High Resolution NMR Techniques for Solids

Solid state NMR, especially of spin-1/2 nuclei, has developed greatly mainly thanks to Waugh and others, who succeeded in designing pulse techniques which drastically improved the resolution of solid state spectra and in addition, significantly enhanced the sensitivity for nuclei with low natural abundance. 

An alternative technique for the removal of dipolar broadening is by introducing a time dependence into the spin system by applying a train of rf pulses while the sample remains stationary. Pulse repetition rates can easily be made greater than the static dipolar line width so in effect line narrowing can be achieved with pulse spacings of a few microseconds equivalent to rotation speeds in excess of 100 kHz. The first line 

Dipolar broadening by unlike spins is removed by strong irradiation of these spins at their resonance frequency. This is analogous to proton decoupling in liquids the indirect spin-spin coupling of the order of a few hundred hertz also disappears, but the power has to be set much higher so as to wipe out dipolar 

All of these quantities can (in principle) be obtained in the gas phase or in molecular beams with high-resolution microwave spectroscopy and molecular beam magnetic or electric resonance. Such data provide isolated molecule values (usually selected as to vibrational and rotational state) with which to compare liquid crystal and solid state values. These techniques reveal the desired tensor components by their relationship to the principal inertial axes of the molecule. 

In partially oriented systems (in liquid crystal solution or in strong electric fields) only linear combinations of tensor elements, such as — or Jaa — Jbb + Jcc)l may be obtained. The spin Hamiltonian for partially oriented molecules, spectral analysis, determination of NMR constants, and results from many applications have 

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Over the last decade, developments in high-resolution NMR techniques for solids have been extended to mercury nuclei. Solid-state NMR studies provide more definitive characterization of mercury complexes since their interpretation is not compromised by exchange processes or solvent coordination. While medium effects on spectra are not absent in the solid-state, they are generally more defined and therefore studied more readily. Furthermore, the chemical shift anisotropy, coupling constant anisotropy, and dipolar coupling constants obtained by solid-state NMR provide an additional probes into structure and bonding. Comparison of solid-state NMR spectra of structurally characterized complexes with solution-state NMR spectra promises to reveal significant differences between the solution and solid-state structures. 

Keifer, P.A. High-resolution NMR techniques for solid-phase synthesis and combinatorial chemistry. Drug Discovery Today 1997,11, 468 78. 

According to Maciel [22] "The availability of high-resolution NMR techniques for solids permits the use of NMR as a structure-determination bridge between the solid and liquid states of matter". Structural information is provided which is not available from X-ray diffraction. For decomposition studies, variable temperature facilities are required. Haw [20] has described the problems associated with such measurements, including rotor design and also temperature measurement and control. 

However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp lines for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 

NMR spectra of solids with the quality of high resolution can be achieved when the above mentioned line broadening phenomena are removed or at least considerably reduced. Only in these cases spectra can be obtained which enable, for example, crystallographically nonequivalent sites of a zeoiitic material framework to be resolved as individual resonance lines. High resolution NMR spectra of solids can be received by use of one or more experimental techniques described in the following ... 

Recently, high resolution NMR studies of solids have been realized by using advanced pulse and mechanical techniques, and so have provided a variety of structural and dynamic information about polymer systems. Further, it can be said that solid state NMR has provided characteristic information that cannot be obtained by other spectroscopic methods, and that it has become a very powerful means for elucidating the structure and dynamics of polymer systems. 

In this section, we will concern ourselves mainly with relaxation processes in solid polymer samples, as distinct from high-resolution NMR used for diagnosing the chemical structure of molecules. The latter can be done on solid samples using cross-polarization, magic-angle spinning (CP-MAS), but these techniques can eliminate valuable information on the very relaxation processes of interest to us. An interesting exception, however, is the WISE experiment, which is discussed below. 

In conclusion, I hope that I have shown that fast-atom bombardment mass spectrometry is a potentially useful tool for the synthetic chemist working in many areas of inorganic, organometallic, and coordination chemistry. In addition, as further fundamental research is done in the field with these applications in mind, the technique should become as routine as IR or NMR. Combined with developments in high-resolution NMR of solids, FAB should provide particularly useful data on supported catalytic reactions. Certainly FAB and the other complementary mass spectrometric techniques, mentioned in less detail, constitute a major way of quickly characterizing new compounds. 

Formerly (also in the second edition of this book) the field of NMR was subdivided into the so called High-Resolution NMR (restricted to solutions) and the "broad band" or "wide line NMR (characteristic for solids). Due to the great advances in NMR techniques the barrier between these two - once quite separate - disciplines has almost disappeared. 

During last two decades, 31P high-resolution NMR spectroscopy has become a very powerful tool for the investigation of solids. SS NMR spectroscopy has been treated extensively, and several treatises provide detailed information about the technique.10-12 Four years ago, Iuga et al. in the Annual Reports on NMR Spectroscopy published a review showing the state of the art in 31P SS NMR.13... 

Applications of solid-state MAS NMR, discussed in the forthcoming sections, include the measurement of H chemical shifts, allowing distinct Bronsted acid sites to be identified and differentiated the study of different A1 coordination environments in aluminas by Al MAS NMR and the characterization of vanadium sites in vanadium oxide catalysts by MAS NMR. An example of the effect of MAS, as compared to static NMR measurements, is shown in Figure 5.2 for V2O5. Both and Al are quadrupolar nuclei, hence anisotropic effects are not removed through MAS alone. Other techniques may therefore be necessary to achieve high-resolution spectra, particularly for non-crystalline samples. 

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