Diradicals nonconjugated

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

In the preface to Diradicals, Borden writes It seems almost as hard to define what diradicals are as it is to study these reactive intermediates [1]. Salem and Roland described a diradical as an atom or molecule in which two electrons occupy two degenerate or nearly degenerate molecular orbitals [2]. A few examples befitting this description include the conjugated non-Kekule hydrocarbons trimethylene-methane (TMM, 1), tetramethyleneethane (TME, 2), and wc/a-quinodimethane 3, as well as the nonconjugated 1,3- and 1,4-diradicals (diyls) trimethylene (4) and tetramethylene (5), and the now very familiar benzene 1,4-diyl 6 (Fig. 1). 

D. A. "Nonconjugated Diradicals as Reactive Intermediates" in Diradicals, W. T. Borden (ed.) Wiley-Intersdence, New York, 1982. 

As noted in the introduction, nonconjugated diradicals are those in which the partially filled orbitals reside on two different carbons that are connected by one or more saturated carbons or by unsaturated carbons in which the it bonds do not overlap the two radical centers. Two types are discussed here 1,3-diradicals, in which the radical centers are separated by a single carbon, and 1.4-diradicals, in which two carbons separate the radical centers. The archetypal 1,3-diradical is trimethylene (TM), the diradical formed by cleavage of a C-C bond in cyclopropane and tetramethylene, the diradical formed by breaUng a C-C bond in cyclobutane, is the archetypal 1,4-diradical. 

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