Magic sandwich

Figure 19 (A) A H magic-sandwich echo spectrum of a single crystal sample of 98.3%-...

Usually a homonuclear decoupling pulse sequence does not simultaneously decouple heteronuclear dipolar interactions, albeit in many cases the heteronuclear dipolar couplings need to be retained. With a significantly modified magic-sandwich sequence, Schmidt-Rohr proposed a pulse sequence that can eliminate both, at least up to first order.63 This sequence is particularly useful for enriched 13C pairs and can be readily combined with 2D experiments. It would be interesting to investigate whether such a sequence or similar ones can work for other nuclei particularly large-7 species. 

Fig. 24. Top Magic-echo sequence, TREV-16TS, for solid-state imaging with (a) ideally pulsed (b) nonideally pulsed and (c) sinusoidal field gradients. On-resonance RF irradiations along x and — x are sandwiched by 90° pulses (magic sandwich). The hatched magic sandwich consists of four magic sandwiches. Bottom Projections of the one-dimensional test sample (admantane powder) obtained with the TREV-16TS sequence (r = 30 ps) and the field gradients shown in Fig. 18. The RF field was set at 100 kHz. (Adapted from Matsui et al.296 with permission.)...

The MSHOT3 (magic-sandwich higher-order truncation) sequence comprises three magic-sandwich units phase shifted by 120° with respect to one another. 

Fig. 3.4.3 Pulse sequence for generation of the magic echo. The magnetization dephases for a time 2r by free precession before and after refocussing by the magic sandwich of length At.

Fig. 8.8.3 [Haf3] Scheme for magic-echo phase encoding with slice selection by the SLISE technique and spectroscopic detection. The strength of the field gradient is varied in constant increments. The gradients may he left on during the magic sandwich. The amplitude of the magic-echo is acquired as a function of the gradient amplitudes. The durations of all time intervals are kept constant. All narrow rf pulses are 90° pulses.

More sophisticated NMR techniques have been also applied to measure the crystallinity in PVA cryogels [58]. For instance, pulsed mixed magic-sandwich echo sequences have been applied in a low-field NMR spectrometer [58]. This kind of pulse sequence provides near-quantitative refocusing of the rigid contribution to the initial part of proton free induction decay [58], allowing for a more quantitative determination of crystallinity in these gels. The results so obtained essentially confirm those shown in Fig. 10. 

Principles of the Method. The method is based on the mixed echo composed of the Hahn and the magic echo (33,71,72). The Hahn echo explores the magic echo shape generated by an accordion magic sandwich (cf Fig. 7). 

One possibility to have the amplitude of the Hahn echo encoded by the homonuclear dipolar interactions is to employ a mismatched magic sandwich... 

MS), ie, 90S - (t + nh.) - 90J - Burst Pulsex (2r - raA) - Burst Pulsejf (2t -n A) -90y - (r + rA) - Mixed Echo (see Fig. 7), where ra is a positive and negative integer number and A is the time decrement and increment, respectively. We call this mismatched MS composite pulse with variable n the accordion magic sandwich (hereafter the method will be called AIMS). 

A. A. Nevzorov and S. J. Opella, A "Magic Sandwich" pulse sequence with reduced offset dependence for high-resolution separated local field spectroscopy, J. Magn. Reson. 164 182-186 (2003). 

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