Super-exchange

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

The two spins 5a and 5b are assumed to be local, associated with the two sites a and b, respectively. The parameter J is the so-called exchange-coupling constant, which expresses the strength of the (super)exchange interaction between the... 

Elmah, A. 2000. The magnetic super-exchange coupling in copper(II) acetate monohydrate and a redetermination of the crystal structure. Turkish Journal of Physics 24 667-672. 

Next we shall show that the electronic energy transfer rate can be put into the spectral overlap form. Notice that by ignoring the super-exchange term we have... 

The theory of inter-atomic exchange intoactions (super-exchange) has been reviewed in many excellent articles see for instance Anderson (24), Martin (25), and Ginsberg (26) and the book edited by Willet, Gatteschi, and Kahn (27). Hay, Thibeault and Hoffmann (28) and Kahn and Briat (29) have developed theories which view the exchange interactions fiom a molecular orbital perspective. 

Speromagnetism (Fig. 6.5 d) is a property of some amorphous or poorly ordered materials containing metal-O-metal bonds which, if the metal carries a magnetic moment, can support super-exchange reactions (Coey Readman, 1973). 

Neglecting the super-exchange contribution the thermal average rate constant for the singlet-singlet transfer can be expressed as... 

It should be mentioned that several copper oxide compounds are described by this model. These compounds contain CuO chains and Cu — O — Cu angle 6 is near 90° [7]. In this case the usual antiferromagnetic super-exchange of two nearest-neighbor magnetic Cu ions is suppressed and the exchange integral J12 is... 

Although no evidence has been found that the monomeric bacteriochlorophyll unit, which is intercalated between the special pair and the pheophytin (see Fig. 1), actually acquires an electron, it should be noted that also this unit may act as a mediator via a super-exchange mechanism (Kuznetsov and Ulstrup, 1982 Miller and Beitz, 1981), which is conceptually related to through-bond interaction. 

If a pair of cations is separated by two anions, delocalization superexchange is still possible, though very much weakened, since the cation d orbitals spread out over the entire anion with which they form a partial covalent bond. Correlation superexchange is also weakened by the anion-anion correlation factor. Polarization super-exchange may be competitive in this case, but it is cooperative. It follows that the rules for the sign of 180° cation-anion-anion-cation interactions are the same as those for 180° cation-anion-cation interactions, but the magnitude is reduced, probably by an order of magnitude. 

From the data given in equations (24)-(28), we may conclude that super-exchange-mediated ET through the DNA duplex is significant and that (3 for DNA may be comparable to the upper limit of j3 observed for conjugated hydrocarbon bridges. However, it now seems clear that DNA does not behave as a molecular wire in ET reactions, as has been posited.178... 

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