Magnetic anisotropy parameters

Table 44 Relationships among magnetic-anisotropy parameters for nearly-octahedral dn complexesa...

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

Note 3 For mesophases eomposed of cylindrically symmetric molecules there is a precise relationship between the magnetic anisotropy, A, and the second-rank orientational parameter P ). 

Figure 2. The nearest-neighbor concurrence C(l,2) for different values of the anisotropy parameter y = 1, 0.7, 0.3, 0 with an impurity located at = 3 as a function of the reduced coupling constant A = 7/2/i, where J is the exchange interaction constant and h is the strength of the external magnetic field. The curves correspond to different values of the impurity strength a = 0,0.5,1,1.5 with system size iV = 201.

The magnetic properties of iron oxides can be determined using Mossbauer spectroscopy, neutron powder diffraction and magnetometry (see Chap. 7). The characteristic parameters are the magnetic moment, the permeability, the saturation magnetization, the magnetic anisotropy constants and the Bhf (Tab. 6.2). 

The crystal field does, however, have a dramatic effect on the magnetic anisotropy of lanthanide complexes. For complexes of less than cubic symmetry the three principal values of the susceptibility tensor are unequal. For uniaxial symmetry, Xx — Xy Xz and for biaxial symmetry Xx Xy Xz- Very extensive studies632-640 have been carried out on the single crystal susceptibilities of the D3d lanthanide hexakis(antipyrene) triiodides over the temperature range 80-300 K, and crystal field parameters were obtained. This crystal field-induced anisotropy is responsible for the effectiveness of lanthanide complexes as NMR shift reagents, and single crystal anisotropies of lanthanide complexes have been determined in this connection also.563... 

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. 

If the energy of the crystal-field splitting AEcf produced by the Bq parameters is much smaller than the thermal energy (kT 200 cm-1 at 300 K), all the crystal-field levels of the ground state have comparable thermal populations and the T 2 term of the power series is adequate for modeling the magnetic anisotropy (Bleaney, 1972 Mironov et al 2002). Substitution of eqs. (35)-(37) into eq. (28) gives eq. (40) in terms of Aq2 r2) which can be transformed into eq. (41) with conventional B2 parameters... 

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