Coupling anti- ferromagnetic

We also mention the octanuclear chromic wheels [CrF(02CCMe3)2]8 (41) and [Cr8(0H)8(02CPh)i6] (42), which contain eight Cr(III) ions anti-ferromagnetically coupled. 

Type 3 Cu(II) dicopper centre strong absorption in the near UV (Amax 330 nm) no EPR spectra, the two coppers are anti-ferromagnetically coupled. 

The active site in the aerobic, inactive D. vulgaris enzyme shows no EPR signals. The FTIR spectrum indicates the presence of a bridging CO, in addition to the end-on bound CN/CO pair to each iron atom (Fig. 7.2, trace A). We presently assume that both Fe atoms in the bimetallic site are low-spin ferric and that their S = 1/2 systems are anti-ferromagnetically coupled to a diamagnetic state. 

ANTI-FERROMAGNETIC COUPLING j (below NEEL TEMPERATURE Tn) j HEISENBERG J< 0 (H = - 2JS Sj) j... 

Spectral studies of the native enzyme are difficult to interpret due to simultaneous contributions from all three types of copper. The EPR spectrum (Fig. 38) of native laccase98) shows features attributable to the type 2 (g = 2.237, gx = 2.053, A = 206 x 10-4 cm-1) and type 1 (gz = 2.300, gy = 2.055, g = 2.030, Az = 43 x 10-4 cm-1) coppers. The type 3 site is EPR-non-detectable and diamagnetic (-2 J >550 cm-1)56,57). This site should be compared to the met derivative of hemocyanin and tyrosinase only in the sense that it appears to contain two anti-ferromagnetically coupled copper(II) s lacking an EPR signal. A characteristic Blue band of the type 1 copper is seen in the optical spectrum at 614 nm (c = 5700 M-1 cm-1). The shoulder observed at 330 nm (e 2800 M-1 cm-1) was originally associated with the type 3 copper site, as it reduces with two electrons at... 

Thereafter, crystals were brought back to the aerobic 25% MPD solution, buffered with 50 mAf sodium phosphate, pH 5.5. This procedure is based on Avigliano et al. s (157) method of preparing T2D ascorbate oxidase in solution and was modified by Merli et al. (159) for use with ascorbate oxidase crystals. The 2.5-A-resolution X-ray structure analysis by difference-Fourier techniques and crystallographic refinement shows that about 1.3 copper ions per ascorbate oxidase monomer are removed. The copper is lost from all three copper sites of the trinuclear copper species, whereby the EPR-active type-2 copper is the most depleted (see Fig. 10). Type-1 copper is not affected. The EPR spectra from polycrystalline samples of the respective native and T2D ascorbate oxidase were recorded. The native spectrum exhibits the type-1 and type-2 EPR signals in a ratio of about 1 1, as expected from the crystal structure. The T2D spectrum reveals the characteristic resonances of the type-1 copper center, also observed for T2D ascorbate oxidase in frozen solution, and the complete disappearance of the spectroscopic type-2 copper. This observation indicates preferential formation of a Cu-depleted form with the holes equally distributed over all three copper sites. Each of these Cu-depleted species may represent an anti-ferromagnetically coupled copper pair that is EPR-silent and that could explain the disappearance of the type-2 EPR signal. 

Compounds 25-27 were characterized by 111, 31P NMR, and X-ray crystallography. They are diamagnetic and the large Zr-Zr separations of 3.64 25, 3.57 26, and 3.61 A 27 indicate that there is no direct metal-metal bonding, and therefore the anti-ferromagnetic coupling must be ligand-mediated. 

Various approaches can be used to obtain a controlled alignment of the magnetization in magnetic multilayers. The most important material systems for devices are anti-ferromagnetic coupled multilayers and spin valves, which is discussed below. 

The low temperature peak (201 C -CuHZSM-5/I 208 C -CuNaZSM-5/I) is due to reduction of a oxidic clusters containing anti-ferromagnetically coupled Cu +ion. The formation of clusters just in these samples could have been expected because in these samples (unlike to the other zeolite samples) copper has been introduced by impregnation. 

Figure 15.9 Spinel unit cell showing anti ferromagnetic coupling between A (T-sites) and B (O-sites) sublattices.

The spectroscopic and magnetic data leave little doubt that a type of oxidative addition mechanism is operative in O2 binding by hemery-thrin. The two electrons necessary to reduce O2 are furnished by the two Fe(II) ions, which from the magnetic and spectral data are anti-ferromagnetically coupled Fe(III) ions in the [02 "-Fe(III)-Fe(III)] oxidative addition product. Two-electron reductive elimination of O2 by this unit completes the reversible process. The oxidative addition process is shown in Figure 17. 

The EPR analysis of the MOF compound Cu3(BTC)2(H20)3 H2O (BTC = benzene 1,3,5-tricarboxylate) revealed the presence of cupric ions in two different chemical environments [188] Cu2° clusters in the paddle-wheel building blocks of the MOF giving rise to an anti-ferromagnetically coupled spin state and Cu monomeric species accommodated in the pores of the system. In a next step [189], the authors substituted Cu ions with Zn thus forming paramagnetic binuclear Cu-Zn clusters that allowed EPR monitoring of the interaction of the Cu ions with adsorbates such as methanol. 

---