Steric Control over Spin Delocalization

The ii-stilbene molecule is planar in crystalline form, in gas phase, and presumably in solutions, although the phenyl group may be rotated as much as 32° to reduce nonbonded repulsions between hydrogen atoms (Waldek 1991, Meier 1992). This modest twisting still allows sufficient 

3 Unpaired Electron Localization in the Field oe Two or More Atoms 

This section is devoted to odd-electron multicenter bonds. Two-center three-electron bonds were described by Pauling (1931). Since the first prediction by Pauling, a great deal of interest has been expressed in such systems. The following text provides a list of important references. 

Clark (1988) calculated the stabilities of diverse species with odd-electron o bonds. Cataldo et al. (2001) produced evidence for the existence of the anion-radical with an intramolecular one-electron bond between two phosphorus atoms in a macrocyclic structure of the metacyclophane type. Dutan et al. (2003) observed a similar situation for the anion-radical of a di (m-silylphenyl-enedisiloxane) analog. 

According to ESR spectra and MO calculations for these anion-radicals, the unpaired electron is indeed associated with the nitrogen-nitrogen n orbital (Bushby and Ng 1996). 

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A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

The cation-radical of permethyldithia[6]radialene provides one, even more pertinent, example of the steric control over spin delocalization (Gleiter et al. 1996). As described, the unpaired electron is delocalized only in one half of this cation-radical, namely, within the limits of the 2,3-dithiatetramethyl-butadiene unit. Owing to the steric demand of the isopropilidene groups, two of the four methylene groups are twisted, whereas the other two are coplanar (see Scheme 3.16). It seems... 

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