Relaxation metals

Carotenoids incorporated in metal-substituted MCM-41 represent systems that contain a rapidly relaxing metal ion and a slowly relaxing organic radical. For distance determination, the effect of a rapidly relaxing framework Ti3+ ion on spin-lattice relaxation time,and phase memory time, Tu, of a slowly relaxing carotenoid radical was measured as a function of temperature in both siliceous and Ti-substituted MCM-41. It was found that the TM and 7) are shorter for carotenoids embedded in Ti-MCM-41 than those in siliceous MCM-41. 

Perhaps not surprisingly, the most thorough NMR studies of Knight shifts, Korringa relaxation, metal-insulator transitions, and the NMR of the dopant nuclei themselves have been carried out for doped silicon. Since few semiconductors other than PbTe, which presents a considerably more complicated case, have been studied in such detail, it is worthwhile here to summarize salient points from these studies. They conveniently illustrate a number of points, and can shed light on the behavior to be expected in more contemporary studies of compound semiconductors, which are often hindered by the lack of availability of a suite of samples of known and widely-varying carrier concentrations. 

We have seen that copper(II) is a slowly relaxing metal ion. Magnetic coupling of copper to a fast relaxing metal ion increases the electron relaxation rate of copper, as clearly shown by the NMRD profiles of tetragonal copper(II) complexes reacting with ferricyanide (105) (Fig. 38). The electron relaxation time, estimated from the relaxation rate of the water protons coordinated to the copper ion, is 3 x 10 ° s, a factor of 10 shorter than in the absence of ferricyanide. 

A major aspect of the problem is the relationship between J and the effect on the electronic relaxation rates of the slow—relaxing metal ion. More experimental data are probably necessary to make general statements. Finally, we have always assumed the zero field splitting of uncoupled ions to... 

Further discussion of self-diffusion in relaxed metallic glasses and other disordered systems may be found in key articles [7, 10, 14, 18, 19]. 

Small solute atoms in the interstices between the larger host atoms in a relaxed metallic glass diffuse by the direct interstitial mechanism (see Section 8.1.4). The host atoms can be regarded as immobile. A classic example is the diffusion of H solute atoms in glassy Pd8oSi2o- For this system, a simplified model that retains the essential physics of a thermally activated diffusion process in disordered systems is used to interpret experimental measurements [20-22]. 

As discussed earlier, the enzymic reaction catalyzed by glutamine synthetase requires the presence of divalent metal ions. Extensive work has been conducted on the binding of Mn2+ to the enzyme isolated from E. coli (82, 109-112). Three types of sites, each with different affinities for Mn2+, exist per dodecamer n, (12 sites, 1 per subunit) of high affinity, responsible for inducing a change from a relaxed metal ion free protein to a conformationally tightened catalytically active protein n2 (12 sites) of moderate affinity, involved in active site activation via a metal-ATP complex and n3 (48 sites) of low affinity unnecessary for catalysis, but perhaps involved in overall enzyme stability. The state of adenylylation and pH value alter the metal ion specificity and affinities. 

The electron spin(s) of a given metal ion may relax faster if coupled to another metal ion experiencing more efficient relaxation mechanisms. This electron relaxation enhancement depends on the electron relaxation time of the more rapidly relaxing metal ion and on the magnitude of the coupling constant. In the case of isotropic coupling between two metal ions, Eq. (11) has been proposed, which describes the increase in the electron relaxation rates of the more slowly relaxing metal ion (Banci et al., 1991),... 

Geometry optimizations were performed on a super-cell structure using periodic boundary conditions. The (111) surfaces were generally modeled using a 3 x 2 /3 super cell. The metal slab was chosen to be three atomic layers thick, and a 15 A vacuum layer was used to ensure that there were no interactions between the surface adsorbates on one layer and the next slab. The first metal layer was allowed to relax, while the bottom two layers of the platinum atoms were held fixed in their bulk position. All atomic coordinates of the adsorbed species and the metal atoms in the relaxed metal layers were optimized to a force of less than 0.025 eV A-1 on each atom. 

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