Non-Kekule hydrocarbons

Cyclobutadiene (l)15 is the first member of the series of cyclic conjugated hydrocarbons (Kekule compounds) cyclopropenylidene (2), of cyclic conjugated carbenes trimethylenemethane (3), of the non-Kekule hydrocarbons. 

One of the ways to generate the tetramethylenethane-type diradical 350, an important reference compound in connection with non-Kekule hydrocarbons [147], consists in the thermal isomerization of hydrocarbon 34 at -100 °C [43, 148], Under the reaction conditions, the six-membered ring of 350 ruptures to yield [4]dendralene (3,4-bismethylene-1,5 -hexadiene) (351). 

The double degeneracy of NBMOs in m-[8] has nothing to do with the geometrical symmetry of the molecule, but, rather, with the connectivity of the two radical centres, or the phase relationship of the atomic orbitals in the conjugated system. Therefore, the term topological symmetry has been proposed to describe the connectivity of the carbon atoms carrying the n-electrons and the periodicity of the Jt-orbitals in this class of non-Kekule hydrocarbons. 

Cramer, C. J. and Smith, B. A. 1996. Trimethylenemethane. Comparison of Multiconfiguration Self-consistent Field and Density Functional Methods for a Non-Kekule Hydrocarbon J. Phys. Chem. 100, 9664. 

Scheme 3 Example applications of spin parity to alternant non-Kekule hydrocarbons. All of these are predicted triplet ground states by Hund s law-based models such as that of Longuet-Higgins.65.

W. T. Borden, H. Iwamura, J. A. Berson, Acc. Chem. Res. 27, 109 (1994). Violations of Hund s Rule in Non-Kekule Hydrocarbons Theoretical Prediction and Experimental Verification. 

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). 

Hund s Rule in Non-Kekule Hydrocarbons Theoretical Predictions and Experimental Verifications. 

The ground states of such diradicals can be predicted, using either MO or valence-bond theory. The predictions made by these two methods are almost invariably the same. The advantages of each approach have been discussed, and the computational and experimental research on the most important non-Kekule hydrocarbon diradicals and heteroatom... 

Both semiempirical and ab initio calculations predict a singlet ground state for TMB. The singlet is calculated to lie below the triplet by 2 kcal mol at the TCSCF/ROHF level and by 5 kcal mol when correlation is provided for all the jr electrons. Thus, TMB is a non-Kekule hydrocarbon diradical that is unequivocally predicted to have a singlet ground state. 

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