Magnetic parameters, simulations

In the case of paramagnetic complexes their experimental magnetic parameters are determined by computer simulation of the powder spectra [59], Together with the corresponding calculated values, obtained using a relativistic spin-unrestricted ZORA approach, they all are collected in Table 2.8. 

Magnetic nanoparticles in the sol-gel silica glass were obtained by heat treatment at 1000°C in air during 6 h and identified by X-ray and Mossbauer spectroscopy as y-Fe203 (maghemite) [11], The magnetic parameters of maghemite used in computer simulations of the SPR spectra, are Ms = 370 kA m 1 and = —4.64 kJ m"3 (cubic symmetry) [35],... 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

Typical values of magnetic parameters are given in Ref. 22, and in the extensive tables of Ref. 31. Recently, we have performed careful simulations of frozen solutions of several nitroxides (Fig. 1) at Q-band in second derivative display [32], and Tables 1-3 contain data from this paper. 

Lund, A., Macomber, L. D., Danilczuk, M., Stevens, J. E., and Schlick, S. 2007. Determining the geometry and magnetic parameters of fluorinated radicals by simulation of powder ESR spectra and DFT calculations the case of the radical RCF2CF2 in Nafion perfluorinated ionomers. The Journal of Physical Chemistry B 111 9484-9491. 

Today the spectral profiles can be simulated for any motional regime by a numerical integration of the stochastic Liouville equation, as discussed in Chapter 12 and in the references therein. The noticeable improvement in the techniques of calculation of the magnetic parameters and their dependence on the solvent, and of the minimum energy conformation of the molecules, have opened the possibility of an integrated computational approach. Since it gives calculated spectral profiles completely determined by the molecular and physical properties of the radical and of the solvent at a given temperature, this method is a step forward in the direction of a sound interpretation of complex spectra. 

Since all magnetic parameters are most often not available directly from the spectrum, computer simulations - combined with the optimization procedure and theoretical calculations - are the only methods that enable a complete analysis of the experimental data (see Section 23.2.1.4) [16]. 

Canne and co-workers have presented EPR studies of three prokaryotic enzymes of the xanthine oxidase family, namely quinoline 2-oxidoreductase, quinaldine 4-oxidase, and isoquinoline l-oxidoreductaseJ In quinoline 2-oxidoreductase a neutral flavin radical was observed, while in quinaldine 4-oxidase an anionic radical was detected. The rapid Mo(V) signal was observed in all three enzymes with only small differences in magnetic parameters. From spectra simulations of Mo (/ = 5/2) substituted quinoline 2-oxidoreductase, a deviation of 25° between the maximal g and Mo-hfc tensor component was derived. The Mo(V) species was detected in small amounts upon reduction with substrates in quinoline 2-oxidoreductase and quinaldine 4-oxidase, but showed a different kinetic behaviour with an intense EPR signal in isoquinoline 1-oxidoreductase. The two [2Fe-2S] clusters produced different EPR signals in all three enzymes and, in isoquinoline 1-oxidoreductase, revealed a dipolar interaction, from which a maximum distance of 15 A was estimated. 

The development of Remote Field Eddy Current probes requires experience and expensive experiments. The numerical simulation of electromagnetic fields can be used not only for a better understanding of the Remote Field effect but also for the probe lay out. Geometrical parameters of the prohe can be derived from calculation results as well as inspection parameters. An important requirement for a realistic prediction of the probe performance is the consideration of material properties of the tube for which the probe is designed. The experimental determination of magnetization curves is necessary and can be satisfactory done with a simple experimental setup. 

Strongly supporting this spectroscopic data, Mossbauer spectroscopy of the as-isolated Rr shows the presence of two types of iron centers a magnetic component that can be well simulated by the parameters of Rd, and a diamagnetic component attributed to the diiron-oxo cluster and resulting from the antiferromagnetic coupling of the two irons. 

Thus, the starting parameters for the computer-simulation of spectrum IB were chosen to agree with the value of hyperfine fields at 613 K as measured by Rlste and Tenzer, using neutron scattering measurements (36). In addition, the magnetic relaxation rate depends on temperature, as discussed in the Theory section of this paper. 

However, when it comes to the simulation of NFS spectra fi om a polycrystalline paramagnetic system exposed to a magnetic field, it turns out that this is not a straightforward task, especially if no information is available from conventional Mossbauer studies. Our eyes are much better adjusted to energy-domain spectra and much less to their Fourier transform therefore, a first guess of spin-Hamiltonian and hyperfine-interaction parameters is facilitated by recording conventional Mossbauer spectra. 

We see that the models which best reproduce the location of all the six data points are the tracks which do not fit the solar location. The models whose convection is calibrated on the 2D simulation make a poor job, as the FST models and other models with efficient convection do therefore this result can not be inputed to the fact that we employ local convection models. A possibility is that we are in front of an opacity problem, more that in front of a convection problem. Actually we would be inclined to say that opacities are not a problem (we have shown this in Montalban et al. (2004), by comparing models computed with Heiter et al (2002) or with AH97 model atmospheres), but something can still be badly wrong, as implied by the recent redetermination of solar metallicity (Asplund et al., 2004). A further possibility is that the inefficient convection in PMS requires the introduction of a second parameter -linked to the stellar rotation and magnetic field, as we have suggested in the past (Ventura et al., 1998 D Antona et al., 2000), but this remains to be worked out. 

Simulating the INS spectra using the Stevens parameters extracted by Riley and coworkers provides a nice reproduction of the INS spectra. However, as previously discussed, it was necessary to adjust these parameters in order to reproduce the static magnetic data (see above, Figure 5.8). Fitting the linear region of the relaxation time for the Er(trensal) SMM yields an energy barrier of 20(1) cm-1... 

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