- Non-Kekulé molecule
A non-Kekulé molecule is a conjugated hydrocarbon that cannot be assigned classical Kekulé structures. Since non-Kekulé molecules have two or more formal radical centers, their spin-spin interactions can cause electrical conductivity or ferromagnetism (molecule-based magnets), and applications to functional materials are expected. However, as these molecules are quite reactive and most of them are easily decomposed or polymerized at room temperature, strategies for stabilization are needed for their practical use. Synthesis and observation of these reactive molecules are generally accomplished by matrix-isolation methods. The simplest non-Kekulé molecules are biradicals.
Tschitschibabin biradical (1907) Yang biradical (1960) Coppinger biradical 1962
A well studied biradical is trimethylenemethane or TMM:
This molecule can be obtained from photolysis of a diazo precursor with expulsion of nitrogen or from photolysis of 2-methylenecyclobutanone with expulsion of carbon monoxide. In 1966 Paul Dowd determined with electron spin resonance that this compound also has a triplet state. In a crystalline host the 6 hydrogen atoms in TMM are identical. Recombination of the two radicals to a cyclopropane full valence compound is only possible when the triplet state converts to the higher energy singlet state first.
The most common use of the TMM framework is in transition metal chemistry and specifically the use in trimethylenemethane cycloaddition reaction. In 1979, Trost et al. published a palladium catalyzed [3+2] cycloaddition of trimethylenemethane. 
Quinodimethanes & PAH's
Non-Kekulé quinodimethanes are biradicaloids of a six-membered ring with methylene substituents. Non-Kekulé polynuclear aromatics are composed of several fused six-membered rings. The synthesis of Triangulene, the simplest non-Kekulé polynuclear aromatic, was first tried by Eric Clar in 1953, but it was not even observed until the syntheses of trioxytriangulene by Richard J. Bushby in 1995 and kinetically stabilized triangulene by Kazuhiro Nakasuji in 2001. A related class of biradicals are para-benzynes.
Teranthene biradical Singlet. max. 3 stabilizing Clar sextets, stable rt, air. 50% biradical, molecular section of graphene Bisphenalenyl biradical Singlet. max. 6 stabilizing Clar sextets, stable rt, air. 42% biradical Pleiadene generation and dimerization
In the oxyallyl diradical (OXA) one methylene group in TMM is replaced by oxygen. This reactive intermediate is postulated to occur in ring opening of cyclopropanones, allene oxides and in the Favorskii rearrangement. The intermediate has been produced by reaction of oxygen radical anions with acetone and studied by photoelectron spectroscopy . The experimental electron affinity of OXA is 1.94 eV.
Non-Kekulé molecules with two formal radical centers (non-Kekulé diradicals) can be classified into non-disjoint and disjoint by the shape of their two non-bonding molecular orbitals (NBMOs).
Both NBMOs of molecules with non-disjoint characteristics such as trimethylenemethane (TMM) have electron density at the same atom. According to Hund's rule, each orbital is filled with one electron with parallel spin, avoiding the Coulomb repulsion by filling one orbital with two electrons. Therefore, such molecules with non-disjoint NBMOs are expected to prefer a triplet ground state.
In contrast, the NBMOs of the molecules with disjoint characteristics such as tetramethyleneethane (TME) can be described without having electron density at the same atom. With such MOs, the destabilization factor by the Coulomb repulsion becomes much smaller than with non-disjoint type molecules, and therefore the relative stability of the singlet ground state to the triplet ground state will be nearly equal, or even reversed because of exchange interaction.
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- ^ Reactive Intermediate Chemistry Robert A. Moss Ed., Wiley-Interscience 2004 ISBN 0471233242
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