Non-Kekule molecules

Dimethylenefuran-, 3,4-dimethylenethiophene- and 3,4-dimethylenepyr-rolediyl radicals as non-Kekule molecules with tunable singlet-triplet energy spacings 97ACR238. 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

A non-Kekule molecule conceptually formed by fusion of two TMM units and also predicted" " to have a triplet ground state is 2,4-dimethylenecyclo-butane diyl (20), which ultimately was prepared by two independent syntheses. Matrix ESR spectroscopy and gas-phase photodetachment photoelectron spectroscopy (PES) (see Section 4.1.4) eventually agreed that the ground state is triplet. 

Space limitations of this chapter preclude a thorough treatment of this topic. Readers who wish more information will benefit from the many reviews available elsewhere.We will concentrate here on its practical applications to non-Kekule molecules, its strengths, and its limitations. 

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... 

In principle, the domain of non-Kekule molecules should extend over territory as broad as that of ordinary Kekule molecules. Aside from the potentially huge applications in synthesis and in materials science, there are many intriguing theoretical and mechanistic questions to be explored. 

A related area is the chemistry and physics of electronically excited non-Kekule molecules. Many of the non-Kekule molecules have absorption in the near-UV or even in the visible. The description of the excited states of these species offers a... 

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