DANTE sequence

The excitation profile of multiple bands was also known for periodic RF pulses, such as the DANTE (delays alternating with nutation for tailored excitation) sequence.26 Similar to the PIP, all the phases and strengths of the effective RF fields can be obtained by expanding the periodic pulse into a Fourier series and properly rearranging the terms afterwards.27 Detailed calculation, comparison with the PIP, and the excitation profiles by a periodic pulse of fix sin(7tt/T) Ix and a DANTE sequence are presented in Section 3. 

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. 

As a second example, a DANTE sequence is taken into account with a period T= 200 ps, the pulsewidth of a single pulse d= 10 ps, pulse strength fix = 1 kHz, and pulsewidth r = 5 ms. As in the previous case, the expansion... 

Table 5. The frequencies / , phases 6 , scaling factors Xn of the effective RF fields, and the amplitudes A of the excitation bands created by a DANTE sequence...

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. 

The DANTE-Z sequence has been employed successfully as a ID substitute in pseudo 3D experiments [7] and also as a band-selective technique in multidimensional experiments [8] in order to improve the spectral resolution. The efficiency of the DANTE-Z procedure over the simple DANTE sequence is illustrated by the spectra shown in fig. 2. 

Fig. 2. Left Experimental profiles of the conventional DANTE sequence (top) and of the DANTE-Z sequence (bottom). The sample used was 5% H2O in D2O with a tiny amount of copper sulfate added (leading to a T of approximately 3 s). The different traces were obtained by shifting the carrier frequency in 50 Hz steps without readjustment of the spectrometer phase. For each experiment, four scans were acquired in order to perform the complete phase cycling of DANTE-Z. Right (a) The conventional H spectrum of a small protein (toxin 7 60 residues) in D2O at 318 K (b) selection of the aromatic region by the conventional DANTE sequence (c) same as (b) using the DANTE-Z procedure. Experiments were performed at 200 MHz using a routine AC200 Bruker spectrometer.

Fig. 9.2. Saturation effect of the modified DANTE sequence on a signal off-resonance with respect to the carrier frequency. The trajectory of the in-plane component of the magnetization is shown. The z magnetization is tilted by the first small angle pulse toward the x axis, and starts precessing toward the y axis. The duration of the pulse corresponds to the duration of a 90s precession, so that at the end of the first pulse the projection lies on the y axis. The phase of the following pulses is rotated in phase with the precession of the signal, in such a way as to follow the spin magnetization in its spiral movement toward the xy plane.

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.

Aspects of the experimental implementation of the technique have been studied in detail [41] and it has been shown that double sideband modulation and pulse shaping can be combined to improve the performance of selective pulses in solid state 2H NMR. Applications of the selective inversion-recovery experiment using a DANTE sequence to study ultraslow motions have been demonstrated [42,43]. 

The DANTE-Z sequence effectively suppresses the sine oscillations that occur using the simple DANTE sequence or Gaussian pulses. 

Use the DANTE sequence provided by your instrument manufacturer. 

Mohebbi and Shaka (1991b) also developed selective homonuclear Hartmann-Hahn experiments based on zero-quantum analogs of DANTE sequences (Bodenhausen et al., 1976 Morris and Freeman, 1978) and binomial solvent suppression methods (Plateau and Gueron, 1982 Sklenaf and Starcuk, 1982 Hore, 1983) (see Section X.C). 

In the homonuclear case, the synchronous and matched irradiation at two frequencies and Vj can be implemented by placing the transmitter midway between the selected frequency ranges Rj and Rj and by modulating a band-selective multiple-pulse sequence with cos r(i, — Vj)/2]. This method can be regarded as an extension of the doubly selective HOHAHA experiment (Konrat et al., 1991), where a weak square pulse, rather than a multiple-pulse sequence, is amplitude-modulated. Implementations based on the principle of interleaved DANTE sequences (Morris and Freeman, 1978 Patt, 1992 Kupce and Freeman, 1992c) are also feasible. 

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 ... 

Multiple site excitation can be achieved by interleaving several frequency shifted DANTE sequences [5-7]. 

Fig. 5.3.12 Selective excitation by the DANTE sequence, (a) The DANTE sequence and the excitation spectrum. Adapted from [Cal2] with permission from Oxford University Press.

A solid-state variant of the DANTE sequence (Fig. 5.3.15) is obtained by replacing the rf pulses and the free precession periods of the original sequence by line-narrowing multi-pulse sequences [Carl, Corl, Flepl, Hep2J. Such DANTE sequences can be used for selective excitation in solid-state spectroscopy (cf Fig. 7.2.8) and for slice selection in solid-state imaging (Fig. 5.3.16). 

Fig. 5.3.15 [Hep21 Dipolar decoupled DANTE sequence for selective excitation for abundant nuclei in solids. The sequence is composed of a. series of phase-toggled MREV8 cycles separated by n normal MREV8 cycles.

The effectiveness of slice selection by the dipolar-decoupled DANTE sequence is illustrated in Fig. 5.3.16 with ID images of a ferrocene cylinder without (a) and with... 

The basic principle is depicted in Fig. 6.2.16(a) [Cho2]. The rf excitation is a train of small flip-angle pulses forming a DANTE sequence, i.e., the sum of flip angles is 90° (cf. Section 5.3.4). They are applied while the read gradient Gx is tuned on. The response to the DANTE excitation is refocused in a Hahn echo, which itself consists of a train of echoes. This echo train is detected in the presence of the read gradient Gx... 

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