Photolysis chemically induced dynamic nuclear

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

Since the heroic early mechanistic investigations, there have been two developments of major significance in radical chemistry. The first was the advent of electron spin resonance (ESR) spectroscopy (and the associated technique of chemically induced dynamic nuclear polarisation, CIDNP) [24], which provided structural as well as kinetic information the second is the more recent development of a wide range of synthetically useful radical reactions [20]. Another recent development, the combination of the pulse radiolysis and laser-flash photolysis techniques, is enormously powerful for the study of radicals but beyond the scope of this book. 

Palm and Dreeskamp (42,43) recently reported a CIDNP (Chemically Induced Dynamic Nuclear Polarization) study of the photolysis of a-(3,5-dimethoxyphenoxy)-acetophenone. From the polarization of the methylene protons they concluded that at least two of the products (diphenyl-1,4-butadione and a-(3,5-dimethoxy-4-hydroxyphenyl)-acetophenone) are formed from the singlet excited state. 

A minor product observed in the photolysis of DBK in aqueous HDTCl (Fig. 5) is the isomeric ketone 4-methylphenylacetophenone (PMAP, 3) 15 18). The question arises as to whether the formation of 3 is due to some special orienting feature of the micelle which operates on the ratio of coupling products 1 and 3. Although 3 has not been detected by the conventional analytical methods for product analyses, (e.g., vpc), some 3 is detectable via chemically induced dynamic nuclear polarization (CIDNP) measurements 28). Thus, it may be that the ratio of formation of 1 to 3 is similar in both micellar and homogeneous solutions, but that the absolute yield of both combination products is so much smaller in homogeneous solution that 3 is not detectable by conventional analyses. 

NMR imaging was used to study the spatial distribution of molecules with chemically induced dynamic nuclear polarization. It is shown that heating of a system during the photolysis can cause a highly nonuniform distribution of reaction products due to a convective effect. 

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