Nonselective pulse

Figure 6.2 Pulse sequences for some common 3D time-domain NMR techniques. Nonselective pulses are indicated by filled bars. Nonselective pulses of variable flip angle are shown by the flip angle )8. Frequency-selective pulses are drawn with diagonal lines in the bars. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al, 14, copyright (1989), with permission from Academic Press, Inc.)...

Different assignment strategies can be employed, depending on whether selective or nonselective pulses have been used in recording 3D spectra. A homonuclear 3D NOESY-TOCSY spectrum in which the NH/ region has been recorded is presented in schematic form in Fig. 6.9. 

Nonselective pulse A pulse with wide frequency bandwidth that excites all nuclei of a particular type indiscriminately. 

Presaturation Selective irradiation of a nucleus prior to application of a nonselective pulse causes it to be saturated, so its resonance is suppressed. This technique may be employed for suppressing solvent signals. 

The ID NOESY-TOCSY experiment [39] shown in fig. 1(c) is a straighffor-ward concatenation of ID NOESY and TOCSY experiments [34] (figs 1(a), (b)). Since the NOE transfer takes place along the z axis, and thus has no phase memory, no phase correction for the second selective pulse is needed to compensate for the change of the r.f. frequency during the tnoe interval. Nevertheless, any possible phase differences between the selective and consecutive nonselective pulses must be taken into account in both steps, by adjusting the phase of soft pulses. 

Another commonly used technique is the water flip-back pulse, a shaped pulse designed to selectively rotate only the water magnetization by 90°, putting it back on the +z axis after a hard (nonselective) pulse has rotated all of the sample magnetization into the x-y plane. Water can be viewed as a wild and powerful bucking bronco—it must be tamed and never allowed to get out of its pen. The best place for water is on the +z axis where it will not do any harm. This is the rationale behind the flip-back pulse every time water is moved from the +z axis, use a selective pulse to put it back there. 

As expected from Table 11.1, application of the nonselective pulse eliminates all terms on the diagonal and generates single quantum I and S coherneces. 

The product operator formalism is normally applied only to weakly coupled spin systems, where independent operators for I and S are meaningful. That means that it is permissible to treat evolution under chemical shifts separately from evolution under spin coupling. It also means that a nonselective pulse can be treated as successive selective pulses affecting only one type of spin. To simplify the notation and to facilitate the handling of the transformation of each product operator, such separations are almost always made. 

As a second example, we look at echoes. We saw in Chapter 9 that a 180° pulse refocuses not only chemical shifts and the effects of magnetic field inhomogeneity but also spin coupling provided that the pulse does not also disturb the spin state of the coupled nucleus (see Fig. 9.2) However, in a homonuclear spin system a nonselective pulse does effect spin states. We found in Chapter 7 that dipolar interactions have the same mathematical from as indirect spin coupling, and it is known that a 180° pulse does not produce an echo in a solid because spin states are disturbed. However, it is possible to obtain a solid echo or dipolar echo by applying the pulse sequence 90, T, 90r It is very difficult to rationalize an echo from... 

Fig. 5.3.11 The effect of prograde and retrograde composite pulses on the magnetization as a function of the actual flip angle with reference to a nonselective pulse P. (a) 90° pulses Bz and C2, respectively [Wiml]. (b) 180° pulses and Q, respectively [Shal],...

This requires the use of a multiple-pulse scheme with a nonvanishing effective r.f. field. Such a nonvanishing effective field can be added to a sequence without an effective field by either adding an additional small-angle pulse after a full cycle of the sequence, or by increasing the power of one of the pulse phases by a small amount. There have been quite a number of multiple-pulse sequences developed for the liquid-state TOCSY experiment [3, 22], which provide offset compensation at low r.f. powers. However, the requirements in the present context are different from the ones in TOCSY due to the fact that the dipolar coupling is not isotropic like the J-coupling and is not, therefore, invariant under nonselective pulses. 

The evolution period may be followed by a mixing period which consists of applying a single nonselective pulse (as in homonuclear shift correlated spectra or homonuclear multiple-quantum spectra) or by a number of nucleus-selective pulses (as in heteronuclear shift correlated spectra). 

This special pulse sequence uses strong nonselective pulses and gives general enhancement rather than specific sensitivity enhancement, as in SPT. The pulse sequence has a basic polarization-transfer portion that produces a net inversion of one of the proton spin states. Proton transverse magnetization is created by the initial proton 90° pulse and precesses for a period t. The magnetization is then refocused as a spin echo at a time 2r by the action of a 180° proton pulse. Application of a 180° pulse at the midpoint of the 2r delay ensures that the echo is modulated by the scalar coupling, 7ch- If t is chosen to be equal to 1/(4Jch) then at time 2r, the... 

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