Quadrupole powder pattern, first-order

Figure 12 Offset dependence of the intensity of the CT resonance in MAS NMR spectra of Na2MgEDTA-4H20, recorded with a single HS pulse placed before the spin-echo sequence. The experimental data (square points) were recorded using a 7mm probe (with 7kHz MAS rate) the solid red line is just a guide for the eyes. Also shown (blue line) is the simulated first-order quadrupole static powder pattern for the single Mg site in this compound.

Fig. 14. The pulse sequence for recording the double-quantum 2H experiment.37 The entire experiment is conducted under magic-angle spinning. This two-dimensional experiment separates 2H spinning sideband patterns (or alternatively, static-like 2H quadrupole powder patterns) according to the 2H double-quantum chemical shift, so improving the resolution over a single-quantum experiment. In addition, the doublequantum transition frequency has no contribution from quadrupole coupling (to first order) so, the double-quantum spectrum is not complicated by spinning sidebands. Details of molecular motion are then extracted from the separated 2H spinning sideband patterns by simulation.37 All pulses in the sequence are 90° pulses with the phases shown (the first two pulses are phase cycled to select double-quantum coherence in q). The r delay is of the order 10 gs. The q period is usually rotor-synchronized.

Fig. 7.2. Powder patterns for I = 3/2 showing (a) NMR spectrum in the presence of first-order quadrupole effects and (b) the effect on the central component in the spectrum of second-order effects. The spectra are shown relative to the central point, with Ap and denoting first- and second-order quadrupole splittings,...

Figure 2.9. The quadrupole perturbed powder patterns for an I = nucleus. A. The outer satellite transitions to first-order. B. The second-order quadrupolar broadening of the central transition with...

This term is anisotropic and produces a powder pattern. It has been derived under the assumptions that first-order perturbation of the S-states is sufficient, that the J tensor is axially symmetric and that the unique axis of J is aligned with the intemuclear vector. Under MAS this term will be scaled but, as it is not proportional to P2(cos0), it cannot be completely removed. Hence the MAS spectrum will still have some residual width, but the most profound effect is to leave an isotropic term which can be calculated by averaging the powder lineshape. Hence for a J-coupled system with an axially symmetric quadrupole interaction, the spectrum is shifted from the isotropic chemical shift by ... 

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