Anisotropy spin-lattice relaxation time

The process of spin-lattice relaxation involves the transfer of magnetization between the magnetic nuclei (spins) and their environment (the lattice). The rate at which this transfer of energy occurs is the spin-lattice relaxation-rate (/ , in s ). The inverse of this quantity is the spin-lattice relaxation-time (Ti, in s), which is the experimentally determinable parameter. In principle, this energy interchange can be mediated by several different mechanisms, including dipole-dipole interactions, chemical-shift anisotropy, and spin-rotation interactions. For protons, as will be seen later, the dominant relaxation-mechanism for energy transfer is usually the intramolecular dipole-dipole interaction. 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

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. 

Other experimental methods for obtaining chemical shift anisotropies are the spin-lattice relaxation, molecular beam, and clathrate methods. The spin-lattice relaxation time, T, measures how soon a nuclear spin or assemblage of spins returns to its equilibrium position by releasing eneigy to the total system. Any mechanism that provides a fluctuating magnetic field can, in principle, contain the proper Fourier... 

Since nuclear spin-lattice relaxation times, Ti, are such a critical parameter in determining the recycle time of FT NMR experiments several studies have examined the magnitude and mechanism of Se relaxation For small molecules the spin rotation mechanism dominates and for larger molecules where this mechanism is not so effective, the chemical shift anisotropy mechanism becomes more effective. Interestingly, the dipole-dipole mechanism has not been found to be an efficient relaxation... 

Motions shorten spin-lattice relaxation times. Analysis can provide insight into molecular motions. Dynamics of a radicals can be studied by analysis of contributions due to rotational modulation of hyperfine and Zeeman anisotropies. 

The advent of pulse Fourier Transform (FT) NMR techniques in the middle 1970s set the stage for the use of Si NMR for qualitative and quantitative analysis in liquids. However, for solid samples the effects of H- Si magnetic dipole-dipole interactions and Si chemical shift anisotropies and the time bottleneck of long Si spin-lattice relaxation times render the direct application of the liquid-state Si NMR technique essentially useless yielding broad, featureless spectra of low intensity. For undo tanding the aspects for solid-state Si NMR, those for solid-state NMR will be briefly presented. 

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