And hyperfine interactions

It is much more difficult to observe the Mossbauer effect with the 130 keV transition than with the 99 keV transition because of the relatively high transition energy and the low transition probability of 130 keV transition, and thus the small cross section for resonance absorption. Therefore, most of the Mossbauer work with Pt, published so far, has been performed using the 99 keV transition. Unfortunately, its line width is about five times larger than that of the 130 keV transition, and hyperfine interactions in most cases are poorly resolved. However, isomer shifts in the order of one-tenth of the line width and magnetic dipole interaction, which manifests itself only in line broadening, may be extracted reliably from Pt (99 keV) spectra. 

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. 

The special case of isotropic g and hyperfine interaction will now be considered. This simplification is valid when the anisotropic interactions are averaged by rapid tumbling. The quadrupole interaction will be omitted because it is purely anisotropic. The resulting simplified spin Hamiltonian is given in Equation (9). 

The spin Hamiltonian for a biradical consists of terms representing the electron Zeeman interaction, the exchange coupling of the two electron spins, and hyperfine interaction of each electron with the nuclear spins. We assume that there are two equivalent nuclei, each strongly coupled to one electron and essentially uncoupled to the other. The spin Hamiltonian is ... 

The experience gained in these studies has been invaluable for the development of the BERTHA molecular code much of the material of the present article was first presented in [29] and [30], together with applications to the study of magnetic and hyperfine interactions in atoms and small molecules, NMR shielding constants for H2O and NH3, and P-odd interactions in chiral molecules such as CHBrClF. A detailed study of the water molecule [31] examined the convergence of the DHF and DHFB calculations with a series of uncontracted correlation consistent basis sets due... 

B. Bleaney, Magnetic resonance spectroscopy and hyperfine interactions 323... 

Note that in all the cases considered in Table 2 the ethyl protons of the initial Et3GeCOPh demonstrate positive polarization (A). Therefore, the analysis of these effects in accordance with the existing rules11 allows us to conclude that partially reversible photodecomposition of the ketone both in the presence and in the absence of the radical traps occurs from the triplet excited state with the formation of the triplet radical pair comprised of Et3Ge and COPh radicals. The analysis employed the following -factor and hyperfine interaction values of the radicals g(Et3Ge ) = 2.0089, g( COPh) = 2.0008 and Ah(CH2) < 0.5 mT (for Et3Ge )12. 

The magnetic dipolar and hyperfine interactions of the nucleus with the electronic moments can be expressed by ... 

The effective spin Hamiltonian (SH) that includes Zeeman and hyperfine interaction for a single unpaired electron and N nuclei is ... 

Kent, T.A., Emptage, M.H., Merkle, H., Kennedy, M.C., Beinert, H., Munck, E. (1985). Mossbauer studies of aconitase. Substrate and inhibitor binding, reaction intermediates, and hyperfine interactions of reduced 3Fe and 4Fe clusters. J. Biol. Chem. 260 6871-81. 

Nagel, S. (1985b). Cluster calculation of electronic structure and hyperfine interactions for a-FcjO,. J. Phys. Chem. Solids 46, 905-19. 

The isotropic g and a values are now replaced by two 3x3 matrices representing the g and A tensors and which arise from the anisotropic electron Zeeman and hyperfine interaction. Other energy terms may also be included in the spin Hamiltonian, including the anisotropic fine term D, for electron-electron interactions, and the anisotropic nuclear quadrupolar interaction Q, depending on the nucleus. Usually the quadrupolar interachons are very small, compared to A and D, are generally less than the inherent linewidth of the EPR signal and are therefore invisible by EPR. They are readily detected in hyperfine techniques such as ENDOR and HYSCORE. All these terms (g. A, D) are anisotropic in the solid state, and must therefore be defined in terms of a tensor, which will be explained in this section. 

TABLE 2. g eigenvalues and hyperfine interactions for PhjAs trapped in a single crystal ... 

Of the several less common spectroscopic methods to combine with electrochemical intermediate generation such as luminescence, Raman, NMR, or X-ray absorption spectroscopy, the EPR method is presented here because of its relative simplicity and pronounced selectivity. Only paramagnetic compounds with a certain, not too rapid relaxation rate from the spin-excited state give detectable signals for EPR spectroscopy, which helps to disregard many simultaneously present species. On the other hand, the rather slow time frame (At 10 s) and the sensitivity of the EPR method to electronic influences from the participating atoms via g-factor shift and hyperfine interaction can render EPR a very valuable method to determine the site of electron transfer (ligand or metal) as well as the spin and thus valence distribution. 

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