Quadrupolar, chemical shift anisotropy

The process of T relaxation can have contributions from a number of different mechanisms, e.g. dipole-dipole, scalar coupling of the first or second kind, quadrupolar, chemical shift anisotropy (CSA) and spin-rotation. The temperature dependence of the T relaxation time depends upon the relative contribution of each of these mechanisms. For example, if spin-rotation is the dominant relaxation mechanism, then the Ti value decreases linearly with increasing temperature, whereas if dipole-dipole is dominant the T value increases with increasing temperature in the extreme narrowing limit. Examples of temperature measurements using T are included in Sections 3.2.5 and 5.1. 

NMR quadrupolar/ Chemical shift anisotropy/ Magnetic dipolar lineshapes Infrared/ Raman spectroscopy Inelastic x-ray/Neutron scattering... 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

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). 

The magnitude of the chemical shift anisotropy depends on the bonding situation and the nucleus gyromagnetic ratio. Since the bonds formed by lithium in organolithium compounds or other lithiated systems are mainly ionic, the anisotropy of the lithium chemical shift is generally small. It is more pronounced for Li than for Li. Li spectra are dominated by the quadrupolar effect and the CSA contribution to the Li lineshape is often negligible. Exceptions are compounds with poly-hapto bound lithium, such as... 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

The anisotropic nature of the dipolar, quadrupolar and chemical shift anisotropy interactions requires that the isotropy of molecular orientation relative to the applied magnetic field be broken in order to allow their direct observation in terms of shifts in the frequencies of resonances.20,32,38 40 For high resolution NMR studies this has meant, thus far, that some degree of alignment of the molecule needs to be established. Almost all molecules will align to a small extent due to the anisotropy of their magnetic... 

The following strategy therefore emerges for the study of quadrupolar nuclei observe the central transition of nuclei with noninteger spin, use MAS (to remove dipolar coupling, chemical shift anisotropy, and first-order quadrupolar effects), and work at high fields (to minimize second-order effects). 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Broad Lines. - The width of an n.m.r. line, At>1/2, is defined as the width in Hz at half signal height. Narrow lines, i.e., Ap1/2 < 10 Hz, are desirable in order to make use of chemical shift information and to follow chemical change. N.m.r. line widths in the liquid and the physisorbed state tend to be very narrow, with Ar>1/2 of the order of 10-1Hz. This fortuitous state arises because the molecular motion is sufficiently rapid and random in a liquid to average out the line broadening features present in solids, namely dipolar interactions, chemical shift anisotropy, quadrupolar interactions, and paramagnetic interactions which render the spectrum unusable under conventional or liquid-state experimental conditions. The mechanisms of each of these features will be described. The treatment will perforce be cursory, but an indication will be given to where a full theoretical treatment can be found. 

The specific line broadening effects which MAS reduces most effectively are the chemical shift anisotropy and quadrupolar coupling. Although dipolar broadening is also reduced, it is not entirely eliminated, and other techniques... 

Besides the opportunity to measure chemical shift anisotropies, MAS NMR also offers the possibility to measure the scalar couplings to other isotopes with magnetic moments. Examples to determine the spatial components of these couplings can be found in the literature48,49. Even couplings to quadrupolar nuclei not observable in solution can be deduced from the MAS spectra48. 2D techniques such as HETCOR are also applicable for solids49. 

The important nuclear spin interaction in the context of probing molecular reorientation is provided by the quadrupolar interaction (Q) or the chemical shift anisotropy (CSA), and we assume that the spin system is prepared by, for example, isotopic labeling in such a way that only a single interaction is relevant. The ubiquitous presence of dipolar broadening is assumed to be small. For most NMR experiments it suffices to consider the secular part of the... 

The most commonly encountered broadening interactions in solids are the chemical shift anisotropy, the direct dipole-dipole interaction, and the quadrupolar interaction. The chemical shift and quadrupolar interactions are inhomogeneous They involve spins individually, whereas the dipole-dipole interaction involves pairs of spins. 

MAS is routinely used in solid-state NMR spectroscopy for eliminating the elfects of chemical shift anisotropy, heteronuclear dipolar interactions and first-order quadrupolar interactions. In this method the sample is rotated abont the axis inclined at 54.74° with respect to the external magnetic field Bo, so that the average of the geometric term in nnclear spin interactions (3cos 0 — 1) = Except for some specific applications of static wide-line solid-state NMR spectroscopy (i.e., for or for the determination of CSA patterns), the MAS is an essential feature of solid-state NMR spectroscopy and can also be nsed in variable temperature mode. The MAS methodology can also be nsed for high-resolntion studies of viscous hquids and soft solids. 

K Zeeman b K-Dipole—Dipole I" "K-Quadrupolar b K Chemical Shift Anisotropy b "Uspin Rotation "b .. 

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

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