Anisotropy mechanical

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. 

Yang, D. W., Konrat, R., and Kay, L. E. (1997). A multidimensional NMR experiment for measurement of the protein dihedral angle psi based on cross-correlated relaxation between (H alpha-13C alpha) XH dipolar and 13C (carbonyl) chemical shift anisotropy mechanisms. J. Am. Chem. Soc. 119,11938-11940. 

A contribution caused by spin-orbit coupling and closely related to magnetocrystalline anisotropy is magnetoelastic anisotropy. Mechanical stress creates a strain which amounts to a lattice distortion and yields a correction to the magnetocrystalline anisotropy. Surface anisotropy is a manifestation of magnetocrystalline anisotropy, too (sections below and Ch. 3). 

Finally, we address the apparent trend of the kj-values for the DCA-PSZ pair to increase slightly at the high held end. Actually, this trend can be reproduced by the contribution k ta of the g tensor anisotropy mechanism, however, using an orienta-tional correlation time Tc of 8 x 10 s that is only about half the value extracted from a ht of the spin-rotational mechanism, as described above. It is possible that distortional motions of the nonplanar azine ring—which has been invoked to account for the deviation of the isotropic hyperhne constants of the azine radicals... 

HETCOR113 or hetero-TOCSY 114,115 see also a review by Pardi87 and references therein. Unfortunately, these methods are not routinely used for structure determination of nucleic acids, except for relatively short oligonucleotides, because of the relatively small chemical shift dispersion of 31P and its fast relaxation via the chemical shift anisotropy mechanism. 

The final example is the dences of the 13C resonance in Fe(CO)j- by Spiess and Mahnke (1972). The temperature dependence of Tj can be made to reverse upon application of a large field because the chemical shift anisotropy mechanism takes over at high field while the spin rotation is the main mechanism at low field. 

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... 

Fig. 7. 49.8 MHz Si H NMR spectrum of the complex [(Ph2PC7H7)Pt(C=C-SiMe3)2] dissolved in CD2CI2. The Si NMR signals are split due to P- Si spin-spin coupling and accompanied by the Pt satellites. These Pt satellite signals are significantly broader than the respective central signals owing to efficient nuclear spin relaxation via the chemical-shift-anisotropy mechanism (short ( Pt)). Adapted from ref. 31b.

Additive gases are also important for anisotropy mechanism for instance deposition of SiO , film is essential for sidewall passivation in silicon etching in SF6/O2 plas-... 

The rate of nuclear spin-lattice relaxation due to the chemical shift anisotropy mechanism de-... 

Relaxation by spin-rotation, dipole-dipole, scalar coupling, or shielding anisotropy mechanisms. 

Relaxation of the cadmium-tetraphenylporphyrin pyridine adduct studied by Jakobsen et al. (28) is strongly dominated by the chemical shielding anisotropy mechanism. Even at the moderate field strength of 4.7 Tesla ( Cd 44 MHz), this mechanism is responsible, within experimental error of 10%, for all observed relaxation (T = 28.5 sec). 

Recent improvements have increased the sensitivity of NMR instrumentation, making relaxation rate experiments of cadmium-metalloprotein systems feasible if not commonplace. Bailey et al. (41) have measured a 1.2 sec ( 20%) relaxation time for cadmium occupying the zinc binding sites in bovine superoxide dismu-tase. Theoretical considerations indicate nearly equal contributions to the relaxation rate from proton-cadmium dipolar and from chemical shielding anisotropy mechanisms. 

Although other relaxation mechanisms may be important for some nuclei, the dipolar relaxation mechanism of P that is coupled to protons and the chemical-shift anisotropy mechanism are most important for nucleic acids. In the case of dipolar relaxation, expressions for the spin-lattice relaxation time T, spin - spin relaxation time Tj, nuclear Overhauser effect (NOE), rotating frame spin-lattice relaxation time in an off-resonance radiofrequency (rf) field T°, and off-resonance intensity ratio R are given by (Doddrell et a/., 1972 Kuhlmann eta/., 1970 James eta/., 1978 James, 1980)... 

We can consider how much of the P relaxation should be attributed to the dipolar mechanism and how much to the chemical-shift anisotropy mechanism. Paramagnetic contributions to the relaxation from transition-metal ions are assumed negligible paramagnetic metal ions could influence... 

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