Selective excitation DANTE

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. 

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... 

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 double-selective TOCSY-ROESY and TOCSY-NOESY techniques are particularly useful. They allow one to measure NOE and ROE correlations in spectra with high degree of overlap as often found in carbohydrates. In addition to the DANTE, DANTE-Z [66], and Gaussian pulses as described earlier for selective excitation, self-refocusing shaped pulses such as BURP (EBURP and UBURP) [67] have also been used for this purpose [64]. 

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... 

Si-NMR spectra were recorded on a Bruker VJM-250 (liquid) or a Bruker CXP-300 Fourier transform magic-angle-spinning (FT MAS) solid state spectrometer. Resonances are relative to tetramethyl-silane (TMS). Dynamics of the silicate solutions were studied by selective excitation techniques by using DANTE-type (131 pulse sequences. 

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

Figure 21 Saturation transfer experiment the downfield branch of the Pa doublet of 1 was inverted using a DANTE selective excitation pulse. The exchange of the magnetization was followed as a function of mixing time. T was found to be 1.26 s and exchange = 2.85 s at 300 K. (From J. M. Brown, and P. A. Chaloner and G. A. Morris (1987)/. Chem. Soc. Perkin Trans. 2, 1583. Reproduced by permission of The Royal Society of Chemistry)...

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. 

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.

DANTE-type frequency-selective pulses have also been used to selectively excite specific nuclei in the spin system and this has been employed to simplify MQ spectra. This approach is limited to relatively simple spin systems where the resonances of some nuclei are well separated from the normally overlapped signals in the spectrum.28,29... 

In the early days of selective excitation, spectrometers were not equipped to generate amplitude modulated rf pulses and the DANTE method (Delays Alternating with Nutation for Tailored Excitation) was devised, [49] requiring only short, hard pulses. Although largely superseded by the amplitude modulated soft pulses, DANTE may still be the method of choice on older instramentation or on those newer instmments which lack waveform generators. 

When implementing this sequence it may be necessary to add attenuation to the transmitter to increase the duration of each pulse so that the shorter elements do not demand very short (< 1 xs) pulses (note the similarity with the requirements for the DANTE hard-pulse selective excitation described above). The binomial sequences can be adjusted to provide an arbitrary overall tip angle by suitable adjustment of the tip angles for each element. For example, inversion of all off-resonance signals can be achieved by doubling all elements relative to the net 90 condition. Exactly this approach has been exploited in the gradient-echo methods described below. 

DANTE Delays alternating with nutation for tailored (selective) excitation 9.3.2... 

The improved DANTE-Z sequence only differs from the original sequence by an additional pulse whose phase is cycled by a tt phase shift and a final 90° pulse to transfer the z-magnetization to detectable transverse magnetization. In Check it 5.3.3.2 the modification to the phase cycling in the original DANTE sequence is examined and the results of selective excitation using the DANTEl-1 and DANTE363 sequence compared. 

Simulate the 1D selective COSY spectrum of glucose using the configuration file ch5333.cfg. The experiment uses a DANTE-Z element for the selective excitation of the proton at 3.9 ppm. Note depending upon the computer speed this simulation may take several minutes. 

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. 

Selective excitation was accomplished by using a delays alternating with nutations for tailored excitation (DANTE) sequence of 20 short rf pulses of x =0.6 is duration [4]. The pulse separation used was 20 ps resulting in the total duration of the DANTE sequence of x,=400 [iS and the corresponding spectral excitation width A of the center band of 2.5 kHz. The excitation sidebands were separated from the centerband by (Xp) = 50 kHz and did not affect the NMR spectrum. After a recovery period,x of at least 35 ps a final 90° pulse was applied followed by the detection of the free induction decay. 

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