Antiferromagnetic coupled pairs

Fig. 6.11. Predicted temperature dependence of the shifts of nuclei sensing either the S = % or the S = 2 spin of an antiferromagnetically coupled pair with J — 300 cm-1.

These results indicate that the antiferromagnetic transition in TbB44Si2 is actually of dimer-like nature, where non-magnetic substitution leads to broken pairs resulting in free spins (Mori, 2004). The antiferromagnetically coupled pairs do not feel the effects of dilution of other terbium sites, and therefore, are stable below Hq which is a unique value regardless of the doping content. 

The emission can be further resolved when the emission spectra are studied at liquid helium temperature, when the R line peaking at 685 nm is assigned to single chromium ions and the one peaking at 705 nm to antiferromagnetically coupled pairs of... 

These last results show that, for example, although the ground S = 0 state of an antiferromagnetically coupled pair of octahedrally-coordinated Ni2+ or Cr3+ ions get no contribution though Eq. (37), we do now get a contribution of / = per ion, where A is the hgand field... 

The iron center of protein R2, which is one of the focal points of this review, is an antiferromagnetically coupled pair of high-spin ferric ions in the active state. Figure 3 shows the known redox states of the iron/ tyrosyl radical site in the protein. The crystal structure of the met form, i.e., the diferric form without radical, around the iron site (Figs. 

Table 2 The relation between and J for antiferromagnetically coupled pairs of Si spins.

Four strongly downshifted signals in each spectrum, between 50 and 110 ppm, were assigned to the four CB protons of the cysteines coordinating the Fe ". The contact shifts of the protons reflect the coordination of cysteine to the Fe " of the antiferromagnetically coupled Fe "-Fe" pair as the cysteine protons sense the spin down orientation of the Fe " (S = ) site. This is supported by the observation that the temperature dependence of the cysteine H" protons (measured between 276 and 308 K) follows Curie behavior (decreasing contact shift with increasing temperature). 

D. Dolphin Clearly the fact that the a3-Cug pair can be antiferromagnetically coupled shows that these two centres can communicate with each other. It is not clear, however, that this particular "intermediate" has any enzymatic significance. 

The observation that XmT of (bpym, Se) is temperature-independent between 50 and 110 K and that no maximum in the susceptibility occurs at temperatures below 50 K suggests that no intramolecular antiferromagnetic coupling in pairs remains in this temperature regime. In fact the magnetisation curves [9] of (bpym, S) and (bpym, Se) at 1.9 K clearly indicate the different nature of the spin pairs involved in each compound in the ground state (Fig. 8). 

Type III copper is characterized by antiferromagnetic coupling of a pair of copper atoms and strong absorbance at 330 nm. A single type III pair is found in hemocyanin, in which it is involved in O2 transport, and in tyrosinase, in which an oxygen is inserted into substrate. A pair of copper atoms is also found in the multi-copper ascorbate oxidase, but it is coupled to the type II copper in a trinuclear arrangement. 

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