Tetraradicals

In this section we consider the spin Hamiltonian appropriate to a biradical with weak dipolar coupling and see how ESR spectra of such species should appear. Obviously, it is possible to find triradicals, tetraradicals, etc. treatment of such species is similar, though of course somewhat more complicated. 

Two isomeric ring homologues of naphthoquinomethane also were synthesized during that phase, one (17) a triplet biradical in its ground state, and the other (18), a ground-state quintet tetraradical. ... 

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... 

Figure 27 (a) The cyclohexadiene (CHD)/cZc-hexatriene (cZc-HT) photoconversion problem involves the formation of a common excited state intermediate (cZc-HTF )) and its decay via a conical intersection point (CICHD). (b) Because CICHD has a tetraradical electronic nature, relaxation on S0 may occur along three different routes (a, b, and c) associated with different bond formation modes and different recouplings. (From Ref. 82.)... 

Iwamura et al.[93 have reported the preparation of a branched-chain nonacarbene possessing a nonadecet ground state with the goal of constructing superparamagnetic polycarbenes. The preparation of three-directional, diradicals was reported by Bock et alJ94J and a tetrahedral, tetraradical has been synthesized by Kirste et al. The synthesis, characterization, and physical properties of the perchloro-2,6-bis(diphenylmethyl)-pyridine-a,a -ylene biradical have also been reported. 

Three limiting topologies, based upon 1,3-phenylene coupling unit, can be considered for Class I polyarylmethyl tetraradicals quasi-linear, star branched, and macrocyclic (calix[4]arene based). Structures of polyarylmethyl tetraradicals are illustrated in Fig. 19. 

Star-branched tetraradicals 33-35 possess S — 2 ground states, with relatively strong ferromagnetic couplings, as determined by EPR spectroscopy and magnetization studies (Fig. 21).61,87 EPR spectra for sterically hindered tetraradicals were... 

Fig. 20 Polymer 30. Precursors 31 and 32 to quasi-linear annelated tetraradicals and hexa-radicals, homologs of diradical 10.

Trianions and tetraanions corresponding to triradicals 25-28 and tetraradicals 33-35 were studied by cyclic voltammetry at 200 K.88 Especially useful data were obtained in the-1.0 to —2.2 V potential range (versus SCE) corresponding to the oxidation of polyanions to polyradicals and reduction of polyradicals to polyanions. Typically, three waves were resolved in this potential range with the relative peak currents 1 1 1 and 1 2 1 (or 1 1 2) for trianions and tetranions for trianion, corresponding to triradical 28, only two waves (relative peak currents, 2 1) were resolved. 

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