Solids high-power decoupling

Carbon-13. The molecular structure of phosphino-thioformamides (27) and their chalcogenides have been studied in the solid and liquid states.57 The, 3C n.m.r. spectra of solid methylphenyl phosphonium salts have been studied using high power decoupling cross-polarisation and slow magic angle rotation. 

We have seen how by magic-angle spinning (MAS) and high-power decoupling the resolution is rendered similar to that in the solution phase. The intensity of signals due to less abundant nuclei is enhanced by the use of cross-polarization (CP) pulse sequences. Excellent spectra of a variety of nuclei in the solid state have been obtained... 

The solid state high resolution n.m.r. spectra were run on a CXP 200 BruKer spectrometer in which the 31P nucleus resonates at 81 MHz. Samples used were finely powedered and hand-tamped in glass tubes. The spectra were recorded using the Proton Enhanced Nuclear Induction Technique (6) on the same basis of a one shot cross polarization and high power decoupling during acquisition. A capillary tube of trimethylphosphate inserted in the powder sample is used as internal reference. 

High resolution solid-state MAS Ge NMR spectra of organogermanium compounds have been observed for the first time the chemical shifts and linewidths of tetraphenylgermane (38) and tetrabenzylgermane (137) were recorded with and without high-power decoupling. ... 

Dipolar interactions (leading to coupling patterns between nuclei which are not directly bonded) can be minimized by using high-power decoupling fields. If the decoupling power usually employed for liquids is about 5 W, typically 50 -100 W are required for solids. 

In some cases, CP is not necessary to obtain a suitable solid-state NMR spectrum. In these cases, the SPE/MAS sequence may be used and for quantitative analysis only the X-nucleus Ti time needs to be determined. The standard inversion-recovery experiment can be used to measure this value, keeping in mind that MAS and high-power decoupling is still necessary. As before, once the X-nucleus Tl time is determined, 1-5 X T may be inserted as the delay time between successive pulses for quantitative data acquisition. 

So, in this section and in the last, we have discussed two line narrowing methods which are fairly difficult to implement in typical solids where the lines are quite broad to begin with. It turns out that a combination of both techniques, i.e., MAS and high power decoupling, can work quite well in reducing the linewidth because each technique reduces the linewidth enough to make it easier for the other technique to work. The sensitivity problem, due to the fact that the nucleus of interest is rare, still remains and will be addressed in the following section. 

Most solid-state NMR experiments on rare nuclei employ heteronuclear high-power decoupling for satisfectory resolution. Alla and Lippmaa" ... 

Solid-state C NMR spectra of oil shales, obtained by CP/MAS with high-power decoupling are broad because of the multitude of resonances from the different carbon types found in these complex materials. A number of af roaches have been taken to improve the resolution of solid-state NMR of fi il fuels. Such tedmiques include variable temperature studies, variable frequency studies, mathematical enhancements and deconvolution techniques, and relaxation rate methods. The most popular method of enhancing solid-state NMR spectra is a relaxation rate method called dipolar dephasing (DD), which is sometimes referred to as interrupted decoupling. The exploitation of relaxation methods in CP NMR of fossil fuels has been reviewed elsewhere. ... 

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