QCPMG experiment

QCPMG experiments, both for static and for spinning samples, have been extensively used for S/N ratio enhancement in experiments involving many half-integer spin quadrupolar nuclei, including Mg (as exemplified in the next section), S, Cl, and Sr. In many of these... 

Dynamical effects of a two-axis 3-by-2-site jump process as can be observed in DMS-d6 were investigated by both QCPMG and MAS simulations. Besides rather interesting line broadening effects when both rate constants were in the intermediate regime, it was observed that the QCPMG experiment is more sensitive towards motional effects than MAS if either of the two rate constants is in the fast regime. 

Two experiments were explored the single-pulse MAS and the QCPMG experiment. The latter may be presented as... 

From these results, it is evident that QCPMG experiments may be performed when the second-order quadrupolar effect is adequately small. In the present calculations, the CQ-limit for the QCPMG spectra is around 750 kHz at 14.1 T but this limit is of course higher at higher magnetic field strengths. 

Figure 3 Maximum intensity as a function of log(fc) for simulated 14N (43.34 MHz) spectra using parameter set P3. The intensity profile for a two-site jump is shown by a solid line for the MAS experiments and by squares for the QCPMG experiments. For the six-site jump process, the triangles correspond to the intensity profile for the MAS experiment and the filled circles for the QCPMG experiment.

Figure 12 Maximum intensity as a function of log(fc) for simulated 39K spectra shown in Figures 10 and 11. (A) The intensity profiles for a two-site jump using the QE (solid triangles), the QCPMG (solid line), the QCPMG-MAS (solid circles) and the single-pulse MAS (solid squares) experiments. (B) The intensity profiles of the QCPMG experiment for a two-site (solid line), three-site (solid triangles), four-site (solid squares) and a six-site (solid circles) jump process.

In this work, it has been demonstrated that periodic modulation of the Hamiltonian describing a dynamic process introduces spectral effects that depend upon the nature of the modulations. For both MAS and QCPMG experiments, sidebands are broadened in the intermediate dynamic regime and a significant narrowing of the sideband manifold is observed for higher order jump processes due to averaging of the anisotropic part of the quadrupolar tensor. 

Both experiments are applicable for acquisition of 2H and 6Li spectra and for these nuclei only first-order EFG- and CSA-terms in the Hamiltonian are required. For CqS below 750 kHz, both 14N QCPMG and MAS experiments are applicable at 14.1 T but above this Cq limit the hardware demands make it very difficult to employ the QCPMG experiment and single-pulse MAS must be the method of choice for such applications. For larger CqS, indirect detection of either 14N SQ or DQ coherences using rotor-synchronized acquisition is suggested. In this context, the DQ lineshape is not as severely broadened as the SQ lineshape as it is not affected by the first-order quadrupolar Hamiltonian. 

Several convenient routes for signal enhancement in quadrupolar SSNMR experiments involve manipulation of the ST populations [46], which results in an enhanced CT population difierence, thereby boosting experimental S/N and reducing experimental times. The apphcation of double-frequency sweeps (DFS) before a Hahn-echo or QCPMG experiment results in the DFS-echo [61,62] and DFS—QCPMG [50,63] pulse sequences, respectively, which Rossini et al. showed are very usefijl for the acquisition of challenging SSNMR spectra [64]. Utilization of... 

Zhu et al. used SSNMR Hahn-echo and QCPMG experiments... 

F.H. Larsen, I. Faman, A.S. Lipton, Separation of Ti and Ti solid-state NMR Un-eshapes by static QCPMG experiments at multiple fields, J. Magn. Reson. 178 (2006) 228-236. 

Although QCPMG experiments are well suited for acquiring Zr powder patterns in a reasonable amount of time, for broad powder patterns, the nature of VOCS dictates that the spectroscopist must be present to retune the spectrometer after the acquisition of each subspectrum, increasing experimental time. A pulse sequence featuring relatively broader excitation... 

Figure 12 Static Zr SSNMR spectra of [lnd2ZrCl2l. WURST-QCPMG spectra were acquired at a field of 21.1 T (A, C), and QCPMG experiments were acquired at 9.4 T (E). These spectra were constructed from several subspectra using the VOCS method of frequency-stepped acquisition. A one-site simulation of the 21.1 T spectrum is indicated by the unbroken gray line in (A), while a two-site simulation of the 21.1 T data is depicted by the unbroken and dashed gray lines in (B) and the unbroken gray line in (Q. A one-site simulation of the 9.4 T data is shown in (D), with a two-site simulation included as the unbroken gray line in (E). Note the satellite transitions in (E) that obscure the Zr powder pattern at 9.4 T. Reprinted with permission from Ref. [46]. Copyright The American Chemical Society 2009.

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