Magnetic susceptibility relaxation

The Curie spin relaxation is also called magnetic susceptibility relaxation. Vega and Fiat (52) have also considered the Curie spin relaxation originating from anisotropic magnetic susceptibility, rather than an isotropic one as assumed in Eqs. (21-23). 

The magnetic susceptibility relaxation is usually more important for than for Ti. In fact, this mechanism is often dominant in determining the proton linewidth in paramagnetic proteins at high magnetic fields (3). Gillis and co-workers have recently developed a theory for the related case of proton linewidth in colloidal solutions of so-called superparamagnetic particles (54,55). 

Observation of magnetic susceptibility relaxation after perturbation of a spin equilibrium would be the most direct way to measure the dynamics of the equilibrium. This does not appear to have been reported as measured in solution. In principle susceptibility relaxation as a function of frequency could be measured much as dielectric relaxation is examined. The requirement is for a sufficiently strong magnetic field with very sensitive detection. A nonequilibrium magnetic susceptibility has been generated by light at low temperatures in the solid state (39). 

In a solution where a nonzero volume change between the electronic isomers, HS and LS, is encountered, the position of the spin equilibrium will depend on pressure. The volume change, usually denoted here AF°, may be obtained from the study of the pressure dependence of equilibrium properties such as the magnetic susceptibility or the electronic spectrum. In favorable cases, A F° values may be derived from the amplitude of sound absorption observed in ultrasonic relaxation measurements of a spin equilibrium as will be shown in the... 

It is thus evident that, in order to interpret the results obtained by one of the relaxation methods, a thorough investigation of the unperturbed equilibrium properties is required. In general, solution magnetic susceptibilities measured by the NMR method of Evans [83, 84] are used to this end, the equilibrium constant for the equilibrium in Eq. (37) being determined by ... 

Fe(6-Mepy)2(py)tren] (004)2 Doped in PSS. Magnetic susceptibilities measured for a microcrystalline sample of the complex produce a magnetic moment value = 0.36 pg at 10 K and 0.61 pg at 150 K, followed by a gradual increase to Peff = 2.80 pe at 311 K [138]. Thus 26% of the complexes are in the HS state at 300 K if a magnetic moment of 5.1 Pe is assumed for the pure HS compound. On the other hand, the complex doped into a polystyrene sulfonate (PSS) film does not provide any evidence for a thermal population of the HS state up to 340 K as demonstrated by variable-temperature UV-VIS and Mossbauer spectra. In fact, all the complexes doped into the PSS film are in the LS state at temperatures below 340 K. However, if irradiated by a single pulse of a Q-switched Nd/YAG laser (532 mp), the complex is excited from the LS ground state to the HS J2 states via an intermediate MLCT state and the metal states. The subsequent back relaxation from the excited T2 state to the... 

In summary, it is non-trivial to implement magnetic resonance pulse sequences which allow us to monitor unambiguously the decrease in absolute concentration of reactant species and associated increase in product species, but measures of relative concentrations from which conversion and selectivity are calculated are much easier to obtain. However, if such measurements are to be deemed quantitative the spectra must be free of (or at least corrected for) relaxation time and magnetic susceptibility effects. 

The convenient NMR observables depend on the characteristics of the system studied, but generally the protons of the liquid are readily detected. The apparent NMR linewidths are often determined by the magnetic susceptibility inhomogeneities in the sample and do not directly reflect the dynamics of the liquid. This report will focus largely on theoretical approaches to understand the spin-lattice relaxation rate constants for both classes of microporous materials. The magnetic held dependence of the spin-lattice relaxation rate constant generally provides a useful dynamical characterization of the liquid and often a structural characterization of the confining media. 

In the case of magnetically coupled systems. Curie relaxation is simply additive as long as the magnetic susceptibility is the sum of the two components. When the magnetic susceptibility is not a simple sum of the components, the magnetic susceptibility contribution of each metal ion should be evaluated. 

---