Pulsed field gradient for coherence selection

The 15N,1H shift correlation maps are most conveniently recorded with a sensitivity-enhanced HSQC sequence with incorporated water flip-back pulses for reduced saturation transfer and pulsed-field gradients for coherence selection. The pulse sequence of the experiment is shown in Fig. 14.4 A. 

Fig. 11. (A) A conceptual diagram of the WATERGATE subunit, the ir RF pulse is some type of selective tt pulse. (B) Multiple solvent suppression using the double-pulsed field gradient echo. Ihe selective ir pulses are SLP pulses to enable multiple solvent suppression. To avoid breakthrough of undesired coherences, different values for the gradients are used in the first and second echo. The phase cycling is given elsewhere. ...

When collected in a phase-sensitive mode, HMBC cross peaks are found to have a mixed phase character. That is, we cannot phase HMBC cross peaks so that they are purely absorptive. The use of pulsed field gradients for the purpose of coherence selection in the HMBC experiment (gHMBC) renders a nonphase-sensitive 2-D data set. This latter method is generally preferred because phasing of the spectrum is not required. 

Fig. 2. Pulse sequence for selective reverse INEPT using pulsed field gradients to select the coherence transfer echo. The 180° pulse pair in the middle of the 2r delay is not normally needed for t < 50 ms, and the second proton 180° pulse and first t2 delay maybe omitted if a linear phase gradient in the resultant spectrum can be tolerated. The second field gradient pulse has an area (7c/th) times that of the first.

The idea of back transformation of a three-dimensional NMR experiment involving heteronuclear 3H/X/Y out-and-back coherence transfer can in principle be carried to the extreme by fixing the mixing time in both indirect domains. Even if one-dimensional experiments of this kind fall short of providing any information on heteronuclear chemical shifts, they may still serve to obtain isotope-filtered 3H NMR spectra. A potential application of this technique is the detection of appropriately labelled metabolites in metabolism studies, and a one dimensional variant of the double INEPT 111/X/Y sequence has in fact been applied to pharmacokinetics studies of doubly 13C, 15N labelled metabolites.46 Even if the pulse scheme relied exclusively on phase-cycling for coherence selection, a suppression of matrix signals by a factor of 104 proved feasible, and it is easily conceivable that the performance can still be improved by the application of pulsed field gradients. 

Pulsed field gradients are added to existing NMR pulse sequences in order to suppress artifacts and/or to select certain coherence transfer pathways. The application of gradients for these two purposes maintain different requirements and benefits. 

The availability of pulsed field gradient (PEG) techniques has had the most significant impact in terms of making it possible to perform 2D- and 3D-NMR experiments on polymer samples. These methods have taken the place of traditional radiofrequency phase cycling methods for coherence selection. By optimizing the use of the spectrometer s dynamic range, PEG techniques not only save time but also drastically reduce artifact noise. 

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