Chemical shift anisotropy orientation experiments

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],... 

For the most part, exchange experiments in the solid state use either chemical shift anisotropy (for spin4) or quadrupole coupling (for spin > i) under static conditions, i.e. no sample spinning, to generate frequencies uj and 102 that depend on molecular orientation. The projections onto the two spectral frequency axes are then the corresponding powder patterns resulting from the... 

Equation (3) has several other important implications which can be directly confirmed by finite-frequency probes. One example is the motion-narrowing effect in NMR experiments which is expected to disappear when l/r is below the chemical-shift-anisotropy (CSA) width. Indeed the NMR results of Tycko et al. [16] indicate that for a CSA width of 18.2 kHz the line broadens below 190 K and develops a powder pattern at lower temperature. This is in fair agreement with the 200 K calculated from Eq. (3). They also concluded that the thermal activation energy is around 260 meV below TV, again close to the values we calculated. The glassy dynamics can be probed by other experiments such as sound attenuation, microwave absorption, and thermal conductivity. In particular the characteristic temperature will depend on probe frequency. Such studies are essential to fully understand the low-temperature orientational dynamics. 

Owing to the orientation dependence that it imparts to the NMR frequency, the chemical-shift anisotropy (CSA) has proven useful not only in studies of slow dynamics but also for characterizing segmental orientation distributions and fast segmental reorientations. While static powder patterns provide this CSA information in the most accessible form, site resolution by MAS is indispensable in all but the simplest unlabeled systems. The two requirements can be combined in two-dimensional (2D) separation experiments. Recently, a robust sequence, termed separation of undistorted powder-patterns by effortless recoupling (SUPER), was introduced that makes CSA measurements under standard MAS conditions routine.28 It enables identification of functional groups and measurements of orientation distributions, segmental dynamics, and conformations. 

The isotropic spectrum in the other dimension of the experiment is produced as follows. MAS averages the anisotropic parts of the chemical shift anisotropy to zero by continuous rotation about a vector oriented at the magic angle with respect to the applied magnetic field, B0. In fact, however,... 

One potential problem with chemical shift anisotropy lineshape analysis (or indeed analysis of lineshapes arising from any nuclear spin interaction) is that the analysis results in a description of the angular reorientation of the chemical-shielding tensor during the motion, not the molecule. To convert this information into details of how the molecule moves, we need to know how the chemical-shielding tensor (or other interaction tensor) is oriented in the molecular frame. A further possible complication with the analysis is that it may not be possible to achieve an experiment temperature at which the motion is completely quenched, and thus it may not be possible to directly measure the principal values of the interaction tensor, i.e. anisotropy, asymmetry and isotropic component. If the motion is complex, lack of certainty about the input tensor parameters leads to an ambiguous lineshape analysis, with several (or even many) possible fits to the experimental data. 

The chemical shift anisotropy arises from the nonspherical electron density around the nuclei, and is particularly prominent for aromatic and carbonyl (C=0) carbon types. These carbon types experience different shieldings of the magnetic field depending on whether the bond axes are parallel or perpendicular to the external magnetic field. For polycrystalline or amorphous materials all orientations are possible, including these two extremes. 

These slow motions which induce a change in the orientation of the chemical shielding tensor of a particular spin can also be studied by spin echo spectroscopy [47]. The experiment consists of producing and digitizing a whole spin-echo train while simultaneously decoupling the protons in a static sample. The Fourier transform of the train gives a spectrum in which the powder pattern of the chemical shift anisotropy is split into a number... 

Fast spinning MAS spectra, even in the case of J-resolved 2D experiments do not allow to distinguish between the presences of one or two chemically equivalent nuclei since under fast spinning MAS conditions the situations is comparable to liquid state NMR i.e. chemical shift anisotropies are averaged out. However, under static conditions, the chemical shift tensors of two chemically equivalent nuclei do not coincide and both nuclei have in general dilferent chemical shifts at certain orientations of the molecules with respect to the magnetic field axis, i.e. they are magnetically inequivalent. Thus under static conditions, the spin system may be described as an AB or AX system, instead of an A2 spin system. 

These angles were determined in turn from distances between backbone carbonyl spins, measured with the DQ filtered dipolar recoupling with a windowless sequence experiment, and by determination of the mutual orientation of chemical shift anisotropy tensors of carbonyl spins on adjacent peptide planes, obtained from the DQ CP MAS spectrum. It was found that peptides composed of periodic sequences of leucines and lysines were bound along the length of the peptide sequence and displayed a tight a-helical secondary structure on the gold nanoparticles. 

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