DANTE pulse train

The principle of multiple selective excitation has been incorporated into a few ID and 2D experiments, the schemes of which are shown below (fig. 1). Depending on the experiment, either a DANTE pulse train (ID TOCSY [2]), frequency selective 180° pulses (ID NOE [3], ID INADEQUATE [4], ID C/H COSY [5] and 2D TOCSY-COSY [6]) or frequency selective 90° pulses (2D HMBC [11]) are applied to selectively perturb and uniquely label selected spins. Besides the DANTE pulse , composed itself of a series of non-selective rectangular pulses, Gaussian-shaped 180° and... 

Fig. 1. A comparison of the profiles relative to the three components of magnetization after application of an inverting DANTE pulse train.

Fig. 2. (a) 400 MHz partial H spectrum of a trisaccharide in CDClj. (b)-(e) ID TOCSY difference spectra with selective inversion of Hj// by a DANTE pulse train, and mixing times of 30, 60, 120 and 180 ms, respectively. IOtz values (of 16 scans each) were used for the 2-filter. (Reproduced with permission from Wessel et al. [26a]. Copyright 1991 Verlag... 

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

The constant amplitude pulses are usually easier to implement and they do not require special hardware. A typical example and also the most frequently used kind of constant amplitude, frequency selective pulse is the DANTE pulse train [4]. In the simplest case it consists of a sequence of square pulses interleaved with periods of free precession. Unfortimately, the excitation profile of the DANTE sequence has extensive sidelobes. It also produces sidebands at frequencies ... 

Fig. 6.2.16 [Cho2] 2D DANTE-imaging. (a) Timing of signals. The rf excitation is a DANTE pulse train applied in a read gradient Gx- The Hahn echo of the response is detected while both the phase-encoding gradient Gy and the read gradient Gx are turned on. The slice-selection gradient is Gj. (b) Traces in k space.

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

We have so far been concerned with 2D spin-diffusion spectroscopy. There are, however, two ID experiments that are likely to be applied to catalytic problems selective excitation 72,85,861 and rotational resonance (87-93]. Selective excitation of selected resonances using the DANTE pulse trains [94] can be used to measure specific C - C connectivity in complex, multiple C-labeled solids (86). Rotational resonance can be achieved by adjusting the MAS rate to an integer fraction of the chemical shift difference between two selected carbon resonances n being a small integer. 

A simplification of the COSY H-n.m.r. spectra of oligosaccharides by use of a 2-ditnensional version of 3-dimensional HOHAHA COSY has been described. A new and economical (in spectrometer time) technique for assigning oligosaccharide n.m.r. specua, which employs selective excitation combined with a DANTE pulse train and chemical shift selective filtering (CSSF), was found suitable for problems of medium complexity, where each spin network contains at least one separate multiplet. It has been applied to hepta-O-acetyl-a-cellobiosyl ande as an example. ... 

Fig. 1. Pulse sequences of ID selective experiments. The shaped 90° pulses were half-Gaussian. The 180° selective pulses were produced by a DANTE-Z pulse train [44]. T denotes the trim pulse of phase ip. tnoe stands for the NOE-mixing time.

The quality of the profile pertaining to the selected region can be improved by a modulation of the pulse lengths within the pulse train, which mimics the amplitude modulation of a simple soft pulse [8]. Simple modulation schemes can be devised for attenuating or even suppressing the side-lobes in the vicinity of the selected region (as shown in fig. 3). Alternatively, more elaborated modulation schemes as those of the BURP family [9] can be run in the DANTE-Z mode [10]. This mode actually offers a... 

Lu et al. have analyzed the direct excitation of wide ID spectra of nuclei with spin I=1/2 or 1 in rotating solids submitted to pulse trains in the manner of Delays Alternating with Nutations for Tailored Excitation (DANTE), either with one short rotor-synchronized pulse of duration tp in each of rotor period or with N interleaved equally spaced pulses tp in each rotor period. ... 

It is well-known that the excitation profile by a periodic pulse also has a pattern of multiple bands in response to the multiple effective RF fields. The DANTE sequence,26 for instance, was one of the most frequently used periodic pulse in the past for selective excitation of a narrow centre band. It is constructed by a long train of hard pulses with a certain delay between two adjacent pulses. The advantage of using the DANTE sequence over the weak, soft RF pulses relies on that it is not necessary to change the RF power level in the pulse sequence. Consequently, phase distortions and certain delays accompanied by the abrupt changes of the RF power level are avoided. 

Fig. 9.1. (A) Gaussian (a) and sine (b) excitation profiles. (B) Composite (G3) Gaussian pulse. (C) Train of soft pulses modified after the DANTE sequence to achieve selective off-resonance excitation. (D) Redfield 21412 sequence. (E) Binomial 11, 121, 1331, 14641 sequences. (F) JR (a) and compensated JR (or 1111) (b) sequences. (G) Watergate sequence. (H) Weft (Superweft) sequence. (I) Modeft sequence. (J) MLEV16 sequence. (K) NOESY sequence with trim pulse. (L) MLEV17 sequence with trim pulses. (M) Clean-TOCSY sequence.

FIGURE 43. Assignment of the two lines in the 29Si NMR spectmm of methyl 3o, 12a-bis(trimethyl-siloxy)-5/i-cholenate, 37, by selective INEPT. Top trace 29Si INEPT spectmm two middle traces selective INEPT spectra measured with selective excitation of H lines indicated by arrows in the bottom trace with partially assigned ll NMR spectmm (25 mg of the sample in 0.7 ml of CDCI3, H frequency 200 MHz, 29Si frequency 39.7 MHz, 5 mm broad-band probe, selective pulse by DANTE train, r = 70 ms, A = 149 ms). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 304... 

The excitation sculpting method was expended to the selective inversion of z-magnetisation. The method utilises DANTE train of hard pulses combined with gradient suppression of transverse magnetisation. Application of the selective inversion to z-magnetisation reduces relaxation loses and extends selective methods to larger molecule. Use of shaped pulses in DANTE train produces band-selective inversion. The proposed inversion method can be... 

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