Chemical-shift anisotropy mechanism

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. 

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.

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

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... 

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. 

When other relaxation mechanisms are involved, such as chemical-shift anisotropy or spin-rotation interactions, they cannot be separated by application of the foregoing relaxation theory. Then, the full density-matrix formalism should be employed. 

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

NMR spin relaxation is not a spontaneous process, it requires stimulation by a suitable fluctuating field to induce an appropriate spin transition to reestablish equilibrium magnetization. There are four main mechanisms for obtaining relaxation dipole-dipole (most significant relaxation mechanism for I = 1/2 nuclei), chemical shift anisotropy, spin rotation, and quadrupolar (most significant relaxation mechanism for I > 1/2 nuclei) (Claridge, 1999). 

Apart from the relaxation mechanism described here, other mechanisms such as relaxation involving cross-correlation between dipole-dipole coupling and chemical shift anisotropy (CSA) can also provide structural information [48, 49]. The expression for this relaxation rate in case of axial symmetric CSA tensors is... 

Another CCR mechanism is the interaction of the magnetic dipole with the chemical shift anisotropy (CSA) tensor, e.g., the interaction with the carbonyl CSA-tensor in proteins. The dipole-CSA CCR-rate is also dependent on the projection angle 9 between the magnetic dipole and CSA tensors ... 

J-splitting, when it exists, imposes the definition of new spin quantities. These quantities also evolve according to relaxation phenomena and may interfere (by relaxation) with the usual magnetization components. This latter interference stems precisely from cross-correlation rates, i.e., relaxation parameters which involve two different mechanisms, for instance the dipolar interaction and the so-called Chemical Shift Anisotropy (27,28) (csa)... 

It can be seen that, in all cases, relaxation rates are directly proportional to (Aa). Because Aa reflects the anisotropy of the shielding tensor and because the chemical shift originates from the shielding effect, the terminology Chemical Shift Anisotropy is used for denoting this relaxation mechanism. Dispersion may be disconcerting because of the presence of Bq (proportional to cOq) in the numerator of and R2 (Eq. (49)). Imagine that molecular reorientation is sufficiently slow so that coo 1 for all considered values of coo from (49), it can be seen that R is constant whereas R2 increases when Bq increases, a somewhat unusual behavior. 

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