Biradical Kekule

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

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). 

A distinction is made between conjugated Kekule biradicals such as 3, for which at least one Kekule structure can be drawn, and non-Kekule biradicals such as... 

Since the time when the thermally induced methylenecyclopropane rearrangement (A - B) of Feist s esters was first observed by Ullman in 1959, rearrangements of a large number of methylenecyclopropane derivatives have been subjected to kinetic, stereochemical, and theoretical studies. The main objectives of these efforts were to understand the role and nature of the trimethylenemethane biradical intermediate (C ) in the rearrangement process. Considerable attention has focused on the theoretical and experimental elucidation of the relationship among structure, spin state, and reactivity of this simple non-Kekule molecule as well as on its applications as a synthetic and practicaP" intermediate. 

Kubo T, Shimizu A, Uruichi M, Yakushi K, Nakano M, Shiomi D, Sato K, Takui T, Mraita Y, Nakasuji K (2007) Singlet biradical character of phenalenyl-based Kekule hydrocarbon with... 

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