Anisotropic solid-state chemical

Nuclear Magnetic Resonance Imaging of Anisotropic Solid-State Chemical Reactions... 

Butler, L. G., Cory, D. G., Dooley, K. M., Miller, J. B. and Garroway, A. N. (1992). NMR imaging of anisotropic solid-state chemical-reactions using multiple-pulse line-narrowing techniques and H-1 T1-weighting. J. Am. Chem. Soc., 114, 125-35. [238]... 

This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties. 

Molecules in the solid state are in fixed orientations with respect to the magnetic field. This produces chemical shift anisotropic powder patterns for each carbon atom since all orientations are possible (Fig. 2). It was shown as early as 1958 that rapid sample rotation of solids narrowed dipolar-broadened signals [18]. Several years later, it was recognized that spinning could remove broadening caused by CSA yet retain the isotropic chemical shift [19]. 

Recently, solid state nmr spectroscopy has been used to probe hydrogen-bonded materials and first reports showed that there was a direct relationship between / o o anisotropic chemical shift as bonds become... 

The NMR observables of a substance as a liquid, solution, or in the gas phase of paramount importance to the inorganic chemist are chemical shifts and nuclear spin-spin coupling constants. In the solid state, however, these effects generally are dwarfed by anisotropic dipolar interactions so... 

Fluids generally give simpler spectra than rigid matrices, because rapid molecular reorientation reduces the influence of anisotropic interactions or eliminates it altogether. When one is interested only in chemical connectivity patterns, as in most solution NMR or EPR studies, the simplicity can be helpful, but for other applications the richness of solid-state spectra can be indispensible. 

Figure 38. (left) Solid-echo 2H NMR spectra of glycerol-/ (7 = 189 K) [305]. A collapse of the solid-state spectrum is observed upon heating the corresponding time constants of the a-process are indicated, (right) Hahn-echo 31P NMR spectra of w-tricresyl phosphate (m-TCP, Tg = 210K) determined by the anisotropic chemical shift interaction [324]. 

While cooling, when the limit xa = 1 /, (cf. Eq. 15) is reached, the central Lorentzian NMR line, which is characteristic of a liquid (t-,8, solid-state spectrum, in the case of 2H NMR the Pake spectrum. The breadth of the solid-state spectra makes it difficult to measure the corresponding (short) free induction decay (FID), so that it is necessary to use echo-techniques (cf. Section II.D.2). Figure 38 (left) shows solid-echo 2H NMR spectra of glycerol-. The crossover from a Lorentzian line to the Pake spectrum is observed some 20% above Tg. Below Tg the spectrum is independent of temperature. In Fig. 37 (right), the corresponding 31P NMR spectra of m-tricresyl phosphate (m-TCP) are displayed. The characteristic spectral shape is now determined by the anisotropic chemical... 

Recently, solid state nmr spectroscopy has been used to probe hydrogen-bonded materials and first reports showed that there was a direct relationship between R0...0 and cr, the anisotropic chemical shift as bonds become shorter the chemical shift moves downfield. Twenty-four compounds were examined and some, such as potassium hydrogen malonate, had a o-value below 20 ppm (Berglund and Vaughan 1980). 

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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