Spin-lattice relaxation measurement

Canet D, Levy G C and Peat I R 1975 Time saving in C spin-lattice relaxation measurements by inversion-recovery J. Magn. Reson. 18 199-204... 

III. Spin-Lattice Relaxation Measurements 1. Experimental Methods... 

Poly(benzyl ether) dendrimers synthesized by Frechet el al. have been studied with many techniques in order to reveal their conformational properties. Size exclusion measurements performed by Mourey et al. [154], rotational-echo double resonance (REDOR) NMR studies by Wooley et al. [155] and spin lattice relaxation measurements by Gorman et al. [156] reveal that back-folding takes place and the end-groups can be found throughout the molecule. The observed trends are in qualitative agreement with the model of Lescanec and Muthukumar [54],... 

Another possible solution to the problem of analyzing multiple-layered membrane composites is a newly developed method using NMR spin-lattice relaxation measurements (Glaves 1989). In this method, which allows a wide range of pore sizes to be studied (from less than 1 nm to greater than 10 microns), the moisture content of the composite membrane is controlled so that the fine pores in the membrane film of a two-layered composite are saturated with water, but only a small quantity of adsorbed water is present in the large pores of the support. It has been found that the spin-lattice relaxation decay time of a fluid (such as water) in a pore is shorter than that for the same fluid in the bulk. From the relaxation data the pore volume distribution can be calculated. Thus, the NMR spin-lattice relaxation data of a properly prepared membrane composite sample can be used to derive the pore size distribution that conventional pore structure analysis techniques... 

Spin-lattice relaxation measurements (7j) are also common,78 83 though these of course monitor more rapid motions. Spin lattice relaxation times have nevertheless proved a simple but effective means of distinguishing different structural regions within polymer samples, i.e. crystalline and amorphous regions and interfacial regions between crystalline and amorphous parts. 

Complementary spin-lattice relaxation measurements corroborate the observations made using the 2H line-shape measurements. Based on these measurements the low temperature relaxation times are dramatically shorter in the intercalated sample as compared to the bulk, indicating enhanced polymer re-orientation dynamics in the intercalated samples. Furthermore, the temperature dependence of the relaxation time in the bulk and intercalated sample show dramatic differences. While the relaxation time for the intercalated sample passes smoothly from low to high temperatures, the bulk sample shows a break between the crystalline state and melt state, with the melt state relaxation times at least one order of magnitude faster than those observed in the intercalated sample at the same temperature. 

NMR spin lattice relaxation measurements provide very direct information about the Fourier transform of the spin susceptibility x( w) in a one-dimensional conductor [39]. The spin degrees of freedom constitute a relaxation channel for nuclear spin due to the modulation of the hyperfine interaction by the electron spin time dependence, which is given generally... 

The attractiveness of surface/pore characterization via NMR spin-lattice relaxation measurements of pore fluid lies in the potential advantages this technique has as compared to the conventional approaches. These include rapid analysis, lower operating costs, analysis of wet materials, no pore shape assumption, a wide range of pore sizes can be evaluated (0.5 nm to >1 /im), no network/percolation effects and the technique is non-destructive. When determining specific surface areas, NMR analysis does not require out-gassing and has the potential for on-line analysis of slurries. 

Early studies involving NMR include the work by Hanus and Gill is [6] in which spin-lattice relaxation decay constants were studied as a function of available surface area of colloidal silica suspended in water. Senturia and Robinson [7] and Loren and Robinson [8] used NMR to qualitatively correlate mean pore sizes and observed spin-lattice relaxation times. Schmidt, et. al. [9] have qualitatively measured pore size distributions in sandstones by assuming the value of the surface relaxation time. Brown, et. al. [10] obtained pore size distributions for silica, alumina, and sandstone samples by shifting the T, distribution until the best match was obtained between distributions obtained from porosimetry and NMR. More recently, low field (20 MHz) NMR spin-lattice relaxation measurements were successfully demonstrated by Gallegos and coworkers [11] as a method for quantitatively determining pore size distributions using porous media for which the "actual" pore size distribution is known apriori. Davis and co-workers have modified this approach to rapidly determine specific surface areas [12] of powders and porous solids. 

