Spin-lattice relaxation dipolar mechanism

The main contribution to the spin-lattice relaxation of C nuclei which are connected to hydrogen is provided by the dipole-dipole interaction (DD mechanism, dipolar relaxation). For such C nuclei a nuclear Overhauser enhancement of almost 2 will be observed during H broadband decoupling according to ... 

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

This simple theoryis based on the expectation that, to a reasonable degree of approximation, proton-proton, dipolar contributions to the measured spin-lattice relaxation-rate are pairwise additive and decrease as a simple sixth power of the interproton distance. The simplified version of the dipole-dipole mechanism is summarized in the following two equations for spin i coupled intramolecularly with a group of spins j... 

Differing 7j values for CH3, CH2, and CH carbon nuclei within a molecule can arise not only by methyl rotation or anisotropic molecular motion, but also from the segmental mobility of partial structures, even when the dipolar mechanism predominates. Thus the spin-lattice relaxation times of methylene carbon atoms in long alkane chains pass through a minimum at the middle of the chain. In the presence of heavy nonassociating... 

Carbon-13 relaxation-rates of monosaccharides are dominated by dipolar-relaxation mechanisms,18,22 and primarily give information about molecular motion,75,76 in addition to the somewhat trivial distinction between C, CH, CH2, and CH3 groups. However, by measuring spectra with a suitable pulse-sequence, the differences in spin-lattice relaxation-rates can be used for the assignment of signals from overlapping CH and CH2 groups.77... 

Spin-lattice relaxation can occur by several mechanisms, in addition to dipolar interactions ... 

Sturz and DoUe measured the temperature dependent dipolar spin-lattice relaxation rates and cross-correlation rates between the dipolar and the chemical-shift anisotropy relaxation mechanisms for different nuclei in toluene. They found that the reorientation about the axis in the molecular plane is approximately 2 to 3 times slower than the one perpendicular to the C-2 axis. Suchanski et al measured spin-lattice relaxation times Ti and NOE factors of chemically non-equivalent carbons in meta-fluoroanihne. The analysis showed that the correlation function describing molecular dynamics could be well described in terms of an asymmetric distribution of correlation times predicted by the Cole-Davidson model. In a comprehensive simulation study of neat formic acid Minary et al found good agreement with NMR relaxation time experiments in the liquid phase. Iwahashi et al measured self-diffusion coefficients and spin-lattice relaxation times to study the dynamical conformation of n-saturated and unsaturated fatty acids. 

Relaxation of nitrogen nuclei apparently is governed by modulation of the electron-nuclear dipolar interaction, the so-called END mechanism. The nitrogen nuclear relaxation probability can be greater than the electron spin-lattice relaxation probability. See, for example, the paper by Popp and Hyde [45]. One consequence of this process is that it can alter the apparent relaxation time of the electron since it gives rise to parallel relaxation pathways. One must distinguish between apparent and actual electron spin-lattice relaxation probabilities. 

We already pointed out that it is much less likely for systems with > to be characterized by a spin temperature. Even for I=% systems, the existence of a uniform spin temperature is not automatic. What is necessary is some mechanism by which a large number of spins can come to thermal (in the spin temperature sense) equilibrium with each other in times short compared with the spin-lattice relaxation time T. Such a coupling between spins usually exists in solids as dipolar coupling which can be characterized by the spin-spin relaxation time Tg. In most solids, Tgspin temperature. In nonviscous liquids, the condition T Tg prevents the existence of a homogeneous spin temperature. For more on spin temperature and how it can be used, see Hebei (1963), Redfield (1969), and Goldman (1970). 

Pulsed NQR measurements of the spin-lattice relaxation time, T, also give detailed information on the mechanisms and dynamics of molecular motion in solids. For example, quadrupole relaxation times for solid triethylenediamine show a Ti minimum at a temperature close to 260 K. when 7j equals 0.048 On either side of the minimum, 7j depends exponentially on temperature, according to an Arrhenius-type equation with an activation energy of 34 kJ moP . The mechanism of the relaxation is shown to be hindered rotation of the triethylenediamine molecule about its threefold symmetry axis, which modulates the dipolar coupling between N and the adjacent CH2 protons. 

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