Jump and Return

Simplified INEPT (with the first 180° pulses in both channels omitted) or INEPT combined with a jump and return pulse can be also used for determination of signs of spin-spin couplings between rare nuclei as described in detail by Kupce and Wrackmeyer143. 

Special pulses containing one or several notches in their excitation profiles have been designed for purposes of solvent suppression. Obviously such pulses could be used also for suppression of parent lines in applications involving isotopi-cally diluted nuclei. One of the simplest experiments of this kind is the jump and return experiment [24] and corresponding higher order binomial pulses with alternating phases. Similarly to binomial excitation pulses these constant amplitude solvent suppression pulses create sidebands and excitation sidelobes. Corresponding amplitude modulated pulses [25,26] provide a better alternative. 

These filters are usually employed to suppress strong and undesirable resonances, e.g. parent signals of isotopically diluted spin systems. In a typical experiment the undesirable magnetization is inverted and allowed to relax until the equilibrium state is reached. At this point the inverted magnetization is not observable and the basic experiment may be started. In the simplest case a jump and return inversion pulse or binomial pulses can provide the necessary selective inversion. A simple example of application of a T relaxation filter is the J/-BIRD (BI linear Rotation Decoupling) HMQC... 

A very efficient suppression of parent resonances can be achieved using the T filter. This, however, requires a rather careful tuning of the relaxation delay T (see Figure 8). If the jump and return inversion pulse is employed, the pulse sequence can be regarded as a selective version of the BIRD experiment [57-59]. Obviously, multiple-frequency selective inversion pulses may be necessary in the case of complex proton spectra. Usually the /-BIRD HMQC experiment gives cleaner spectra as compared with equivalent heteronuclear singlequantum coherence (HSQC) experiments, presumably because of fewer 180° pulses which are frequently a source of various artefacts. 

Many different ways of effecting water suppression in the NOESY sequence have been implemented, for example, presaturation, jump and return, l-T-echo, WATERGATE, SS pulses and excitation sculpting. The basic NOESY sequence using presaturation and two variations using WATERGATE are illustrated in Fig. 24. In the basic NOESY sequence (Fig. 

Different principles are used (1) presaturation method presaturation of the signal, (2) jump-and-return method excitation of all signals followed by transfer of solvent coherence into not-detectable equilibrium magnetization of the solvent, (3) WATERGATE gradient driven destruction of solvent coherence... 

Variants Presaturation technique [5.13], jump-and-return technique [5.14],... 

Despite these disadvantages the jump-and-return method is a useful technique if the overall experiment time should be kept as short as possible. 

As shown in Check it 5.2.33 in the jump-and-return method it is very critical that the hard pulse pi and p2 are exactly 90° because an error of as little as 0.5° can cause a large residua] solvent signal. 

The jump-and-return method can also be implemented in 2D experiments in a similar manner to the presaturation method. Check it 5.23.4 shows the jump-and-return method as part of a 2D homonuclear COSY experiment for peracetylated p-D-glucose. 

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