Magnetic hyperfine interaction in paramagnetic

In order to complete the discussion of magnetic hyperfine interactions in paramagnetic heme proteins as detected by Mossbauer spectroscopy, reference should be made to the work of Champion et al. (58). There the influence of the halides F, Cl, and I on ferric chloroperoxidase was studied. The results presented in this work indicate that halide anions,... 

The third term of the nuclear Hamiltonian contains two contributions. The nuclear Zeeman term couples the magnetic moment of the nucleus to the external magnetic field Bo. Furthermore, there is a term that describes the interaction of the nuclear spin with the internal magnetic hyperfine field. For paramagnetic samples this is often done in terms of the hyperfine coupling tensor, which multiplied by the spin... 

In Eq. (6) the index i designates the components x, y, z of the electron spin, and N is the sum of all the populated states AEn is the energy of the IVth electronic spin state relative to the lowest one. The nuclear. Hamiltonian, containing the magnetic hyperfine interaction between nucleus and paramagnetic valence shell, the nuclear Zeemann term ( —gNjMN o ), and the quadrupole interaction Hq, is given by... 

In paramagnetic substances, it is necessary to compare tr with tl If tr < < tl so that the orientation of the electronic spins fluctuates very rapidly, the internal magnetic field seen by the nucleus will average to zero and there will be no magnetic interactions. This appears to be the case with the ferrous hemoglobins and one observes unbroadened, nearly symmetric, quadrupole doublets. If on the other hand tr > > TL, magnetic hyperfine interactions can occur and one may expect line broadening and perhaps even a fully resolved hyperfine spectrum. 

The conditions necessary for observation of proton magnetic resonance spectra in paramagnetic systems are well established (1). Either the electronic spin-lattice relaxation time, T, or a characteristic electronic exchange time, Te, must be short compared with the isotropic hyperfine contact interaction constant, in order for resonances to be observed. Proton resonances in paramagnetic systems are often shifted hundreds of cps from their values in the diamagnetic substances. These isotropic resonance shifts may arise from two causes, the hyperfine contact and pseudocontact interactions. The contact shift arises from the existence of unpaired spin-density at the resonating nucleus and is described by 1 (2) for systems obeying the Curie law. 

Time-dependent phenomena can influence the Mossbauer spectrum whenever they make the position of the Mossbauer nucleus or the properties of the nuclear environment and, hence, the hyperfine interactions change with time. Time-dependent effects can influence both the spectral lineshapes and the values of the Mossbauer hyperfine parameters. The nuclear transitions and the hyperfine interactions have characteristic times, and each type of relaxation phenomenon must be considered in the context of the appropriate time scale. In case of super-paramagnetic relaxation, the magnetic hyperfine interaction fluctuates with time. The magnetic hyperfine field acting at a given Mossbauer... 

The spin state of a paramagnetic system with total spin S wiU lift its (25 + l)-fold degeneracy under the influence of ligand fields (zero-field interaction) and applied fields (Zeeman interaction). The magnetic hyperfine field sensed by the iron nuclei is different for the 25 + 1 spin states in magnitude and direction. Therefore, the absorption pattern of a particular iron nucleus for the incoming synchrotron radiation and consequently, the coherently scattered forward radiation depends on how the electronic states are occupied at a certain temperature. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

The hyperfine interaction is shown in Figure 21 of reference 16. The Ms = 1/2 states of an S = 1/2 paramagnet interact with an I = 1/2 nuclear moment to create the hyperfine interaction. Interactions from ms = — 1/2 to Mi = - 1/2 and ms = - 1/2 to Mi = + 1/2, for instance, create the magnetic field specified as the hyperfine interaction A. Figure 21 of reference 16 describes the behavior for an 1=1/2 nuclear moment. The number of hyperfine lines will be equal to 21 + 1 for nuclear moments greater than 1/2. Each hyperfine line will be of equal intensity when the electron is interacting with its own nucleus. For instance the Cu2+, / = 3/ 2 nucleus will produce four hyperfine lines as described in the next section. 

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