Exchange coupling - strong

The spin Hamiltonian for a biradical consists of terms representing the electron Zeeman interaction, the exchange coupling of the two electron spins, and hyperfine interaction of each electron with the nuclear spins. We assume that there are two equivalent nuclei, each strongly coupled to one electron and essentially uncoupled to the other. The spin Hamiltonian is ... 

Consider now the limiting case of strong exchange coupling. When / > > A, R x J, and the 43 and 31 transitions are forbidden. The 42 and 21 transitions are at equal energy and so we have only ... 

Many multiple copper containing proteins (e.g., laccase, ascorbate oxidase, hemo-cyanin, tyrosinase) contain so-called type III copper centers, which is a historical name (cf. Section 5.8 for type I and type II copper) for strongly exchange-coupled Cu(II) dimers. In sharp contrast to the ease with which 5=1 spectra from copper acetate are obtained, half a century of EPR studies on biological type III copper has not produced a single triplet spectrum. Why all type III centers have thus far remained EPR silent is not understood. 

FIGURE 11.6 EPR of the copper dimer in pure copper acetate powder. Strong exchange coupling gives an S = 0 ground state and an S = 1 excited state at 2J 300 cm 1. At T = 48 K the triplet is hardly populated, and the spectrum is dominated by a trace of monomeric copper. 

As indicated earlier, type 1 structures will usually lead to zero or weak intramolecular exchange coupling, while type 2 may lead to weak to strong ex-... 

The combined effects of the dipolar and exchange interactions produce a complex frequency-dependent EPR spectrum, which can however be analysed by performing numerical simulations of spectra recorded at different microwave frequencies. When centre A is a polynuclear centre, the value of its total spin Sa = S, is determined by the strong exchange coupling between the local spins S, of the various metal sites. In this case, the interactions between A and B consist of the summation of the spin-spin interactions between Sb and all the local spins S, (Scheme II). The quantitative analysis of these interactions can therefore yield the relative arrangement of centres A and B as well as information about the coupling within centre A. 

Cohydrolysis of MnLCls (L= 1,4,7-triazacyclononane) and RuL Cl3 H20 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane) in equimolar amounts in methanol containing an excess of sodium acetate gives rise to the asymmetric heterodinuclear species [L Ru(/i-0)(/u-CH3C00)2MnL]PFg on addition of sodium hexafluorophosphate. A model to interpret the strong spin-exchange coupling in this complex was presented." ... 

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