Iron , nuclear spin relaxation

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

Fig. 7. H water proton relaxivity i.e., the nuclear spin-lattice relaxation rate per mM of metal, plotted as a function of the magnetic field strength expressed as the proton Larmor frequency for aqueous solutions of manganese(H) and iron(HI) ions at 298 K. (A) 0.10 mM manganese(II) chloride in 2.80 M perchloric acid (B) 0.1 mM aqueous manganese(H) chloride at pH 6.6 (C) 0.5 mM iron(HI) perchlorate in 2.80 M perchloric acid (D) 0.5 mM iron(IH) perchlorate in water at pH 3.1 (F) 2.0 mM Fe(HI) in 2.0 M ammonium fluoride at pH 7, which causes a distribution of species dominated by [FeFe]"-.

Nuclear relaxation rates, iron-sulfur proteins, 47 267-268 Nuclear resonance boron hydrides and, 1 131-138 fluorescence, 6 438-445 Nuclear spin levels, 13 140-145 Magnetic properties of nuclei, 13 141-145 Nuclear testing... 

In Mossbauer spectroscopy, we encounter two types of expectation values for the electronic spin4 6 that we illustrate briefly for an iron site with S = 1/2 and g 2, taking the applied field along z. If the spin relaxation rate (spin flips between the Ms= + 1/2 and Ms= —1/2 sublevels) is slow compared to the nuclear precession frequency (which is typically 10—30 MHz Larmor precession around Bint or quadrupole precession), the nucleus senses the Fe atom in either the Ms= + 1/2 or Ms =1/2 state during the absorption process. In this case, we have (Sz) = + 1/2 for spin up and (Sz) = —1/2 for spin down. Each electronic level produces a Mossbauer spectrum, and these two spectra are weighted by the probability (given by the... 

A development in the theory of nuclear relaxation in macromolecules by paramagnetic ions has been suggested by Gueron. (675) In the case of heme proteins there is a net polarization of the iron electronic spin magnetic moment which is oriented along the direction of the magnetic field. Modulation of this dipolar field due to the spin polarization (Curie spin) by rotational diffusion introduces an additional term into the expression for transverse relaxation [equation (18)] giving ... 

The source of 7 rays needed for the teehnique is typically an excited-state nucleus, which is itself formed by a nuclear decay process from another nueleus. The most widely used Fe (where denotes excited state) is formed in an electron-capture proeess from radioactive 27C0 (half-life 270 days). This in turn is readily obtained by cyclotron irradiation of iron. f Co decays to Fe with nuclear spin quantum number 7 = 5/2, for which two relaxation processes exist, one with a 15% probability that leads directly to the Fe ground state (by emission of a 7 photon of 136.32 keV), and another with an 85% probability that leads to a different excited-state nucleus with 7 = 3/2. This is what is actually used for the Mossbauer experiment. It has a transition to the ground state (7 = 1/2) with emission of a 7 photon of 14.41 keV (Figure 6.1). 

Representative and spectra are presented in Figures 5, 6, 7, and 8 for the Amax and Monterey samples. The quantitative reference (peak at 0.0 ppm) in each spectrum is hexamethyldisiloxane ((CH3)3-Si-0-Si(0113)3). A paramagnetic relaxation reagent (the paramagnetic relaxation reagents used were either tris(acetylacetonato) iron (III), Fe(acac)3, or tris(acetylacetonato) chromium (III), Cr(acac)3 at concentrations of 2-6 X lO M). was added to decrease spin lattice relaxation times (Ti s) and suppress nuclear Overhauser effects (5). In addition,... 

Another particularly interesting type of experiment gives information about the Co parent atom. At temperatures of < IK the Zeeman levels of Co I — 1) atoms in iron metal are not equally occupied as their separation is kT. Assuming that the spin-lattice relaxation times are longer than the total nuclear decay time, the preferential orientation of the nucleus... 

Low-spin iron (III) ions have an electron hole in the izg orbitals. Therefore, these centers have 5 = 1/2 and spin-orbit interaction contributes considerably to the magnetic hyperfine field. Low-spin ironflll) compounds in solution always show a rather complicated magnetic Mossbauer pattern at temperatures around 4.2 K and low external fields, which means that the relaxation rate of these centers is lower than the nuclear precession rate of 10 s. Sometimes a magnetic splitting is observed even at 77 K. Therefore, in order to pin down 5 and AEq, it is advisory to measure between 100 and... 

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