In NMR work, spin-lattice relaxation measurements indicated a non-exponential nature of the ionic relaxation.10,11 While this conclusion is in harmony with results from electrical and mechanical relaxation studies, the latter techniques yielded larger activation energies for the ion dynamics than spin-lattice relaxation analysis. Possible origins of these deviations were discussed in detail.10,193 196 The crucial point of spin-lattice relaxation studies is the choice of an appropriate correlation function of the fluctuating local fields, which in turn reflect ion dynamics. Here, we refrain from further reviewing NMR relaxation studies, but focus on recent applications of multidimensional NMR on solid-ion conductors, where well defined correlation functions can be directly measured. 

Two further studies, one by Villafranca and Viola,309 who carried out 13C spin-lattice relaxation measurements on the interaction of methyl a-D-glucopyranoside with con A, and the other by Alter and Mag-nuson,310 who characterized the binding of 2-deoxy-2-(trifluoroacetamido)-D-glucose to the lectin, placed the saccharide at 1.0 to 1.4 nm from the Mn2+ion, in agreement with the results of Brewer and coworkers.307,308 Villafranca and Viola s model309 placed C-l, C-2, and C-6 closest to the Mn2+ (see Fig. 3). On the other hand, support for the cavity proposed by Becker and coworkers280 comes from p.m.r.-spectral measurements of the longitudinal relaxation-times of the methyl proton of methyl a-D-mannopyranoside with Zn2+- and Gd2+-substituted con A.311,312... 

EPR signals for both the flavosemiquinone radical and the low-spin ferric heme have been reported (65, 78-82). The flavosemiquinone signal, which is easily observed at 123 K, shows a typical g value of 2.0039 0.002 (65). The bandwidth, which is around 15 G, is very like that of an anionic, or red, semiquinone (65). The EPR signal of the low-spin ferric heme can be observed at low temperatures ( 28 K) and shows g values of 2.99, 2.22, and 1,47 (65), which are similar to those found for cytochrome 65 (81). EPR rapid freezing studies have allowed the amounts of semiquinone and ferric heme to be monitored during reduction of the enzyme by L-lactate (66). This has proved to be extremely useful in the development of kinetic schemes to describe the flow of electrons in the enzyme. The distance between the prosthetic groups in H. anomala flavocytochrome 62 has been estimated from EPR experiments and spin-lattice relaxation measurements (82). Pyruvate was used to stablize the flavosemiquinone and the effect on the signal of this species from oxidized and reduced heme was measured. The results indicated a minimum intercenter distance of 18-20 A (82). 

Pt NMR has been used to study platinum particles embedded in a zeolite and to compare their characteristics with the more common oxide-supported platinum catalysts (Tong etal. 1993). Spin-lattice relaxation measurements indicated that a measurable fraction of the platinum in a zeolite-Y sample is not in a metallic environment, but it was not clear whether the loss of metallic signal refiected very small particle sizes or was due to interactions with the framework and its counter-ions. The latter possibility was supported by the observation that part of the metallic signal was restored by chemisorption of hydrogen (Tong et al. 1993). 

Composite polymer electrolytes based on PVDF (at least 35%) containing lithium salts were studied [140] by a number of techniques including Li NMR. Spin-lattice relaxation measurements were used to show that localised lithium motion is significantly impeded in some samples but not in others. 

Spin lattice relaxation measurements were conducted on two spectrometers with a standard tt—t —tt/2 pulse sequence. The 30 and 90 MHz proton measurements as well as the 22.6 MHz carbon-13 measurements were made on a Bruker SXP 20-100. The 250 MHz proton and 62.9 carbon-13 measurements were made on a Bruker WM-250. 

G. C. Levy and I. R. Peat, "The experimental approach to accurate carbon-13 spin-lattice relaxation measurements," J. Magn. Resonance 18, 500-521 (1975). 

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