Non-TMM Diradicals

Conjugated hydrocarbons for which all Kekul structures leave two n electrons nonbonded are called non-Kekule diradicals. Longuet-Higgins proved that, unless such molecules have rings with 4/i atoms, these molecules do indeed contain two NBMOs that are occupied by a total of two electrons. As shown in Figure 10. trimethylenemethane (TMM), tetramethyleneethane (TME), m-benzoquinodimethane (MBQDM) and tetramethyleneben-zene (TMB) are all non-Kekule diradicals. 

Diradicals represent the most clear-cut application of the SF approach because in these systems the non-d namical correlation derives from a single HOMO-LUMO pair (e.g., n and n in twisted ethylene). In this section we present results for methylene and trimethylenemethane (TMM). 

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). 

Figure 10 Four of the most important non-Kekul6 hydrocarbon diradicals trimethylenemethane (TMM), tetramethyleneethane (TME), m-benzoquinodimethane (MBQDM), and tetramethyleneben-zene (TMB)...

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