Solids proton enhanced spectroscopy

Lippmaa et al. (86) have successfully studied solid state Si NMR of several trimethylsilyl silicate samples using proton-enhanced nuclear induction spectroscopy and magic angle spinning. They showed that the high resolution Si NMR of solids can be useful for structural studies of certain soluble and insoluble silicates, because many of these compounds with well defined molecular structures in the solid state tend to undergo very complicated rearrangements and condensation in solution. (93)... 

A. Pines, M. G. Gibby and J. S. Waugh, Proton-enhanced nuclear induction spectroscopy. A method for high resolution NMR of dilute spins in solids. ]. Chem. Phys., 1972, 56, 1776-1777. 

In the solid state the restricted molecular motions may cause relaxation times to be relatively long. This means that the experiment cannot be repeated as fast as in solution and a smaller number of accumulations is possible in a given time. The signal can be enhanced by a double resonance technique that is called cross polarization (CP) that solves the problem of slow signal accumulation caused by the long longitudinal relaxation times 71 of heteronuclei in the solid state. The polarization required for the experiment comes from the protons. Thermal equilibrium polarization of the protons is restored with the longitudinal relaxation time of the protons, which is much faster than that of heteronudei. Most commonly, the CP technique is combined with MAS and is denoted CP/MAS [28, 29]. Today this is the predominant method for 13C solid state NMR spectroscopy, but is not restricted only to this isotope. 

The problem of long relaxation times of nuclei in solids has been overcome by utilizing cross-polarization (CP) and proton enhanced (PE) spectroscopy. By this process the C spin system transfers its energy to the spin system. By relaxation of the protons, the energy of the C nuclei can then be transferred to the lattice. Since the spin-lattice relaxation times are... 

Pines A, Gibby MG, Waugh JS. Proton-enhanced nuclear induction spectroscopy. chemical shielding anisotropy in some organic solids. Chem Phys Lett 1972 15 373-6. 

Employing CRAMPS-type proton spectroscopy of powdered solids by using a 2D acquisition experiment can enhance the resolution by a factor of 2 or 3. A H FSLG CRAMPS-MAS experiment was used to study sample 6. Figure 7.10 shows the H MAS spectrum of sample 6 recorded at a spinning rate of 10 kHz. 

Spectrometer does not fall far behind the theoretical limitations, if it does at all. The quality of the pulses (uniformity and constancy of the flip angles and the rf phases) is sufficiently high that pulse errors hardly play a role as a resolution-limiting factor. The tightest theoretical limitation is the necessarily finite width of the rf pulses, which is particularly acute for the BR-24 sequence. The next significant step to enhance the resolution in solid state proton m.p. spectroscopy may well require either 90° pulses shorter than, say, 500 ns (this would be the brute force method) or another clever idea. 

Solid-state MAS NMR spectroscopy has had much success in examining amorphous insoluble polymers [59]. In recent years, however, there has been some debate on the reliability of quantitative data derived from CP experiments [60] and work on fossil fuels in particular has highlighted the problem [61,62]. Undoubtedly, the issues arise in the analysis of polymers as well [63-66]. While CP results in signal-to-noise enhancement and hence reduced accumulation times, carbon atoms present with no proximal protons tend to have their peak intensities reduced relative to other signals. Quaternary aromatic carbons are likely to suffer badly in this respect. The modulation of the dipolar interactions by the motion of some moieties can also introduce quantitative errors [67]. The rotation of the methyl group about its 3-fold axis of symmetry is a good example of this. Single pulse excitation (SPE) [60] however overcomes the problems that are associated with CP,... 

Although polarization transfer techniques have been available for over a decade, they have not been widely used to obtain 29Si-NMR spectra. 7-cross polarization (7) (JCP), which evolved from methods used to enhance the solid state spectra of rare spin nuclei, (8) has been applied to 29Si-NMR spectroscopy (9). JCP suffers from several limitations the proton and 29Si pulses must be on resonance, and the Hartmann-Hahn condition (8) (yHHH = VsiHsi) must be established for full enhancement. Neither of these conditions is trivial to obtain, and the difficulty in establishing them has prohibited routine application of JCP methods to 29Si-NMR spectroscopy.3... 

By using incipient wetness impregnation of the solid samples with a solution of organic radical species such as TEMPO or TOTAPOL, surface enhanced NMR spectroscopy by DNP can be carried out to allow fast characterization of functionalized solid surfaces. Polarization is transferred from the radical protons of the solvent to the rare NMR active nuclei at natural abundance on the surface. Lesage et al. [48] have applied surface enhanced NMR spectroscopy by DNP to... 

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