Zero-field splitting parameter space

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

In contrast, when the active space is restricted to the spin-only kets, the influence of all attainable excited states manifests itself in filling the magnetic parameters (tensors). In such a case the g-tensor deviates considerably from the free-electron value, the temperature-independent paramagnetism appears substantial and the spin-spin interaction tensor transforms to high values of the zero-field splitting parameters (D and E). 

In the columns for g and A, there is only one numerical value listed for each, whenever the spin Hamiltonian appears to be isotropic. Where an anisotropic (e.g. axial) spin Hamiltonian is applicable, the principal values of the g and A tensors are always preceeded by the appropriate symbols (e.g. g, gj or A,B). Due to space limitations, special inserts into the general table appear, whenever zero field splitting parameters are required in the description of the EPR spectrum. In this case, the parameters employed are specified in the headline to the insert, consistent with the appropriate spin Hamiltonian, e.g., D and E if the symmetry is lower than axial. 

Abbreviations CF - crystal field SH - spin Hamiltonian ZFS - zero-field splitting MA - magnetic anisotropy TIP - temperature-independent paramagnetism MP - magnetic parameter averaged (gav, /tip), axial (gz> g > D /up) CSC - complete space calculation. 

Since the position of the levels is a function of two parameters, no simple relationship can be deduced for the relative position of the crystal-field levels. If Bl is accidentally equal to zero, an equal spacing will be found between the three levels. The degenerate I 2) level will be at the intermediate position in all cases. Because one can determine the Bq parameter from the splitting of a 7 = I level, the experimental position of two of the three crystal-field levels in the 7=2 level is in theory sufficient to determine Bq. The position of the crystal-field levels in the octagonal D4d symmetry ( =8), is the same as in the hexagonal crystal field, due to the absence of non-zero off-diagonal matrix elements. 

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