Soft pulses

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Figure 2.11. Proton-Proton shift correlations of a-pinene (1) [purity 99 %, CDCls, 5 % v/v, 25 °C, 500 MHz, 8 scans, 256 experiments], (a) HH COSY (b) HH TOCSY (c) selective one-dimensional HH TOCSY, soft pulse irradiation at Sh = 5.20 (signal not shown), compared with the NMR spectrum on top deviations of chemical shifts from those in other experiments (Fig. 2.14, 2.16) arise from solvent effects...

Figure 1.16 Time domain representation and frequency excitation function of a soft pulse. The soft pulse selectively excites a narrow region of a spectral range and leads to a strong offset-dependent amplitude of the excitation function.

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

Describe what is meant by hard and soft pulses. 

Figure 1.24 Fourier components of a long Rf pulse ( soft pulse) are spread over a relatively narrow frequency range. (Reprinted from S. W. Homans, A dictionary of concepts in NMR, copyright 1990, pp. 127-129, by permission of Oxford University Press, Walton Street, Oxford 0X2 6DP, U.K.)...

Hard and soft pulses depend on the pulse width The shorter the width of the pulse, the harder, or the more powerful, it will be. Similarly, a long width pulse is soft and less powerful. For... 

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

The selective INEPT experiment also requires a separated proton resonance for excitation and also uses rectangular soft pulses (Bax and Freeman, 1982). Magnetization is selectively transferred by the soft proton pulses to the corresponding carbons to which the protons are coupled. 

Figure 7.16 When soft pulses are used for excitation and mixing in a 2D experiment, it becomes a 2D soft experiment. The spectrum of the 2D soft experiment has reduced frequency ranges in F, and Fj. The excitation ranges of the selective pulse depend on the type of experiment. For example, in a soft COSY-COSY experiment, one multiplet is excited, while in the soft NOESYexperiment the whole resonance region of a group of signals is excited. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Figure 7.19 Soft H,C-COSY pulse sequence with two soft pulses having the same excitation frequency. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

THREE-DIMENSIONAL EXPERIMENTS USING SOFT PULSES... 

Soft-pulse multiple irradiation In this method, pre-saturation is done using shaped pulses having a broader excitation profile. Therefore, it is a more suitable method for the suppression of multiplets. This technique is very effective, easy to apply and easy to implement within most NMR experiments. In aqueous solutions, however, slowly exchanging protons would be detectable due to the occurrence of transfer of saturation. In addition, the spins with resonances close to the solvent frequency will also be saturated. 

Hard pulse A pulse which is equally effective over the whole chemical shift range. See Soft pulse. 

Soft pulse Pulse designed to bring about irradiation of only a selected region of a spectrum. See Hard pulse. 

Fig. 14.4 Pulse sequences used for the experiments described in this chapter. A [ N HJ-HSQC with water flip back and PFGs. The shaped pulse on the proton channel is a sine-shaped, 1.5 ms soft pulse all other pulses are hard pulses. Gradients are applied as square or sine-shaped pulses. The sign of the last gradient is reversed for anti-echo selection together with the sign of phase 6. B CPMG sequence. C bpPFGLED sequence. The delay T denotes the diffusion delay. Typically, r is set to 1 ms, T to 50-100 ms and Te to 1.2 ms.

This contribution will describe the manipulation of spin multiplets as a whole, and the word selective - or soft - will be used for multiplet-selective pulses, in contrast to band-selective, which refers to a broader bandwidth which may affect several spins, and to transition selective when only one line is affected. The discussion will be based on proton spectra, but all aspects are similar for other nuclei. Soft pulses use lower amplitudes and much longer irradiation times than non-selective hard pulses. Typical durations for soft pulses are of the order of 1 to 500 ms with a peak amplitude... 

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. 

Pig. 1. Pulse sequence for selective reverse INEPT. The time-shared homonuclear decoupling during acquisition is optional, and a variety of simplifications may be made to the sequence depending on the instrument used and on the spin system under investigation, as discussed in the text. A DANTE sequence is shown as the selective 90° carbon-13 pulse, but this may be replaced by a soft pulse or some other form of selective excitation. Phase cycling for this sequence is summarized in table 1. 

---