Dipolar coupling between spin species

The heteronuclear dipolar coupling between spin species I and S is given in Eq. (2) and rewritten below in different form ... 

Heteronuclear correlation (HETCOR) solid-state NMR spectroscopy [54] has been widely used to provide information on the spatial proximity of different nuclei in complex spin systems. When more than one distinct / and/or S spin species is present, a 2D correlation experiment enables the detection of dipolar couplings between specific distinct IS pairs. Typically, the experiment consists of a 90° pulse that creates transverse I spin magnetization, which evolves for a time tj before it is transferred to spin S, usually via CP. The S spin FID is then detected in t2- 2D Fourier transform yields a 2D spectrum, with the appearance of cross peaks between individual I and S resonances from spins which are dipolar coupled. 

Figure 19 Demonstration of the rotational resonance effect for the framework-bound ethoxy species obtained on CsX from ethyl iodide-1,2- C. The rotational resonance condition was achieved when the spinning speed was set to match the frequency difference (4646 Hz) between the signals at 68 and 17 ppm. Rotational resonance reintroduced the C- C dipolar coupling for the rigid ethoxy species, resulting in a splitting pattern that reflects the short intemuclear distance.

Anisotropic A, with principal values b Apart from the spin flip lines due to weak dipolar coupling with distant H atoms of the matrix this case is rare at X-band and lower frequencies, but may become of interest for measurements at high microwave frequency bands. The situation is most likely to occur for species with anisotropic hyperfine couplings of moderate size due to nuclei with large nuclear g-factors like or F. The effective field acting on the nucleus is dominated by the applied field, so that a and the normal selection rule Ami = 0 applies. The splitting between the lines is AT = 1 A 1 and A g = 1 Ag 1 for isotropic and anisotropic g-factors, respectively. 

It is worth noting that the linewidth of this resonance decreases with increasing Tc This behavior is in sharp contrast to the situation described by Eq. (2) and to the dipolar relaxation process, where the linewidth increases with increasing VqTc. From the field dependence of the chemical shift and linewidth described in Eqs. (3) and (4), the values of 7 and for spin 5/2 nuclei in the slow motion limit can be obtained. As the parameter % provides a measure of the coupling between the nuclear quadrupole and the electric field gradient at the nucleus, it reflects the symmetry of the local environment of the nucleus and thus can give information on the nature of the colloidal species, ic is a measure of the mobility of the species involved and, in principle, can provide information on the magnitude of the particle size when Eq. (1) is used. 

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