Rhombic magnetic anisotropy

In almost all cases the admixture of excited states is anisotropic that is, the observed g value varies according to the orientation of the paramagnetic species in relation to the applied magnetic field (orientation-dependent). The g-factor anisotropy is characterized by three principal g values, namely, gxx, gyy, and g--. When these three values are different, the symmetry is defined as rhombic and in the case of axial symmetry, gxx = gyy gzz. In the orientation-independent (isotropic) situation the g factor is represented by a single value. This is also true if the species paramagnetic is in a solution of low viscosity (water) where the molecular tumbling causes all the g factor anisotropy to be averaged out (Knowles et al., 1976 Campbell and Dwek, 1984). 

An analysis of variable-temperature H-NMR spectra showed that Fe(II)-OEC possesses rhombic magnetic anisotropy (85JA4207). 

In rusticyanin and pseudoazurin, the axial Met ligand adopts a different orientation than in azurin, resulting in a stronger Cu(II)-S8 Met bond and a tetragonal distortion. The magnetic anisotropy tensor is rhombic in the Co(II)-substituted proteins, and the pseudocontact contribution to... 

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.

The construction of a numerical effective Hamiltonian from accurate electronic structure calculations permits us to determine the complete Z9-tensor and therewith the orientation of the magnetic axes frame of the system with its easy axis or easy plane, depending on the relative energies of the different Ms components of the triplet. When the magnetic axes frame coincides with the cartesian axes frame, D is diagonal and the energy levels of the triplet can be described with two parameters the axial anisotropy D and the rhombic anisotropy E as defined in Eq.2.16. Hence, the symmetric anisotropic interaction of the 5 = 1/2 spin moments, which by themselves are isotropic by definition, makes that the total spin moment of the system is no longer fully isotropic. 

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