Structures, alternative spin-density distributions

The preparation and the crystal structures of single crystals of alkali aromatic ion pairs are discussed. A close relationship was found between the ion-pair structures in solution and in the solid state. The physical properties of the pseudo-two-dimensional magnetic alkali biphenyl crystals are reviewed. The effect of spin diffusion manifests itself clearly in the line width and line shape of the exchange-narrowed electron spin resonance (ESR) line. The monoanions of cycloocta-tetraene, produced by X-ray irradiation, rotate rapidly about their eightfold axes. At 20 K this rotation is frozen, and an alternating spin density distribution is found around the ring. The equilibrium position of the monoanion is rotated by 22.5 compared with the equilibrium position of the dianion. 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

FIGURE 7.Ans.l (a) Resonance structures of pentadienyl radical and their symmetry properties, (b) The VB mixing of the resonance structures, (c) The quasiclassical (spin alternant) determinant that dominates the ground-state wave function, and the corresponding secondary determinants, and the resulting spin density distribution (p) in the ground state, (d) The spin distribution in the covalent excited state. 

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