Non-Kekule structure

The stable anion-radical in Scheme 3.63 contains two perchlorotriphenylmethyl radical units linked by an all-trani-p-divinylbenzene bridge. At 200 K, the unpaired electron of the anion-radical is localized (within the ESR timescale) on one stilbenelike moiety only. At 300 K, thermal activation forces the nnpaired electron at one strong electrophilic center to move to another one. Such an electron transfer takes place between two eqnivalent redox sites (Bonvoisin et al. 1994). In contrast to this situation, no electron transfer was observed for the anion-radical that contains two perchlorotriphenylmethyl radical units linked by an all-trani -m-divinylbenzene bridge (Rovira et al. 2001). Such results can be ascribed to the localization of frontier orbitals in the meta-isomeric anion-radical because of the meta connectivity of this non-Kekule structure. 

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). 

The electronic structural change of radical-substituted diarylethenes accompanying the photoisomerization is the transformation between a disjoint non-Kekule structure and a closed-shell Kekule structure. One may infer from the above consideration that the interaction between spins in the open-ring isomer of diarylethene is weak, whereas significant antiferromagnetic interaction takes... 

We must emphasize that in the PPP-VB calculations referred to above, we employed all the covalent structures for a given system. However, these results remain practically unchanged when only Kekule structures Eire used. Merely in cases when only one Kekule structure exists is it advisable to employ one or more additional non-Kekule structures. 

Non-Kekule structures are of importance for the design of organic ferromagnets. Having prepared a hexacarbene with a tridecet ground state [21] Iwamura and co-workers have now been able to obtain a branched nonacarbene, 25, which possesses the record number of 18 unpaired electrons [22]. 

The classification into Kekule and non-Kekule diradicals is mainly based on the difference in their resonance structures. From the proceeding discussions, however, such a classification does not closely relate to the relative stabilities and spin preference of TT-conjugated diradicals. For example, some non-Kekule diradicals, such as 1 and 8, prefer a triplet ground state, but some others (like 7) have a singlet ground... 

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

It is true that in some cases, the spectroscopic data on a reactive intermediate are so persuasive that the connection between structure and spectroscopic features is firm. However, in general this will not be the case, and additional spectroscopic or preparative criteria will have to be provided. So we are faced with the question How can we connect the information obtained, for example, from observations in matrices or in solution-phase fast kinetic studies, to molecular structure How do we know that the results of these experiments, using what we hopefully call direct methods, really pertain to the species we are trying to characterize I attempt to deal with this issue in what follows. Since the methods used vary from one class of non-Kekule species to another, specific classes are individually discussed, and special techniques are introduced as needed. Electron spin resonance spectroscopy has played such a pervasive role that it will be useful to give first a brief outline of that method. 

We can summarize this section on TME and its hydrocarbon derivatives with the observation that it is not easy to test the major predictions of the theoretical model of disjoint non-Kekule compounds. Whether or not the singlet is actually the ground state in any given case depends on subtle particularities of structure and conditions of measurement. Much of the contention of the last decade or more in this area focused on these difficulties, but many of those difficulties are suppressed in the case of tetramethylenebenzene (TMB). 

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... 

It may be fairly said that the field of non-Kekule compounds has expanded from a few curious and esoteric molecules to an active domain of inquiry and a potential source of materials for practical application. Theory and experiment have interacted fruitfully in these developments, but some serious limitations remain. For example, for many years, experimentalists have operated under the imperative of structural simplification. The ideal is to construct molecules that embody the features of theoretical interest but are as free as possible of complicating impedimenta such as extra substituents or other structural elements left over from the synthesis itself and not readily removed. 

The amount by which the total energy of the 77-electrons lies below that of the localized orbitals in Kekule structures, termed the delocalization or resonance energy, is decreased if the conjugated planar framework is distorted into a non-planar conformation, because maximum overlap of 77-orbitals is then not possible. Coulson (1958) has shown that the effect of bending a regular planar benzene ring (Fig. la)... 

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