Couplings anisotropic quadrupolar

A method which allows testing NMR data such as dipolar couplings, anisotropic chemical shieldings and quadrupolar interactions has been proposed by Zidek et aV The authors have used the fact that only a limited number of frequencies corresponding to these parameters can be measured indepedently in a rigid fragment of the macromolecule. Additional values can be predicted as linear combinations of a set of independent frequencies. Comparison of the experimental data with the predicted values of the parameters studied provides a test of the reliability of the former. 

Fluorinated calamitic thermotropic L.C. represent an important class of materials for high-tech applications, especially in the field of L.C. displays. NMR spectroscopy is the most powerful technique for studying orientational order in L.C. systems at a molecular level thanks to the possibility of exploiting different anisotropic observables (chemical shift, dipolar couplings, and quadrupolar coupling) and nuclei ( H, c, and F). In a relevant paper, the basic theory and NMR experiments useful for the investigation of orientational order on fluorinated calamitic L.C.s are reported, and a review of the literature published on this subject is given. ... 

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. 

The first such experiment (Fig. 21a), introduced independently by Gan [258] and Bodenhausen et al. [259], used 13C as the so-called spy nuclei in a 2D experiment similar to the HMQC scheme used in solution NMR [260], The transfer of magnetization occurred by a combination of weak ]JCN (through-bond) couplings and second-order quadrupolar-dipolar (through-space) cross-terms between 14N and 13C, which contain the isotropic and anisotropic terms l = 0, 2, and 4 and are referred to as residual dipolar splittings (RDS) [261-263], Under MAS, the 1 = 0 term results in isotropic coupling, which for NH3 groups is expected to be... 

A variety of relaxation time studies have been performed on toluene. The choice of deuterated toluene avoids certain complicating factors which affect proton NMR studies, such as, dipolar or spin-spin couplings. The dominant relaxation mechanism is quadrupolar and the relaxation times are determined by the reorientation of the C-D bond vector. Relaxation times such as T, are sensitive to the motions of the solvent around the larmor frequency, which is on the order of 14 MHz in this study. T2 measurements may probe slower motions if the molecule undergoes slow and/or anisotropic motion. The relaxation time results presented in Figure 3 are significantly shorter than those found in bulk toluene solutions (18.19). In bulk toluene, the T and T2 values are equal above the melting temperature (1.2.). In this polymer system T2 < T indicative of slow and/or anisotropic reorientation. 

Superhyperfine interactions are rarely observed in the ESR spectrum of the VO + ion because the unpaired electron interacts only weakly with the hgand nuclei, so that often the size of the coupling is less than the ESR bandwidth. This problem has been surmounted through the use of ENDOR spectroscopy. In ENDOR spectroscopy, molecules with their V=0 axes either parallel or perpendicular to the direction of the static magnetic field are selectively irradiated. In this way, the anisotropic superhyperfine coupling constants of H and " N and the " N quadrupolar coupling constants can be obtained. 

Residual coupling effects due to quadrupolar nuclei in mas spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to P2(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the... 

For first-order interactions (dipole-dipole coupling, chemical shift anisotropy, first-order quadrupolar interaction) the expansion in Eq. (23) contains only the zero- and second-rank terms. As shown in Fig. 3, the Legendre polynomial of rank 2 is zero at the magic angle (P2(cosXm)=0 for Xm=54.74°). Therefore, under MAS, and provided that the spinning rate is larger than the anisotropic linewidth, the anisotropic terms of the first-order interactions are averaged to zero and isotropic spectra are obtained. 

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