Magnetization-exchange pulse sequence

Fig. 12. The magnetization-exchange experiment.(a) magnetization-exchange pulse sequence (b) = 3 C magnetization-exchange spectra of doubly C-labelled zinc acetate at different mixing times Tm (c) simulated (solid lines) and experimental ( ) magnetization-exchange curves on (top) and off (bottom) the R condition. (Parts (b) and (c) reproduced from ref. 75, 1988, with permission from Elsevier Science.)...

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

A number of recoupling techniques are utilized for specific purposes, but they all rely on synchronizing measurements, and, in most instances, pulse sequences, with the rotation period. For homonuclear recoupling (e.g., 13C-13C or 31P-31P), rotational resonance (R2) is based on magnetization exchange when the spinning rate wr is adjusted so that a multiple of it equals the chemical shift difference between the two dipolar coupled nuclei, and DRAMA (dipolar recovery at the... 

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

One of the drawbacks of WATERGATE is that the water molecules have some transverse magnetization at the end of the pulse sequence, albeit dephased. This saturation can be transferred onto labile amide protons through chemical exchange, which, in a similar manner to presaturation, results in reduced sensitivity. 

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