| Abstract
| - We have analyzed Mössbauer spectra of a model complex ofknown structure with anFe2+(S1=2)−μOH−Cr3+(S2=3/2)center (A) and of itsFe3+(S1 =5/2)−μO−Cr3+(S2=3/2)analog (B). These μ-hydroxo and μ-oxobridged binuclear metal centers display unusual magnetic properties asfound in several diiron−oxo proteins. Ourresults confirm antiferromagnetic spin coupling between Fe and Cr ionswhich results in Seff =1/2 and Seff =1ground states for A and B, respectively. Theisotropic exchange ℋex =JS1·S2 isweaker for the μ-hydroxo (J ≈ 21cm-1) than for the μ-oxo (J ≈ 275cm-1) complex. Spectra recorded at 4.2 K, in fieldsof 0.22−4.7 T, have beenanalyzed with the effective spin Hamiltonian for the ground stateℋeff =βSeff·g̃eff·H++I1·P̃1·I1−βngnH·I1.For complex B, the zero-field splittingSeff·D̃eff·Seffis also included in ℋeff. In applied fields,the 4.2 Kspectra of Fe2+ in A showed hyperfinesplittings which allowed the determination of the followingSeff = 1/2Hamiltonianparameters: 1/3 Trg̃eff ≈ 2.00,= −(18.3,5.6,25.0) T, ΔEQ = +2.87mm/s, η = 0.93, and δFe = 1.21 mm/s.The weak coupling of A allows the zero-field splittingto mix higher spin manifolds with the ground statedoublet,and, to obtain intrinsic parameters, we also calculated the spectra ofFe2+ by diagonalizing the (2S1 +1 = 5) × (2S2+ 1 = 4) matrix of the Hamiltonian ℋ =JS1·S2 ++βSi·g̃i·H}+S1·ã1·I1+I1·P̃1·I1−βngnH·I1.We determined the following parameters for Fe2+: D1 = +4.0 cm-1,E1 = +0.4 cm-1,1/3 Tr g̃1 ≳2.07,ã1/gnβn=−(10.2,3.5,15.6) T. For complex B, we found thatFe3+ has a large quadrupole splitting(ΔEQ = −2.00 mm/s, η =0.22) presumably as a result of anisotropic covalency due to the closeproximity of the bridging O2-. ThislargeΔEQ is comparable to values found indiiron−oxo proteins. Spectra of B in applied fieldsalso displayed hyperfinesplittings, and the following Seff = 1Hamiltonian parameters could be deduced: Deff= +3.9 cm-1, Eeff =+1.7cm-1, 1/3 Trg̃eff = 2.01,= −(33.8,30.9,35.8) T, δFe = 0.52mm/s.
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