Orientation chemical shift anisotropy

Fig. 1. Schematic representation of the NMR absorption of a carbonyl functionality a) Single crystal with two different orientations b) Polycrystallinc sample (contributions from the random distribution of orientations, chemical shift anisotropy, CSA) c) In solution (random motion of the molecules yields the isotropic average chemical shift) (Reproduced by permission of The Royal Society, London).

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],... 

Fig. 2 Schematic representation of the 13C NMR signal of a single crystal containing the functional group AB, oriented (A) perpendicular to the applied field, and (B) parallel to the applied field. The lineshape in (C) represents the NMR signal of a polycrystalline sample with a random distribution of orientations yielding the chemical shift anisotropy pattern displayed. (From Ref. 15.)...

The characterisation of the angular dependence of the interaction of two dipole tensors A1 A2 and B B2 is therefore straightforward, namely it depends on the projection angle of the two bonds between A1 and A2 and between B1 and B2. The orientation and magnitude of the chemical shift anisotropy (CSA) tensor, which also can cause cross-correlated relaxation, is not know a priori and therefore needs to be determined experimentally or... 

Fig. 8.10 31P chemical shift anisotropy (CSA) tensor orientation of a nucleotide from a DNA dode-camer with its helix axis pointing vertically. The DNA structure has been calculated using CSA data obtained under anisotropic conditions... 

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... 

As seen from Fig. 15, in the case of unprotonated aromatic para carbons, a phenyl ring yr-flip does not change the orientation of the chemical shift anisotropy tensor components and, thus, this yr-flip does not affect the NMR response of such para carbons, whereas phenyl ring oscillations do. 

In contrast, for protonated ortho and meta aromatic carbons, both phenyl ring yr-flips and oscillations affect the orientation of the chemical shift anisotropy tensor components and, consequently, their NMR response. The change in the amplitude of the oscillations with temperature for protonated and unprotonated aromatic carbons is shown in Fig. 16. For the two types of carbons the amplitudes of the oscillations are quite similar. 

Fig. 15 Effect of a phenyl ring yr-flip on the orientation of the chemical shift anisotropy tensor components for unprotonated and protonated aromatic carbons...

Additionally, anisotropic chemical shift effects will also be present in the spectrum due to the nonexistence of molecular tumbling. This means that, since in solids the atoms and molecules are not moving, they do not change the orientation. Since the ability of the applied field to generate electron currents depends on the orientation of the nuclei relative to the applied field, the chemical shift depends on orientation, and since in solids this effect is not averaged out, it will contribute to the peak broadening. The chemical shift anisotropy also varies with the angle, 9,... 

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... 

The amount of variation of chemical shift with the orientation is called the chemical shift anisotropy, or CSA. CSA is simply the difference between the smallest fixed-position chemical shift and the average of the other two fixed-position chemical shifts ... 

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