Paramagnetic impurities relaxation

The hyperfine couplings between localized electrons on paramagnetic impurities and nuclear spins often serve as the dominant source of spin-lattice relaxation for... 

F NMR of Fluorine-Doped -Alumina. The samples studied 115) were high surface area aluminum oxides doped with fluorine by addition of aqueous HF to alumina and subsequent dehydration. A suflScient number of paramagnetic impurities were present in the samples to give relaxation times of the order of 0.01 second. The BET surface areas of most of the samples examined were within 20% of 250 meterVgram. 

Fio. 18. Spin-lattice relaxation time Ti for silica gel (SG) and silica-alumina (SA) versus paramagnetic impurity content N. The straight line corresponds to the relation Ti oc (irS). 

The most important relaxation processes in NMR involve interactions with other nuclear spins that are in the state of random thermal motion. This is called spin-lattice relaxation and results in a simple exponential recovery process after the spins are disturbed in an NMR experiment. The exponential recovery is characterised by a time constant Tj that can be measured for different types of nuclei. For organic liquids and samples in solution, Tj is typically of the order of several seconds. In the presence of paramagnetic impurities or in very viscous solvents, relaxation of the spins can be very efficient and NMR spectra obtained become broad. 

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

In commercial zeolites the proton relaxation of adsorbed molecules is controlled by magnetic interaction with paramagnetic impurities (Fe8+) of the zeolite lattice (9). The relaxation rate 1/Ti is proportional to the number of these paramagnetic centers. If the lattice is covered with diamagnetic metal atoms, this interaction should be reduced according to the amount and dispersity of the metal. 

The 13C nuclei of molecules whose protons undergo exchange relax at different rates in H20 and D20 solutions. Thus Tx for the carbonyl carbon of acetic amide is 72 s in D20, but only 37 s in H20 [189]. In this context, the influence of paramagnetic impurities should again be mentioned. Traces of paramagnetic ions (e.g. Cu2 + ) no longer detectable by the usual analytical techniques can drastically lower the 13C relaxation times of complexing substrates (e.g. amino acids). 

Relaxation times Tt and T2 have been determined as a function of temperature and surface coverage in various zeolites, particularly of the faujasite type. The early experiments have been troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca. 6 ppm. Figure 38 shows the variation of Tt and T2 for water adsorbed in a particularly pure sample of zeolite Na-X (248). The authors (248) account for the experimental results using a model of the intracrystalline fluid, which is about 30 times as viscous as bulk water at room temperature. It shows a broad distribution of molecular mobilities (the ratio T,/T2 at the minimum in Tt is much larger... 

Deuterium nuclei in water molecules have negligible asymmetry parameters and residual anisotropies. After correcting for the paramagnetic impurities present, that affect relaxation rates somewhat (35), one obtains from the experimental data a value of the longitudinal relaxation rate for the water deuterons in the bound state of 650 s 1. This value incorporates the quadrupolar coupling constant (above determined) and the correlation time for bound waters. Using the standard expression for quadrupolar relaxation (29,35) yields a value for t 1.6 ns. 

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

The amount of radicals in carbon black filled rubbers decreases significantly upon extraction of free rubber with the aid of a solvent containing a free radical scavenger. The extraction nevertheless causes a substantial increase in the fraction of the T2 relaxation component with the decay time of about 0.02-0.03 ms [62], This increase is apparently caused by an increase in the total rubber-carbon black interfacial area per volume unit of the rubber due to the removal of free rubber. The T2 relaxation component with a short decay time is also observed in poly(dimethyl siloxane) (PDMS) filled with fumed silicas [88], whose particles contain a minor amount of paramagnetic impurities. Apparently, free radicals hardly influence the interpretation of NMR data obtained for carbon-black rubbers in any drastic way [62, 79]. 

HOptner et al.118 have carried out 1FI and 19F SLR time as a function of temperature. Fluorines are known to be relaxed mainly by the reorientational motion of the anions and by the interaction with fixed paramagnetic impurities, the protons are relaxed additionally above 150 K predominantly by highly mobile paramagnetic species, whose concentration could be determined directly via the NMR signal amplitude. Korringa relation observed for proton relaxation shows that it is metallic above 183 K. Further, 1/Ti versus (vL) 1/2 dependence of the proton relaxation supports the ID spin transport and also confirms that only protons of the cation stacks are relaxed by the highly mobile paramagnetic species. 

In the presence of human serum albumin, the H spectrum of acetyl-salicyclic acid is specifically shifted and broadened [119]. The interpretation of changes in T, and T2 require several theoretical assumptions. These have been discussed in detail [120] for JV-acetylsulphanilamide and acetate binding to the active site of carbonic anhydrase. It was concluded that the acetyl groups of these inhibitors have a motion additional to that of the enzyme. It can be shown by NMR that acetate binds to two sites on the enzyme, only one of which is inhibitory to esterase activity (methyls are 4.3 and 4.8 A from the metal in the Mn substituted enzyme [121]). Strict care must be taken to avoid paramagnetic impurities when NMR relaxation enhancement by diamagnetic macromolecules is being studied. A preparation of carbonic anhydrase, for example, can contain 0.24 paramagnetic Cu atoms per Zn atom [122]. 

A number of processes contribute to relaxation. Intra- or inter-molecular dipole-dipole interactions with nuclei are less efficient for than for due to a dependence of the relaxation rate. Spin rotation processes, which are especially important for small molecules and ions such as ND3, [NOal , and N2, are more effective at higher temperatures. Shielding anisotropy, which is important for linear or planar nitrogen groups, such as in dinitrogen complexes, becomes more effective at higher fields, and paramagnetic impurities present in... 

In addition to the solvent-induced electron spin relaxation by independent spin flips at two well-separated radical centers (spin-lattice relaxation), which is quite slow in the absence of paramagnetic impurities (ordinarily, on the order of lO -IO s" in monoradicals), there are two important mecha-... 

In neat 2-fluorophenol liquid a linear decay is, in fact, observed, with Tj = 1.64 sec. By contrast, 2-fluorophenol in coal exhibits a nonexponential decay, suggesting that not all the molecules in the coal are equivalent. The time scale for the decay is more than two orders of magnitude faster in the coal than in neat liquid. The rate of relaxation is found to be independent of the frequency for resonance, demonstrating that molecular motion (and not paramagnetic impurities, for example) is responsible for this rapid decay. The dramatic change in Tj seen in coal is explained by a slowing down of molecular motion. As shown in Figure 2(b), for the very rapid motions encountered in liquids (on about a 10 -... 

Lithium aluminate, LiAlsOg with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. Li and Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that Li relaxes most significantly through interactions with paramagnetic impurities, whereas Li relaxes much more strongly through dipole-dipole interactions. 

From the comparison of the measured and calculated temperature dependences of the relaxation time (see Fig. 20), it follows that the inelastic phonon scattering is the most essential mechanism of the spin-lattice relaxation for Ge. It is evident that only at low temperatures T < 30K) some other mechanisms (the most probable one is the relaxation due to a small amount of paramagnetic impurities) become dominant. At T > 300/C some additional mechanism of relaxation may also exist. The interaction of the nuclear quadrupole moment with vibrations of the nearest four Ge atoms brings about the main contribution to the spin-lattice relaxation rate. The effective modulation of the EFG by the nearest bond charges is greatly reduced because of strong correlations between their displacements. As the main result of the present investigation of spin-lattice relaxation,... 

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