Magnetic moments principles

Fig. 8. Principle of the magnetooptical read-out of domain patterns by the polar Kerr effect. The polarisation plane of the incoming laser beam is rotated clock- or counterclockwise according to the orientation (up or down) of the magnetic moments.

We have investigated ground state properties on a first principles basis. Total energy as well as magnetic moment (for FeaNi) were determined with the FLAPW method and the GGA introduced by Perdew and Wang in 1992 by employing the WIEN95 code developed by Blaha et al. 

This simplified treatment does not account for the fine-structure of the hydrogen spectrum. It has been shown by Dirac (22) that the assumption that the system conform to the principles of the quantum mechanics and of the theory of relativity leads to results which are to a first approximation equivalent to attributing to each electron a spin that is, a mechanical moment and a magnetic moment, and to assuming that the spin vector can take either one of two possible orientations in space. The existence of this spin of the electron had been previously deduced by Uhlenbeck and Goudsmit (23) from the empirical study of line spectra. This result is of particular importance for the problems of chemistry. 

Associated with the spin of an electron is a magnetic moment, which can be expressed by a quantum number of + or —5. According to the Pauli principle, any two electrons occupying the same orbital must have opposite spins, so the total magnetic moment is zero for any species in which all the electrons are paired. In... 

Potassium hexafluororhenate (IV), K2ReF6, is, obtained from potassium perrhenate via K2Re(I)6 and subsequent fusion with KHF2 (45), and shows a magnetic moment of 3.3—3.4 B.M. at 298 °K (31, 46). The electronic spectrum in aqueous solution has been studies in some detail by Jergensen and Schwochau (29), and the principle features of their results are listed in Table 11. (See also Fig. 6). 

Electron paramagnetic resonance (EPR) and NMR spectroscopy are quite similar in their basic principles and in experimental techniques. They detect different phenomena and thus yield different information. The major use of EPR spectroscopy is in the detection of free radicals which are uniquely characterised by their magnetic moment that arises from the presence of an unpaired electron. Measurement of a magnetic property of a material containing free radicals, like its magnetic susceptibility, provides the concentration of free radicals, but it lacks sensitivity and cannot reveal the structure of the radicals. Electron paramagnetic resonance spectroscopy is essentially free from these defects. 

In principle, there are several contributions to nuclear Tm" however, the dipolar coupling term often dominates (10,22). The dipolai contribution depends on the reciprocal of the sixth power of the distance between the resonating nucleus and the relaxing electron, on the square of the magnetic moments associated with the unpaired electrons (ge2 B S(S+l)) and with the nucleus on the magnetic field as expressed by the Larmor... 

Table 8.2. Comparison of SGTE ( ) first-principle and experimental magnetic moments for selected elements (adapted from de Fontaine et al. 1995)...

D spectra are in principle possible for heteronuclei coupled by either dipolar or scalar interactions. However, the magnetic moments of heteronuclei are sizably smaller than that of the proton, and since cross relaxation depends on the square of the magnetic moment it appears that this is a serious limitation for the observation of NOESY or ROESY cross peaks. However, as already discussed, in scalar-coupled systems the relevant coherences build up with sin(nJ/jt). Since Jjj in directly bound 13C- H and l5N- H moieties is of the order of 102 Hz, as opposed to about 10 Hz between proton pairs, it is conceivable that scalar correlation experiments are successful. Heterocorrelated spectra have the advantage of allowing one to detect signals of protons attached to carbons or nitrogens when they are within a crowded envelope. 

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