Electron spin resonance exchange effects

Coyne, L. M., and Banin, A. (1986). Effect of adsorbed iron on thermo-luminescence and electron-spin resonance-spectra of Ca-Fe-exchanged montmorillonite. Clays Clay Miner. 34(6), 645-650. 

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. 

The NMR spectra of these complexes under ambient conditions exhibit sharp, paramagnetically shifted features that can span up to 400 ppm (90, 134). Electron exchange is fast on the NMR time scale, so there is an effective twofold symmetry, which approximately halves the number of distinct features observed. The sharpness of the resonances is due to the short electron spin-lattice relaxation time of the Fe(II) center, which allows even the CH2 protons adjacent to the coordinated nitrogen atoms to be observed. 

The observation by Carver and Slichter of a positive Overhauser effect for the protons indicates that the relaxation rate of the protons is determined primarily by its contact interaction with the unpaired electron. This is rather interesting because the proton resonance shift which also depends on the contact interaction is small compared to the shifts for the and Na nuclei. No estimates of for the protons due to the contact interaction with the electron spin are available. Such estimates would require a knowledge of the exchange rate of ammonia molecules between a monomer cluster and the bulk. 

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