Dipolar-decoupling experiment

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... 

The experiment is applied for the evaluation of C T, values. T, values are usually used to optimize insensitive C experiments, i.e. to adjust the length of the preparation time in other NMR experiments. To deduce structural information it is usual to interpret the dipolar part of the longitudinal relaxation time (T, ). To separate the dipolar contribution from the contributions of other relaxation mechanisms, it is necessary to perform further experiments (gated decoupling experiments) to evaluate the heteronuclear NOE values. T °° may be exploited in a qualitative way to differentiate between carbon nuclei in less or highly mobile molecular fragments. In a more detailed analysis reliable T, values can be used to describe the overall and internal motions of molecules. 

Dipolar dephasing experiments were performed by inserting a delay before data acquisition in which the decoupler was gated off. 

The transverse magnetization of the REDOR experiment decays as a function of the decoupling field [23, 24]. Dipolar decoupling may be strongly interfered with by molecular motion, if any, when the motional frequency is of the same order of magnitude as the decoupling field and, hence, the transverse relaxation times (Ta s) are significantly shortened. In... 

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. 

Fig. 6.4.3. chemical shift powder pattern spectra of labeled gramicidin A. (A) Trpn gramicidin A—experimental data obtained with cross-polarization and H dipolar decoupling at 20.3 MHz for N. (B) spectral simulation with o-n = 36, saturated solution of NH4N03. (C) [ C,]Leuio-[ N ]Trpn gramicidin A—experiment as in (A) displaying a combination of N chemical shift anisotropy and the — " C dipolar interaction. (D) spectral simulation with the same an values as in (B) and with = 0° and Pu = 106°. 

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