Heteronuclear dipolar interaction

However, in the presence of a strong quadrupolar interaction, the second-order cross-terms between the quadrupolar interaction and the CSA [27, 28] or the heteronuclear dipolar interaction [29, 30] also have to be taken into account. For instance, upon including the CSA in the expression of the time-dependent Hamiltonian in (9), the second-order term becomes... 

Table 2 lists a number of publications of recent years on investigations of spatial orderings, interactions, and local structures, most of which have employed methods based on heteronuclear dipolar interactions. 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

Since dipolar interaction is a distance-dependent interaction, the heteronuclear interaction between spins 1 and S can be exploited to get information about the distance between these nuclei. Heteronuclear dipolar interactions that are averaged by magic angle spiiming can be reintroduced by suitable dephasing pulses that are synchronized with the sample spinning. It is a difference spectroscopy... 

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

The first combined 13C- H MAS/CP experiment was performed by Schaefer and Stejskal (23). The double-resonance procedure decouples the strong heteronuclear dipolar interactions and indirect J couplings, while the weak 13C signal is enhanced by proton polarization transfer. The residual spectral width of several kHz arises from 13C chemical shift anistropy and weak 13C- 3C dipolar interactions between 1.1 % abundant 13C nuclei. Both of these interactions are removed by MAS. 

Fig. 84. Two-dimensional 13C NMR spectrum of ammonium hydrogen malonate (NH4HC3H204) (414) using the pulse sequence shown in Fig. 83. Heteronuclear dipolar interactions, to are plotted versus chemical shift frequency, to2. The projection of the 2D spectrum onto the

Cross-polarisation (CP) is a solid-state NMR experiment designed to achieve a higher sensitivity for the rare nucleus through the transfer of polarisation, driven by the heteronuclear dipolar interaction, from an abundant ( ll) to a dilute spin (13 C).118 Transfer of magnetisation is possible... 

Heteronuclear correlation (HETCOR) is an experiment establishing a correlation between the chemical shift of proton and that for another nucleus (i.e. C, P, Nor Si). The experiment relies on the heteronuclear dipolar interactions and uses a CP to transfer magnetization from protons to a rare spin.4,6 We have used 2D HETCOR experiment to characterize support/counter-cation/cluster interaction in the supported mesoporous catalysts.21... 

For the experimental conditions given, the spectra are found to be dominated by heteronuclear dipolar interactions which leads to relatively narrow, well-separated dipolar spinning sidebands. The heteronuclear dipolar couplings could be evaluated by simulating the spectra on this basis (see right-hand column in Figure 14.12). They were found to be between 0.9 kHz and 1.5 kHz for the samples of the series (the inter-crosslink masses Mc were between 6700-11000 g/mol). A practically identical envelope of the spinning-... 

Usually a homonuclear decoupling pulse sequence does not simultaneously decouple heteronuclear dipolar interactions, albeit in many cases the heteronuclear dipolar couplings need to be retained. With a significantly modified magic-sandwich sequence, Schmidt-Rohr proposed a pulse sequence that can eliminate both, at least up to first order.63 This sequence is particularly useful for enriched 13C pairs and can be readily combined with 2D experiments. It would be interesting to investigate whether such a sequence or similar ones can work for other nuclei particularly large-7 species. 

Mastui et al.29S proposed a new version of the magic echo (ME) sequence, called tetrahedral magic echo (TME), which uses the so-called tetrahedral angle 0=109.5° instead of 90° in the ME or modified ME (MME). This is substantially superior to ME or MME line narrowing for solids where heteronuclear dipolar interaction between protons and the other nuclear species is comparable to the homonuclear proton-proton interaction. 

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. 

Since CP is governed by the heteronuclear dipolar interactions, the Tis time constant is related to the intemuclear distances and molecular mobility. The relaxation time describes the decay of intensity for longer contact times. Relaxation is mostly ensured by the Ft- lT homonuclear dipolar interactions. In contrast to the Tis time characteristic of the chemical group under study, relaxation time is a volume property averaged over the distance of ca. 

If the I-S heteronuclear dipolar interactions are weak, e.g. reduced by molecular mobility or spacial remoteness, while the I-I homonuclear dipolar interactions are moderate or strong, then the CP kinetics is likely to follow the classical I-S model. In most cases, it is described by Eq. (2) ... 

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