Magic-echo techniques

Space encoding is achieved most conveniently through magic-echo techniques [Hafl]. Parameter contrast is introduced by suitable filters, which generate a signal decay under an effective Hamiltonian that is sensitive to slow molecular motion. Such filters are the... 

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.

The cw-NMR is orthodox and sufficiently reliable if the signal-to-noise ratio of the spectrum is high enough to estimate its baseline precisely. In the pulse NMR case, on the other hand, since the short-time behaviour immediately after the intense radio-frequency (rf) pulse irradiation is important, as expected from (6.2), recovery time or dead time of the experimental equipment after the pulse, causes a serious limitation of reliability for the estimated moments. To overcome this weak point the improvements in technique, the magic echo sequence [26-29] and in analysis, the functional forms of fitting [30-32] were applied to recover the FID signal at r = 0. Details are described in [2]. 

In the in situ approach, the growth of the particles from the surface of a PDMS sample can be followed by NMR (figure 9.3) utilizing and Si magic-angle spinning, with two-dimensional Fourier transform spin-echo techniques. The spin-spin (T ) relaxation time of the protons in... 

Figure 8.3 Illustration of HR-MAS techniques applied to a resin-bound trisaccharide (a) static XH spectrum of the solvent swollen sample (b) XH spectrum with magic-angle spinning at 3.5 kHz (c) H spectrum with MAS and spin echo pulse sequence.

The main problem is that the SEDOR experiment is carried out imder static conditions, i.e., without magic angle spinning. Therefore, SEDOR measurements only yield average values for the internuclear distance if a sample contains two or more different types of 7-S-spin pairs with different internuclear distances, r/j. Typical double resonance experiments for the measurement of internuclear distances under magic angle spinning conditions are the REDOR (rotational-echo double resonance [50]) and the TEDOR (transferred-echo double resonance [51]) techniques. 

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