Biradicals 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... 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

The existence of the biradicals and the multipHcity of the surfaces on which these are formed have not been demonstrated directly however, experimental results (stereochemistry of the reaction, CIDNP [chemically induced dynamic nuclear polarization], radical trapping experiments, and quantum yield measurements) support their existence. Recently, the mechanism of 1,3-migration and oxa-di-Jt-methane reactions in terms of potential energy surface and decay funnels has been described this also supports the aforementioned mechanistic impHcations. The detailed mechanism, however, depends, in a very subtle way, on the structure of the chromophoric system and the presence of the functional groups. 

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