Combined rotation/multiple pulse

For powdered or amorphous samples, in order to avoid line broadening due to chemical shift anisotropy, MAS is employed (which also averages various hetero-nuclear dipolar couplings). Gerstein and co-workers [25] were the first to combine a multiple pulse homonuclear line-narrowing technique with MAS, and introduced the acronym CRAMPS for Combined Rotation And Multiple-Pulse Spectroscopy [4,25]. 

Another method to remove the anisotropy of spin interactions and to obtain high resolution spectra of solids is by multiple-pulse sequences. The combination of multiple-pulse sequences and MAS (CRAMPS combined rotational and multiple-pulse spectroscopy) is used to obtain highly resolved proton spectra. 

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. 

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. 

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 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.

Although NMR is probably the most widely used NMR technique in chemical applications, it has only been used sparingly in studies of humic substances. Much of the literature in which this technique is used was published prior to 1987 and is well covered in the review by Wilson.(5) Use of NMR is mostly limited to solutions, and it is thus necessary to dissolve humic substances in aqueous solutions. Of course, one must reduce the amount of H s from solvent molecules by use of deuterated water, but it is often difficult to reduce these background signals to negligible amounts. Wilson(5) described the application of water suppression techniques which have been necessary to obtain H NMR spectra of humic isolates from soil. In the case of solid-state NMR of whole soils and humic substances, the development of a new technique called CRAMPS (combined rotation and multiple pulse spectroscopy) has not received the attention it has in the field of coal science, primarily because of the lack of resolution. Thus, recent applications of either solution or solids NMR has been very limited in humic substance science since Wilson s(5) review. 

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