Coupling to other Nuclei

Two additional interactions experienced by the nucleus in the solid state are spin-spin couplings to other nuclei, and quadrupolar interactions, which involve nuclei of spin greater than 1/2. Spin-spin (Hx), or J, coupling,... 

Hj — indirect (electron-coupled) spin-spin couplings to other nuclei. 

The first consideration by looking at Eqs. (7.5), (7.6) and (7.7) is that NOE does not contain any structural information within the present scheme. Indeed, both / >/(/) and /(/). In practice, the two-spins scheme is never a good description of any real system. The nucleus I, on which NOE is going to be observed, is always coupled to other nuclei. When dealing with hyperfine coupled nuclei, often the major source of relaxation is not the coupling with J but the coupling with the unpaired electrons. The expression for t)i(j) then is (Appendix VII)... 

Heteronuclear dipolar coupling can be a major broadening mechanism, especially as in a number of materials protons are present which often have dipolar couplings to other nuclei in excess of 50 kHz. This means that MAS is usually not sufficient to remove the effect of the dipolar coupling in the spectra. C NMR spectra are, for instance, often obscured by dipolar coupling to the protons. In order to remove the carbon-proton coupling the protons are irradiated with an rf field while, at the same time, the carbon spectrum is measured. Consider the CH system where the heteronuclear dipolar... 

A nucleus with a spin 1= 0 has no NMR spectrum, it is NMR silent, and does not couple to other nuclei. Important examples are C, 0, and S. We are all familiar with the consequences from C-NMR. Here the predominant isotope is C, an NMR silent nucleus. The NMR active nucleus C has a natural abundance of only 1.1%. Coupling to H is observed and usually deactivated by broadband decoupling. Couphng to carbon is not observed, except in the case of C enrichment, due to probability constraints. 

As we did in the COSY experiment, we want to allow the magnetization vectors of the protons to process according to different rates, as dictated by their chemical shifts. Therefore, we apply a 90° pulse to the protons, then include an evolution time (ti). This pulse tips the bulk magnetization vector into the Z F plane. During the evolution period, the proton spins process at a rate determined by their chemical shifts and the coupling to other nuclei (both protons and carbons). Protons bound to atoms experience not... 

An additional consequence of quadrupolar relaxation is that spin-spin coupling to other nuclei, which is readily observable if is present, is usually eliminated. Hence a valuable source of structural information is lost. 

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