Heteronuclear Dipolar Decoupling

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 

In addition to providing motional information there is another benefit to be derived from CP studies which will be exemplified further in Section 8 and which depends on the fact that the effectiveness of the CP phenomenon is proportional to r 3, where r is the distance between the / and S nuclei. In other words, the efficiency of transfer of magnetization falls off extremely rapidly as the distance between / and S increases. Bulk Si nuclei in Si02, for example, will be relatively distinct from any proton species, whereas Si nuclei at the surface will be relatively close to surface OH groups (or any chemically bound organic species). The use of cross-polarization Si n.m.r. will therefore enable one to examine Si nuclei at the surface relative to those in the bulk material. 

A further feature of the CP experiment arising from the rare spin and abundant spin situation is that following transfer of magnetization from / to S there is relatively little diminution of the / magnetization. It is perfectly 

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While the combination of the heteronuclear dipolar decoupling and MAS provides a mean to obtain high-resolution isotropic spectra in solids, the serious problem still remains in addition to the relatively small magnetic moment and low natural... 

Acquisition of the free induction decay of the rare spins by continued irradiation of the iH field for heteronuclear dipolar decoupling. 

Fig. 3.3.2 Illustration of cross-polarization (CP) (a) Tuning diagram of rf excitation for cross-polarization of H magnetization to C. DD denotes heteronuclear dipolar decoupling...

It has been shown that a direct spectral optimisation scheme can be implemented to obtain improved heteronuclear dipolar decoupling schemes for solid state MAS NMR experiments. The resulting schemes, which turn out to have a particularly simple form, are shown to be applicable over the whole range of commercially available spinning speeds (from 5 to 35 kHz), and are shown to improve on the performance of the best existing sequences. 

S. Paul, V.S. Mithu, N.D. Kurur, P.K. Madhu, Efficient heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance at rotary resonance conditions, J. Magn. Reson. 203 (1) (2010) 199-202. 

FIGURE 2.3 Heteronuclear dipolar decoupling method for the case of a H- C spin pair. 

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