in the rearrangement of C- or 0-labeled 4-methoxyphenyl acetate is in agreement with the absence of an activation energy barrier for the O—CO bond cleavage and with formation of 2-hydroxy-5-methoxyacetophenone by recombination of a caged radical pair produced from a singlet excited state [40, 41]. 

In the PFR of 1-naphthyl acetate, the yield of 2-acetyl-l-naphthol exhibits a positive external magnetic field effect for the ester labeled by the magnetically active isotope C, but no effect for the C ester. This effect is explained by considering hyperfine coupling between H or C of the naphthoxyl radical and the unpaired electron of the acetyl radical, which is in agreement with formation of the in-cage product from a singlet radical pair [42], 

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