Homonuclear Dipolar Decoupling

The factor of /a arises from the fact that the spin spends an equal amount of time along each of the Cartesian directions. It is clear from this expression that the sequence successfully removes the homonuclear dipolar coupling. However, it is important to realise that such manipulation by pulses has the effect of scaling other operators and not completely removing them. The most important is I, its average value for the above sequence being 

This means that there is an effective field along the unit vector 1/J3( 1,1,1) direction in the Cartesian basis, producing an effective scaling of the frequency of 1/ 3. This basic sequence was developed into longer sequences such as MREV-8 and BR-24 (Table 2.7). 

Sequence Pulse cycles (phase) Cycle time Scale factor for Iff 

All these pulse-cycle techniques are in practice complicated and there are many pitfalls. The potential experimentalist is therefore recommended to refer to the literature4 25 28 and to the summary in ref. 1, p. 196. 

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The inclusion of average Hamiltonian theory derived pulse sequence building blocks specifically designed for the measurement of RDCs is highly desirable. The homonuclear dipolar decoupling sequences used in ref. 190 lead to the less complex multiplet structures observed in isotropic samples with the chemical shift resolution reduced by a factor of 2-3, depending on the multiple pulse... 

Homonuclear dipolar decoupling can be achieved by using so-called multiple pulse sequences (MPS). These are carefully tailored short and intense pulse cycles averaging the homonuclear dipolar interactions by reorientadon of the nuclear spins. 

In this section we list some of the homonuclear dipolar decoupling multiple-pulse sequences for high-resolution NMR in the solid-state which we will analyse... 

Symmetry arguments have been proposed by Levitt and coworkers [77,86-88] that enable a constructive combination of both MAS and RF based on certain symmetry properties of the internal spin Hamiltonians. Two classes of symmetry sequences have been proposed, CN [86] and [87,88]. Selective recoupling or decoupling of spin interactions is possible with a suitable combination of the symmetry numbers, N, k and x- An example of RI82 for homonuclear dipolar decoupling will be discussed later. 

In this section we introduce bimodal Floquet theory (BMFT), based on which we represent the spin Hamiltonians and analyse the various homonuclear dipolar decoupling sequences. 

In homonuclear dipolar decoupling experiments this RF sequence is applied to accomplish efficient decoupling, leaving the isotropic chemical shifts at least partially intact. The actual time dependence of the RF coefficients varies for each experiment. Here we will assume that the Hupit) term is periodic in time with an RF cycle time r ... 

Based on the previous discussion, during homonuclear dipolar decoupling experiments, the zero-order effective Hamiltonian is at least equal to... 

In this section we will deal with homonuclear dipolar decoupling sequences where the RF irradiation is synchronised with MAS. 

Table 2 shows examples of CN] homonuclear dipolar decoupling sequences with values of k according to Eq. 76. The k values are tabulated for both homonuclear dipolar (with interaction terms proportional to and... 

Table 2. Examples of homonuclear dipolar decoupling sequences ( A < 10)...

D. SakellarioLi. A. Lesage, P. Hodgkinson and L. Emsley, Homonuclear dipolar decoupling in solid-state NMR using continuous phase modulation. Chem. Phys. Lett., 2000, 319, 253 -260. 

A. Lesage. D. Sakellariou, S. Hediger, B. Elena, P. Charmont, S. Steuernagel and L. Emsley, Experimental aspects of proton NMR spectroscopy in solids using phase-modulated homonuclear dipolar decoupling. J, Magn, Reson.. 2003, 163, 105-113. 

Spectroscopic imaging with homonuclear dipolar decoupling... 

In 1968, J. S. Waugh et al reported a new method for removal of the homonuclear dipolar interaction using multiple pulses (WAHUHA-4 Waugh, Huber and Haeberlen 4 pulses/cycle), which is shown in Fig. 2. Homonuclear dipolar decoupling by multiple-pulse irradiation is termed MAS in spin space . In general, we consider that the magnetization of the z direction is... 

For such an experiment to work, we have to be able to distinguish the different domains during the evolution and the detection period of the two-dimensional experiment. Since proton spectral resolution in typical solids is very poor, we have to use homonuclear dipolar-decoupling methods to narrow the lines sufficiently to obtain spectral resolution. The 2D spin-diffusion CRAMPS spectrum was first recorded by Caravatti et al. [68] for blends of polystyrene (PS) and polyvinyl methyl-ether (PVME). There are other methods to generate an initial nonequilibrium polarization based on differences in linewidth or relaxation times. The reader is referred to the excellent book of Schmidt-Rohr and Spiess [67] for an overview. 

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