From Introduction To Dynamic Spin Chemistry: Magnetic Field Effects On Chemical And Biochemical Reactions

8.1 Historical Introduction

CIDNP, CIDEP, and MFEs on chemical reactions through radical pairs have been reviewed in the former chapters, but there is another group of reactions which can give similar phenomena. Because biradicals (or diradicals) have two radical centers in one molecule, the reactions through biradicals are also expected to be influenced by external magnetic fields as those through radical pairs. It is noteworthy that biradicals differ from radical pairs in the absence of diffusive separation of the component radicals. From this point, the reactions through biradicals have the following characteristics: (1) Because the lifetime ( ? BR) of biradicals are much larger than that ( ? RP) of radical pairs in fluid solutions, MFEs of biradicals are expected to be much larger than MFEs of radical pairs in fluid solutions and similar to MFEs of radical pairs in micellar solutions. (2) Because the distance between two radicals in a biradical can be changed artificially, different MFEs can be expected in the reactions of biradicals than those of radical pairs.

In 1973, Closs and Doubleday studied the photochemical ?-cleavege of alicyclic ketones in chloroform at room temperature in the presence of magnetic fields below 2.3 T [1a]. Their reactions are illustrated in Fig. 8-1.

Figure 8-1: Reaction scheme of the photochemical ?-cleavege of alicyclic ketones (Ia-e and IV). (Reproduced from Ref. [1a] by permission from The American Chemical Society)

Irradiation of ketones Ia-e and IV yieded the corresponding alkenals. Each irradiation was carried...

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