Combined rotation and

One final technical improvement in soHd-state nmr is the use of combined rotational and multiple pulse spectroscopy (CRAMPS) (2), a technique which also requires a special probe and permits the acquisition of high resolution H and X nucleus nmr from soHds. The combination of these methods permits adapting most of the 1-D and 2-D experiments previously described for Hquids to the soHd phase. 

Reciprocating compressors have unique operating dynamics that directly affect their vibration profiles. Unlike most centrifugal machinery, reciprocating machines combine rotating and linear motions that generate complex vibration signatures. 

Reciprocating Reciprocating pumps are more difficult to monitor because of the combined rotational and linear motions that are required to increase the discharge pressure. Measurement-point location and orientation should be based on the same logic as that of reciprocating compressors. 

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. 

CRAMPS combined rotation and multiple pulse spectroscopy... 

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. 

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

Although the CRAMPS technique (Combined Rotation and Multiple-Pulse Spectroscopy) was developed in the 1970 s23,24,25 it lasted up to 1988 before Bronnimann26 was able to produce well resolved H NMR spectra of the silica surface. Bronnimann s technique was further developed by Haukka and co-workers27,28 in 1993. 

Besides 29Si-NMR, that distinguishes between siloxane bridges, single and double silanols, especially H-NMR-CRAMPS (Combined Rotation and Multiple Pulse Spectroscopy) is a very powerful tool. This technique allows to differentiate the Si-OH and (after hydrolysis) the Ti-OH species, yielding thereby useful information on the structure of the surface groups3. Due to spectral overlap, this distinction is very difficult to observe by infrared spectroscopy. 

CRAMPS Combined Rotation and Multiple Pulse Spectroscopy... 

Two recent studies have examined the NMR spectra of coal macerals and lithotypes respectively. Retcofsky and VanderHardt (12) reported the aromaticities of the vitrinite, exinite, micrinite, and fusinite from Hershaw hvAb coal using non-spinning cross-polarization techniques. The fa values of 0.85, 0.66, 0.85, and 0.93 -0.96 for these macerals demonstrate clear variations between the materials at a given rank. Gerstein et. al. (13) used carbon-13 CP/MAS proton combined rotation and multiple pulse spectroscopy (CRAMPS) to examine Iowa vitrain (Star coal) and a Virginia vitrain (Pocahontas 4 coal) with aromaticities of 0.71 and 0.86 respectively. 

In numerous cases, it may be useful to realize that all these chiral structures may be described in terms of helicity [47], that is the arrangement of atoms can be described as a combined rotation and translation. If all the compounds presented above require a certain rigidity to be chiral, there are however molecules which do not require any rigidity at all to remain chiral. This seems at first paradoxical, but it is the case of the so-called topologically chiral molecules (as opposed to Euclidean chiral molecules). Before examining these objects, it is necessary to briefly review the bases of molecular topology. 

MAS is normally applied concurrently with the dipole line-narrowing methods in order to eliminate the effects of CSA and provide true high resolution NMR spectra in the solid phase. For heteronuclear systems the combined method is usually referred to by the initials CP-MAS, and for homonuclear systems the acronym CRAMPS (combined rotation and multiple pulse spectroscopy) has been coined. 

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. 

In order to obtain optimum line narrowing and improved sensitivity in a solid-state NMR spectrum of a zeolitic material, the experimental techniques discussed in this chapter may be applied in combination, as, e.g., CP/MAS, DD/MAS, CP/DOR or CRAMPS (Combined rotation and multiple pulse spectroscopy). The rotor synchronization technique provides... 

Figure 2. High-resolution NMR of protons in 2,6-dimethybenzoic acid. Combined rotation and mutliple-pulse spectroscopy.

Figure 3, High-resolution NMR spectra of protons in Pocahontas No, 4 vitrain (top) and Star vitrain (bottom). Combined rotation and multiple-pulse spectroscopy, (top) in Pocahontas No. 4 vitrain CRAMPS at t = 36 jjisec f = 2.5 KHz = 0.73 corrected for hydroxyl, (bottom) H in Star vitrain CRAMPS at t = 36 xsec f =2.5 kHz f = 0.23 corrected for hydroxyl.

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