Radical Polyradical

For polyatomic radicals in the gas phase the above Spin-Hamiltonian does not apply and four magnetic hyperfme coupling constants a, b, c, d are needed to describe the interaction between a nuclear and the electron spin. These are defined and explained in the introduction to the tables on inorganic radicals. Polyradicals and certain radicals on transition metal complexes have N unpaired electrons located on different molecular segments k. Their Spin-Hamiltonian is... 

All the theories predict that polyradicals [14] consisting of w-diphenylmethyl units connected at w-positions should have S = mil and spin multiplicities of w + 1. The exchange interactions between the radical centres in [14] are estimated to be 0.2055 and 0.0012 eV for the first and second neighbours, respectively (Tyutyulkov and Karabunarliev, 1986). 

In principle, an organic molecule can accept as many electron pairs as it has low-lying vacant orbitals. In the same way, high-lying occupied orbitals can release not a single, but several electrons. Such multielectron processes can result in the formation of polyion-polyradicals. As will be seen from this section, the main topic of interest in poly(ion-radicals) consists of their spin multiplicity. 

From the materials just mentioned earlier, one can conclude that mutual meta orientation (meta through a benzene) of the spin-bearing moieties is an indispensable condition for the existence of triplet states in aromatic di- or tri-(cation-radical)s. However, in fact, these systems have both singlet and triplet forms, and the questions are about what is the difference in the corresponding energy and which state is more stable. Stability of the polyion-polyradicals is also a very important factor, especially in the sense of practical application. Let us consider several relevant examples. 

The catalyst thus acts as a sort of polyradical influencing the course of the reaction in the same manner in which the introduction of free radicals into a homogeneous medium affects the course of a homogeneous reaction. In both cases, the reaction is accelerated because of the participation of the free valencies. In the case of heterogeneous catalysis, these free valencies are introduced by the catalyst itself. In the final account, it is they who sustain and regulate the process. 

Our choice was the two series of dendritic polymers 5 and 6, depicted in Figure 4, which have all their open-shell centers (or trivalent carbon atoms) sterically shielded by an encapsulation with six bulky chlorine atoms in order to increase their life expectancies and thermal and chemical stabilities. Indeed, it is very well known that the monoradical counterpart of both series of polyradicals, the perchlo-rotriphenyl methyl radical, shows an astonishing thermal and chemical stability for which the term of inert free radical was coined. The series of dendrimer polymers 5 and 6 differ in the nature and multiplicity (or branching) of their central core unit, N, as well as in their branch-juncture multiplicities, N Thus, series 5 has a hyperbranched topology with = 3 and = 4, while dendrimer series 6 has a lower level of branching with = 3 and = 2, and the topology of a three-coordinated Cayley tree. 

R. A. Forrester, K. Ishizu, G. Kothe, S. F. Nelson, H. Ohya-Nishiguchi, K. Watanabe, and W. Wilker, Organic Cation Radicals and Polyradicals, in Landolt Bornstein, Numerical Data and Functional Relationships in Science and Technology, Vol. IX, Part d2. Springer-Verlag, Heidelberg, 1980. 

Fig. 6 High spin polyradical PAM26 (26 radical sites), designed by Rajca group using spin parity modeling.

Activation of elemental metals by mechanical methods (Sec. 3.4) in the presence of organic acceptor molecules in solid phase, described in detail [14], allows us also to obtain metal-polyradical complexes [206,216-219]. The formed radical-pair species have some unusual properties [217,218]. Compared with triplet radical pairs generated photochemically with the same donor-acceptor composition, these mechanically induced species appear to be much more stable [216]. As an example... 

It is again plausible that an additional non-localized bond system develops within the Si layer, causing the color (SiX) derivatives exhibit a metallic lustre and a certain degree of conductivity, which supports this possibility. The dark color of the layered silicon and the relatively great stability of the radical state show that the effect is particularly strongly developed in this substance. The radical electrons presumably undergo substantial resonance interaction with the electron system of the Si-Si bonds, possibly giving a resonance-stabilized polyradical. 

Spin Hamiltonian Valence Bond Theory and its Applications to Organic Radicals, Diradicals, and Polyradicals... 

As the MSADs have the largest coefficients in the ground-state wave function, one may expect that for alternant free radicals or polyradicals, the dominant positive spin density will be largest on the atoms that bear an alpha spin in the MSAD. An example that was analyzed in Chapter 7 is the allyl radical, where the MSAD predicts positive spin densities at positions 1, 3 and a negative density at position 2. Similarly, the MSAD of benzyl radical predicts positive spin densities on the benzylic carbon and on the ortho and para positions. 

Figure 4.3. A 2nd generation, perchlorinated polyradical constructed to investigate solid state radical stability.

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

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