Shaped soft pulses

Associated with the use of selective pulses are a number of experimental factors that have a considerable bearing on the selection (and design) of a soft pulse. In short, the key features are its  

More elaborate pulse shapes have been developed over the years which aim to produce a near top-hat profile yet retain uniform phase for all excited resonances within a predefined frequency window. These operate without the need for purging pulses or further modifications, allowing them to be used directly in place of hard pulses. They are typically generated by computerised procedures which result in more exotic pulse envelopes (and acronyms Fig. 9.11) that drive magnetisation vectors along rather more tortuous trajectories than the simpler Gaussian-shap ed cousins. Trajectories are shown in Fig. 9.15 

Shaped pulse Application Bandwidth factor Attenuation factor/-dB Ref. 

G4 Gaussian Cascade Excitation only, Pure-phase 7.8 25.4 [43] 

G3 Gaussian Cascade Inversion only. Pure phase 3.6 23.1 [43] 

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Nymphaea caerulea, for seven natural anthocyanins stabilizing a DNA triplex, etc. Sequential analysis of the oligosaccharide structures of the flavonol tamarixetin-7-O-rutino-side has been performed by ID multistep-relayed COSY-ROESY experiments. Selective excitation was performed by Gaussian-shaped soft pulses. 

An alternative solution to the problem of selectively observing, for example, the H NMR spectrum of a metal-hydride in protio-, rather than deutero-solvent is to excite just that region of the NMR spectrum that contains signals of interest. This can be done using a shaped soft pulse, using a DANTE (delays alternating with nutation for tailored excitation) type sequence or, on a modem... 

Table 93. Properties of some shaped soft pulses...

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). 

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. 

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.

In this chapter, the discussion will be focused on the ID TOCSY (TO-tal Correlation SpectroscopY) [2] experiment, which, together with ID NOESY, is probably the most frequently and routinely used selective ID experiment for elucidating the spin-spin coupling network, and obtaining homonuclear coupling constants. We will first review the development of this technique and the essential features of the pulse sequence. In the second section, we will discuss the practical aspects of this experiment, including the choice of the selective (shaped) pulse, the phase difference of the hard and soft pulses, and the use of the z-filter. The application of the ID TOCSY pulse sequence will be illustrated by examples in oligosaccharides, peptides and mixtures in Section 3. Finally, modifications and extensions of the basic ID TOCSY experiment and their applications will be reviewed briefly in Section 4. 

An early alternative to soft pulses was the DANTE Delays Alternating with Nutation for Tailored Excitation) experiment, which used a sequence of short, hard pulses of angle a <3C 90°, followed by a fixed delay t to achieve selective excitation. Thus, the pulse sequence is (a-T), ]. Nuclei that are on resonance are eventually driven to the y axis and hence are selected, whereas those more removed from the frequency range are not affected. The sequence of hard pulses can achieve a result similar to that of soft pulses and even can be shaped by modulating the duration of the pulse lengths, but DANTE pulses lead to spectral artifacts not created by soft pulses, such as unwanted sidebands. 

Theoretical analysis of simultaneous application of two soft pulses Adiabatic effect of the shaped pulse presaturation... 

Fig. 1. Pulse sequences for ID- X, "Y H) polarization transfer experiments. If not stated otherwise, narrow and wide bars denote 90° and 180° hard pulses, narrow and wide ellipsoids 90° and 180° shaped pulses. Essential phase cycles for selection of the polarization transfer signal are given on top of the pulses, if no phase is indicated, pulses are applied along the x-axis A denotes a fixed delay of length (n/(X,Y))" (a) UPT (, 0 = 90°), (b) unrefocused INEPT, (c) unrefocused selective INEPT with soft pulses, (d) INEPT with selective excitation via H, "Y cross-polarization si denote WALTZ-17 spinlock pulse trains which were applied for a period t— (2/( H,Y)) ...

The excitation profile of soft pulses is defined by the duration of the pulse, these two factors sharing an inverse proportionality. More precisely, pulse shapes have associated with them a dimensionless bandwidth factor which is the product of the pulse duration. At, and its effective excitation bandwidth, Af, for a correctly calibrated pulse. This is fixed for any given pulse envelope, and... 

Hard/shaped pulse, soft pulse, DANTE pulse train, composite pulse, cw-irradiation, cp decoupling sequence, spinlock, constant or incremented delay... 

Low-power pulse or train of pulses Shaped low-power (soft) pulse Frequency swept (adiabatic) puise... 

Having determined the necessary pulse duration, the transmitter power must be calibrated so that the pulse delivers the appropriate tip angle. As already alluded to, this can be avoided on instruments with linearised amplifier outputs, provided accurate hard-pulse calibrations are known. The calibration of soft pulses differs from that for hard pulses where one uses a fixed-pulse amplitude but varies its duration. For practical convenience, amplitude calibration is usually based on previously recorded calibrations for a soft rectangular pulse (as described below), from which an estimate of the required power change is calculated. Table 10.3 also summarises the necessary changes in transmitter attenuation for various envelopes of equivalent duration, with the more elaborate pulse shapes invariably requiring increased rf peak amplitudes (decreased attenuation of transmitter output). 

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