Residual quadrupolar interaction

Figure 15.15 Natural abundance 2H MAS spectra observed in a series of vulcanised natural rubbers with various vulcaniser (sulfur 1 or 3 wt%) and/or filler (carbon black 0 or 40 wt%) contents. The spinning speed is 0.5 kHz. The number of scans is about 300000. Spectra are simulated with two components (a mobile and a rigid one) with various residual quadrupolar interactions...

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. 

In case of anisotropic motions (an anisotropic fluid), the quadrupolar interaction is partially time-averaged to a residual tensor. This residual interaction A modulates the time relaxation function via a reversible dephasing MYt)=exp[-t/T2]cosAt. For motions uniaxial around a symmetry axis, the effect on the spectrum is to split the resonance line into a symmetric doublet with a splitting (in frequency units) ... 

While the Overhauser shift is due to delocalised electrons, the ENDOR experiments require more localised electrons. Both groups were able to observe ENDOR on the effective mass donor resonance but not on the deep donor signal. An illustrative ENDOR spectrum is shown in FIGURE 4, in which the 69 71Ga ENDOR lines are observed. The 69Ga line is clearly split by quadrupole interactions just as in the Oveihauser shift measurements, while the quadrupolar interaction is much less resolved in the 71 Ga line, due to its smaller quadrupole moment. In the three samples investigated by the two groups, the linewidths were all too broad to resolve any hyperfine interaction and so no definitive identification of the residual donor was possible. 

Based on the Ga quadrupolar interaction and the orientation dependence of the (hyperfine broadened) central linewidth, Koschnick et al argue that their ENDOR signal is due to an interstitial Ga atom which is the residual donor in their film [59], Glaser and co-workers argue that the Ga nuclei observed in their resonance... 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

The residual dipolar coupling also manifests itself in NMR spectra of dipolar-coupled quadrupolar spin pairs.Wu and Yamada also observed residual dipolar coupling attributed to the N- B dipolar interaction in the NMR spectra of MAS samples of triethanolamine borate. More recently, the effect of the N- B, "B- B, and Mn- Mn residual dipolar interactions on the B and Mn NMR line shapes acquired through MQMAS were also demonstrated. ... 

Phase diagram studies by NMR are generally based on the fact that the rapid molecular dynamics causes an elimination of any spin interactions to an extent that is directly related to the degree of anisotropy of the structure. The most important use of NMR is to distinguish between phases that give residual dipolar or quadrupolar interactions and phases that do not in general, this also marks the difference between optically anisotropic and optically isotropic phases. Whether residual couplings are seen depends on the extension of the anisotropic domains (microcrystallite sizes) in combination with the rate of molecular diffusion. 

In the same way, the contribution from the first-order quadrupolar interaction can be successfully removed by applying fast MAS. However, the resulting spectrum measured even at a very high-spinning speed is only partially narrowed and still exhibits some remaining anisotropic broadening the residual is narrowed by about V3 of the original width (Fig. 10). Similarly, Eq. (11) describes harmonic oscillation P4 which indicates that, to get rid of... 

The chemical shielding, dipole-dipole, spin-spin indirect coupling or J-coupling, spin-rotation, and hyperfine couplings represent the major internal magnetic interactions. The quadrupolar interaction has an electrostatic character. All these interactions have a tensorial character, ie, are function on the ori-entation of the principal axes of the tensor relative to the direction of Bq. They are relevant for solid polymers below and around the glass transition temperatures. For polymer in solution or for soft polymers fast molecular motions average these anisotropic interactions to isotropic or residual values which can be zero. Detailed description of the properties of these spin interactions can be found in References 1-9. 

II.5 " H NMR background Fast, anisotropic reorientations of a C-D bond lead to a quadrupolar interaction which is no longer averaged to zero. ° When motions are uniaxial around a macroscopic symmetry axis, such a residual interaction splits the liquid-like NMR line into a doublet whose spacing is in frequency units ... 

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