Biradical disjoint

The NBMOs of the singlet state of such a biradical are called disjoint in recognition of the fact that they can be considered to be confined to different sets of atoms. The net result is that in such molecules, the singlet-triplet gap will be small, and the singlet may become the ground state. Borden and Davidson pointed out that TME (5) is the parent molecule of this non-Kekule class, which we now call disjoint. 

An early attempt to test the disjoint hypothesis compared the magnetic properties of two isomeric tricyclic m-quinonoid non-Kekule molecules 17, formally a biradical with tetraradical resonance structures, and 18, formally a tetraradical (Section 2.3). These molecules belong to the point groups C2 and C2v, respectively, and it will be mnemonically convenient to use those descriptors in what follows. The test derives from the recognition that the connectivities of the two molecules... 

The non-Kekule compounds we have considered so far are all stmcturally related to TMM and TME, and the disjoint and parity methods for predicting qualitatively the ground-state spin give similar results. However, the two methods do not agree in the case of another type of non-Kekule structure, of which the parent compound is the biradical 2,3,4-trimethylenepentane-l,5-diyl (56), commonly called penta-methylenepropane (PMP) (Scheme 5.11). 

Section 4.2, any normalized linear combination of degenerate wavefunctions is an equally valid wavefunction. The linear combinations of the NBMOs with the smallest possible amount of local overlap for 3 and 17 are shown in Figure 5.14. Those of 3 are separated entirely in space to the starred and unstarred sets of atoms. Hence 3 is a disjoint biradical. The same cannot be achieved by any linear combination of the NBMOs of 17 it is classified as a nondisjoint biradical. 

Figure 14.32 C shows the NBMOs of m-xylylene, another prototype high-spin system. This structure also has two more than non- atoms. As such, its NBMO are non-disjoint, and a triplet state for the biradical is preferred. In our discussion of organic magnetic materials in Section 17.3 we will see that this /non- approach to predicting spin states can be extended to a remarkable degree, allowing the rational design of very high-spin organic molecules.

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