Proton dipolar spin-lattice process

The interpretation of carbon Tip data is complicated by the fact that spin-spin (cross-relaxation) processes as well as rotating frame spin-lattice processes may contribute to the relaxation. This arises because the proton dipolar state is strongly coupled to the lattice. During the period in which carbon magnetization decays with H] h off, carbon polarization can decay by motional processes (Tjp) or by polarization transfer to the proton dipolar state (T p ), then to the lattice via the proton dipolar spin-lattice process Schaefer et al., ... 

Figure 8. During the C-13 T,p experiment, the protonated (primed) and unpro-tonated (unprimed) carbons are in contact with not only the lattice but also the proton dipolar reservoir. Here T,n (frot) indicates a dipolar spin-lattice process which depends on spinning speed and orientation.

Both spin-lattice (motional) and spin-spin processes contribute to TjpCC). Experimental cross-polarization transfer rates from protons in the local dipolar field to carbons in an applied rf field can be used to determine the relative contributions quantitatively. This measurement also requires a determination of the proton local field. Methods for making both measurements have been developed in the last few years [1,2]. For polystyrenes, the spin-lattice contribution to TjpCCO s is by far the larger. This means that the TipCCVs can be interpreted in terms of rotational motions in the low-to-mid-kHz frequency range. 

These components will oscillate as a function of t]. The resultant Z-magnetizations will be in a nonequilibrium state and spin-lattice relaxation processes will work toward equilibrium. For the case of protons relaxation is primarily dipolar via other protons, this non-equilibrium magnetization will then be redistributed by mutual spin flip to other protons. The final 90° pulse monitors the extent of magnetization transfer. The 2D experiment samples all degrees of magnetization transfer as it increments t]. ... 

The direct NMR method for determining translational difiFusion constants in liquid crystals was described previously. The indirect NMR methods involve measurements of spin-lattice relaxation times (Ti,Ti ),Tip) [7.45]. Prom their temperature and frequency dependences, it is hoped to gain information on the self-diflPusion. In favorable cases, where detailed theories of spin relaxation exist, difiFusion constants may be calculated. Such theories, in principle, can be developed [7.16] for translational difiFusion. However, until recently, only a relaxation theory of translational difiFusion in isotropic hquids or cubic solids was available [7.66-7.68]. This has been used to obtain the difiFusion correlation times in nematic and smectic phases [7.69-7.71]. Further, an average translational difiFusion constant may be estimated if the mean square displacement is known. However, accurate determination of the difiFusion correlation times is possible in liquid crystals provided that a proper theory of translational difiFusion is available for liquid crystals, and the contribution of this difiFusion to the overall relaxation rate is known. In practice, all of the other relaxation mechanisms must first be identified and their contributions subtracted from the observed spin relaxation rate so as to isolate the contribution from translational difiFusion. This often requires careful measurements of proton Ti over a very wide frequency range [7.72]. For spin - nuclei, dipolar interactions may be modulated by intramolecular (e.g., collective motion, reorientation) and/or intermolecular (e.g., self-diffusion) processes. Because the intramolecular (Ti ) and intermolecular... 

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