PROTON SPIN RELAXATION

For heterogeneous materials, CPMAS spectra cannot be properly understood without a knowledge of proton relaxation times, which are also of intrinsic importance because of their relation to mobility at the molecular level and to domain structure. Three types of relaxation that need to be considered are  

Whereas and Ti are normally exponential or multi-exponential, transverse 

In principle, free induction decays (and/or the spectra produced therefrom) should give better information on domain structure. In several ways it makes more sense to analyse the free induction decays directly, rather than the transformed spectra. However, there is no universally accepted algorithm for such analysis, which is not surprising given the complexity of the situation for polymeric systems. We have adopted a mix of three mathematical functions  

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Beckmann P. A., Bloom M., Ozier I. Proton spin relaxation in dilute methane gas a symmetrized theory and its experimental verification, Can. J. Phys. 54, 1712-27 (1976). 

By measuring the proton relaxation times, and T,p, it is possible to estimate the mobility of polymer chains within the cell wall (11). Proton spin relaxation editing (PSRE) is a method of expressing these results. It separates the components seen in a conventional CP-MAS C spectra into low-mobility and intermediate-mobility components. If PSRE is applied to a experiment (12) the mobility of the... 

The usefulness of NMR in such analysis is because the proton spin-relaxation time constants are different for different components, such as water, liquid fat and solid fat. For example, the signal from solid fat is found to decay rapidly while the liquid signals decay much slower. This phenomenon is the basis for an NMR technique to determine the solid fat content [20], However, as the relaxation time constant of water, for example, could depend on its local environment, such as protein concentration, it may overlap with that of oil and other components. As a result, it could be difficult to formulate a robust and universal relaxation analysis. It... 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

Takigawa M, Yasuoka H, Saito G (1987) Proton spin relaxation in the superconducting state of (TMTSF)2C104. j Phys Soc Jpn 56 873-876... 

In zeolites the mobility of hydrocarbon molecules with double bonds is specifically restricted because of a specific interaction between the 7r-elec-trons and the zeolite (2). As expected, proton spin relaxation of benzene, cyclohexadiene, cyclohexene, and cyclohexane adsorbed on NaY reveals an increasing restriction of mobility with increasing number of -electrons (8, 4, 8). This is shown in Figure 1, where the longitudinal (7 ) and transverse (T2) proton relaxation times are plotted. 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

Elsewhere (31), we have discussed Browns measurements (14) on the transverse proton spin relaxation time as a function of temperature. His data suggest anomalous changes around 18°, 42°, and 60°C. Simpson and Carr (138) have also observed an anomaly near 40°C. on the basis of their spin relaxation studies. Simpson (137), in particular, reviewed other evidence for anomalies in this temperature range and noted that... 

In this section, we will only discuss a specific system the enhancement of proton spin relaxation in aqueous solutions of paramagnetic ions, which to our knowledge is the only case of paramagnetic relaxation, which has been studied with MD simulations. Just as in the description of dipole-dipole relaxation, we will only treat the through space dipole-dipole interaction, and not the scalar interaction. This is experimentally well motivated in the case of Ni ions [23]. 

These results may be interpreted in the following way (22) At room temperature the species exists mainly as dimer (or ion quadruplet) with rapid migration of MeaSi groups on the NMR time scale. As the sample is cooled to -20°C, trimethylsilyl migration becomes slow so that two peaks are observed for the dimer. The related compound LiN(SiMc3)2 shows similar monomer-dimer equilibria in solution in THF and hydrocarbons (72). Below —20°C, monomer (ion pair) is observable in equilibrium with dimer. Between -20°C and -60°C silyl migration is rapid in the monomer but slow in the dimer (compared to the proton spin relaxation time). Finally at -80°C anionic rearrangement for the monomer also becomes slow, so that two pairs of 2 1 peaks are observed. 

Fig. 2. Larmor frequency and angular dependences of the longitudinal proton spin relaxation time Tj for the two /i-alkyl-cyano-biphenyls 5CB and 8CB, respectively, at temperatures in the middle of the nematic mesophases. The upper diagram shows the dispersion profiles Ti(v) with the director H a igned parallel (A = 0) and perpendicular (A = 90 ) to the external 2 eman field, Bq, and suggests to distinguish at least four relaxation regimes. The lower diagram shows ri(A) in different frequency ranges of the dispersion profile. The solid lines are model fits of equation (3a) to the experimental 5CB data points as discussed in the text.

To test the quality of both the established and the disputed relaxation models in the light of the new angular-dependent results, we performed simulations and tried model fits of the pertinent theories for proton spin relaxation by reorientations of dipolar coupled inter- and intra-molecular proton pairs. Assuming fast magnetization transfer between unlike spin pairs and a superposition of the three independent reorientations (M) known to be important mechanisms for common nematic liquid crystals, namely nematic... 

Long contact times are useless the intensity of resonances decreases due to the proton spin relaxation (in the spin-locking conditions, i.e. in rotating frame, it is the proton spin relaxation time T.p ). Usually, a contact time in the range 2-6 ms was the most frequently applied in cross-polarization experiments. However, according to the plot of intensity versus tc illustrated in Figure 12-8, the optimal value for chromanol is ca. 10 ms. 

Solution structures deduced from LIS data. - Andre et a/." have prepared new ditopic trihelicate lanthanide (La-La -L3) complexes, which they consider of potential utility as biomedical probes. The solution structures of these species were determined by analyses of lanthanide-induced chemical shifts and proton spin relaxation data of mixed complexes containing one diamagnetic Lu + ion and one paramagnetic lanthanide ion. 

The intensity of the signal—its relative lightness or darkness in the image—depends on the concentration and spin relaxation times of the various protons. Spin relaxation time is the time it takes for the perturbed magnetization associated with a proton to return to its equilibrium value. The relaxation time is quite sensitive to the environment and is different for water in blood and various tissues. 

This chapter concludes by pointing out that relaxation of multispin proton systems played a major role in the early days of NMR relaxation measurements on liquid crystals [5.34]. In particular, the detection of director fluctuations [5.35] by means of its characteristic frequency dependence in proton Ti [5.36-5.39] started intensive NMR research on liquid crystals. Since there are many inequivalent proton species in a liquid crystalline molecule, it is impossible to distinguish various atomic sites from a broad proton lineshape, which is a result of strong dipolar couplings. Moreover, translation self-diffusion also modulates the intermolecular dipole-dipole interactions and contributes to proton relaxation in liquid crystals [5.40, 5.41]. Partially deuterated liquid crystals may be used to reduce the number of inequivalent proton species. Proton spin relaxation studies remain as a possible method of probing intermolecular interactions or translational motions in liquid crystals. 

Variable frequency proton Ti studies were first used to detect the characteristic dependence of Ti due to director fluctuations [6.20] in liquid crystals. It was recognized soon after that besides the director fluctuations, relaxation mechanisms, which are effective in normal liquids such as translational self-diffusion and molecular reorientation [6.24], also contribute to the proton spin relaxation in liquid crystals. Though the frequency dependences of these latter mechanisms are different from the relaxation, the precise nature of proton Ti frequency dispersion studied over a limited frequency range using commercial NMR spectrometers often may not be unambiguously identified. Furthermore, because of a large number of particles involved in collective motions, the motional spectrum has much of its intensities in the low-frequency domain far from the conventional Larmor frequencies. The suppression of director fluctuations in the MHz region due... 

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