CRAMPS pulse sequence

Lesage et al. have shown that the resolution of the proton NMR spectroscopy of powdered solids can be improved significantly when multi-pulse sequences are employed [44a]. In the approach based on combined rotation and multipulse spectroscopy (CRAMPS) (Figure 7.9) the problem of dipolar line broadening is usually overcome. 

Figure 7.11 Two-dimensional CRAMPS-MAS spectrum of a powdered sample 6 recorded using the pulse sequence of Figure 7.6.

The influence of the homonuclear magnetic dipole-dipole interaction on can be reduced either by an increase of the sample spinning frequency, Vjot, (Eq. (20)) or by the application of a multiple-pulse sequence causing an additional averaging of this interaction (combined rotation and multiple-pulse spectroscopy, CRAMPS 19-21 ). With today s instruments, sample spinning frequencies of up to 40 kHz can be reached using MAS NMR rotors with an outer diameter of 2.0 mm. 

Despite numerous applications, conventional CRAMPS still remains one of the most demanding solid state NMR experiments as it requires the use of specially prepared spherical samples to minimise radiofrequency inhomogeneity effects and the careful calibration and setting of pulse widths and phases. Further modifications of the experiment that do not require the complicated and extended set-up procedures have been suggested recently. These are known as rotor-synchronised CRAMPS, which combines a new multiple pulse sequence [21], and its modification which uses a standard WHH-4 sequence at ultrafast MAS frequencies (e.g. 35 kHz) [22]. 

Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) is a technique in which the dipolar interaction is averaged through a multiple-pulse sequence [54, 55]. The simultaneous spinning around the magic angle, as in MAS NMR, averages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclusively a surface-sensitive technique, the majority of catalytic applications have focused on the study of adsorbed species, and the information on surface structure that can be extracted from their spectra. 

An important consideration in a CRAMPS experiment is the interference between the two averaging processes, i.e., does the physical rotation of the sample by MAS impair the performance of the multiple-pulse sequence, the latter having originally been designed for static samples. Indeed, a low vr, i.e., less than 3 kHz, is used in a conventional CRAMPS experiment, such that, to a first approximation, the sample can be considered to be static during each cycle of the multiple-pulse sequence. In this so-called quasi-static limit, the multiple-pulse sequence can be considered to take care of the... 

Figure 6. Pulse sequences and coherence transfer pathway diagrams for (a) a 2D CRAMPS experiment incorporating a z-filter to ensure that pure absorption-mode line shapes are obtained and (b) a 2D constant-time CRAMPS experiment. The relative performance of the two experiments with respect to yielding high-resolution H NMR spectra in the indirect (F ) dimension is illustrated by Figure 5c,d and is discussed in the text. (Adapted with permission from Figure 2 of ref 78. Copyright 2001 American Chemical Society.)...

For the excitation of DQC under MAS, the interference with the sample rotation must be considered, as was the case for the CRAMPS experiment discussed in the previous section. In particular, if a sequence designed for a static MQ experiment is used without modification, the excitation (and reconversion) time is limited to ttr/2, since the rotor modulation causes the action of the pulse sequence in the second half of the rotor period to be the time reversal of that which occurred in the first half of the rotor period. Starting with the suggestion of Meier and Earl,89,90 who simply proposed the phase switching of the static sequences used by Baum et al.,47,48 every half rotor period to prevent the process of self-time-reversal, many different approaches have been presented that allow excitation (and reconversion) times of one or more rotor periods. Such pulse sequences that counteract the effect of MAS are referred to as recoupling methods,1112 examples that have been used in homonuclear DQ MAS NMR spectroscopy include BABA,91 C7,92 DRAMA,93 DRAWS,94 and HORROR.95 We note that Levitt and co-workers have recently introduced a very helpful classification system, based on symmetry principles, which covers such sequences.96,97... 

Combined rotation and multiple-pulse spectroscopy (CRAMPS). A special pulse sequence, in addition to MAS, is required for high-resolution proton NMR in solids. This technique is known as CRAMPS. 

The H cramps NMR measurement was performed on a Chemagnetics CMX 300 spectrometer operating at 300.16 MHz, equipped with a 5 mm CRAMPS probe. The BR-24 pulse sequence"" was used, and 7r/2 pulse width was 1.3 ps. The rotational frequency was exactly controlled in the range 1.5 to 2.0 kHz, and the cycle time of BR-24 was 108 ps. The recycle delay was 10 s and spectra were usually accumulated 32 times to achieve a reasonable signal-to-noise ratio for the samples. The H chemical shift was calculated with a scaling factor of 0.40 for all samples, which was determined experimentally. The H cramps spectra were recorded first without internal standard, and calibrated afterwards with internal Si-rubber (5 = 0.12) relative to TMS (5 = 0). The typical half-width was 30 Hz, and the total measurement time for one sample was usually 5 min. 

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