Splitting zero-field, rhombic

Fig. 9. Zero-field splitting of the S = 2 ground state of [Fe3S4] clusters. Constructed for D = -2.5 and E/D = 0.23, where D and E are the axial and rhombic zero-field splitting parameters, respectively.

Molecular g value, axial and rhombic zero field splitting, exchange interaction... 

The rhombic zero-field splitting in non-Kramer s doublets is more pronounced the lower the ms value of the doublet. For example, for S = 2 the zero-field splitting of the 1 doublet is 6E, while that of the 2 doublet is 3E2ID (ibidem 212), and obviously 3E2/D 6E because EID < 1/3. Consequently, the effective of lower... 

So the zero-field splitting can be seen to be a function of the rhombicity, and ranges from 2-D in axial symmetry to (4/3)DV3 for maximal rhombicity (EID = 1/3). 

For higher integer spins the number of allowed zero-field interaction terms further increases, and so does the convolution of comparable effects, except once more for a unique term that directly splits the highest non-Kramer s doublet. For S > 3 we have the addition, valid in cubic (and, therefore, in tetragonal, rhombic, and triclinic) symmetry ... 

As an illustration consider then a zero-field Hamiltonian for S = 4 in which we have retained only the familiar axial D- and rhombic E-term plus the cubic terms that split the non-Kramer s doublets in first order ... 

High-spin hemes have a strong axial zero field splitting which splits these states into three doublets, and because the value of the zero field splitting parameter D is positive (typically 5-10 cm ) for hemes the doublet consisting of the Sj = -1-, — states is lowest lying. There is usually also a smaller rhombic term E which cannot split the Kramers doublets but mixes states differing in by 2. The main net effect of E is to remove the equivalence of the x and y (in heme plane) directions so that g, and gy are unequal. 

Recently high-field/high-frequency EPR spectra of [Mn(taa)] were recorded for a powder sample at 1017.6 GHz. Analysis of the spectra showed the presence of a rhombic zero-field splitting in the HS molecule [30], which is consistent with the symmetry lowering by EQ c Jahn-Teller distortions and the dynamic reorientation of deformation dipoles slower than the time scale of 10-12 s. 

The Spin Hamiltonian Formalism Determination of Zero-Field Splitting D and Rhombicity E/D of Paramagnetic Iron Centers by Mossbauer Spectroscopy... 

Mossbauer spectroscopy can also be used to obtain information about fine structure parameters like zero-field splitting D and rhombicity parameter E D. The zero-field splitting is a second-order effect, which can be classically visualized as resulting from the circular currents generated in the atomic shell by the electron spin. In Section 3.10.5, we describe how D and E D can be calculated. 

Figure 11. The best fit has been found for 5 = 0.7 0.01 nuns, Aitg =—3.25 0.01 nuns, " =(2.11, 2.19, 2.00), "T/ nMn = (-45, 10, 19) T, tj = 0.74 0.1, D = 7.2 0.5 cm, and EID = 0.16 0.02. The zero-field splitting parameter D = 7.2 0.5 cm is comparable to that found for rabredoxin from Clostridium pasteurianum (D = 7.6cm Surprisingly, the rhombicity parameter E/D = 0.16 0.02 differs somewhat from that of rubre-doxin from C. pasteurianum (E/D = 0.28). The hyperfine coupling tensor has been determined to be A = (-14.5, -9.2, -27.5) T. The anisotropy of the hyperfine conpling tensor is cansed by spin-orbit confribntions to the internal magnetic hyperfine field.

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