NMR of Solids

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 

Consider two nuclei A and B which possess nuclear moments ha and jup separated by a distance rAB and which are subjected to a magnetic field Ho whose direction defines the 3-axis. Then if ha = hu, a quantum mechaiii- 

Since there is a reciprocal effect of ha on B, a doublet resonance is expected with spacing 

A particularly interesting example of the effect of molecular motions on NMR line widths was observed by Murray and Waugh for Co(NH3)6Cl i (92). Theoretical line shapes for the proton resonance of Co(NH3)6C13 are shown in Fig. 22 for (1) a rigid lattice, (2) rotation about the Co—N bond, and (3) rotation of the entire Co(NH3)63+ ion in the crystal lattice. Experimental points for II1 spectra at 100°K and 300°K are seen to fall quite well on curves calculated for, respectively, models (2) and (3). Thus even at 100°K rotation about the Co—N bond at frequencies in excess of about 104 sec-1 is occurring. Between 100°K and 300°K, the frequency of rotation of the Co(NH3)63+ ion exceeds 104 sec-1. 

There are a number of areas where NMR has made significant contributions that are of more interest to the solid state inorganic chemist than to the molecular inorganic chemist. These applications will not be treated here 

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Gerstein B C and Dybowski C 1985 Transient Techniques in NMR of Solids (New York Academic)... 

Lutnnger, G, Meurer, B, Weill, G Makromol Chem 1989,190, 2815 Majumdar, R N, Harwood, H J Appl Polym Anal Charact 1987,423 A single issue of Magnetic Resonance in Chemistry (1990,28) is devoted exclusively to the NMR of solids, although there are no specific arucles on solid-state fluonne NMR... 

Magnetic resonance techniques, EPR (ESR) and NMR, can be used [341,342] to obtain information about atomic, ionic, molecular and crystallographic states before, during and after solid state reactions. Only a very restricted use has been made of the NMR of solids [342—345]. 

NMR of solids is a very diverse collection of methods, and the practice of applying it to chemisorption is changing dramatically as a result of advances in theoretical chemistry including reliable chemical shift calculations. The wide application of NMR to solid state chemistry grew out of the revival of magic angle spinning in the mid 1970 s. This line... 

It is well known that solid materials are characterized by lack of relative motion between the constituent atoms compared with liquids, which results in NMR linewidths much broader than for liquids. Also, it often happens that solids have much longer spin-lattice relaxation 7 than in liquid samples. Thus, studying NMR of solids is more difficult than liquids and studying flows of solid material would seem to be quite impractical. 

CP-MAS-NMR of solids is now almost a routine technique, but unfortunately there are few modern textbooks which deal with solid state NMR, so... 

NMR of solids usually give broad featureless absorptions due to the dipolar interactions which, at least in the case of protons, are orders of magnitude larger than the characteristic chemical shifts and spin-spin couplings used for structure elucidation... 

In general, high-resolution NMR of solids is found to be very much complementary to diffraction techniques in the investigation of solid-state structures. The applications may be classified into three very general groupings ... 

Details of NMR spectroscopy will not be dealt with here but can be found in references sited in the bibliography. NMR is most often carried out on liquid samples or solutions of pure compounds in deuterated solvents, although the NMR of solids can also be obtained. 

Fig. 10.20. Theoretical spectral patterns for NMR of solid powders. The top trace shows the example of high symmetry, or cubic site symmetry. In this case, all three chemical shift tensor components are equal in value, a, and the tensor is best represented by a sphere. This gives rise to a single, narrow peak. In the middle trace, two of the three components are equal, so the tensor is said to have axial site symmetry. This tensor is best represented by an ellipsoid and gives rise to the assymetric lineshape shown. If all three chemical shift components are of different values, then the tensor is said to have low-site symmetry. This gives rise to the broad pattern shown in the bottom trace.

Solids—Many polymers are either soluble or insoluble. NMR of solids generally give broad lines because of the effects of dipolar coupling between nuclei and the effect of chemical shift anisotropy (CSA). Both of these effects are greatly reduced for polymers in solution and allow for decent spectra of soluble polymers in solution. 

Dipolar broadening constitutes a routinely-overcome barrier in high-resolution NMR of solids, and the consequences are readily removed in the... 

This article will review the impact of two powerful new techniques for characterizing epoxy resins at the molecular level — Fourier transform infrared spectroscopy (FT-IR) and high resolution nuclear magnetic resonance (NMR) of solids. Fortunately, these two techniques are not inhibited appreciably by the insoluble nature of the cured resin. Consequently, substantial structural information at the molecular level can be obtained. In this article, the basis of the methods will be briefly described in order to appreciate the nature of the methods followed by a description of the work on epoxies to date and finally some indication will be given of the anticipated contributions of these methods in the future. 

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. 

Schaefer J, Stejskal EO (1979) High-resolution 13C NMR of solid polymers, in Levy GC (ed) Topics in Carbon-13 NMR spectroscopy, Wiley, New York, vol 3... 

Gerstein BC, Dybowski CR (1985) Transient techniques in NMR of solids. An introduction to theory and practice, Academic, Orlando... 

A single issue of Magnetic Resonance in Chemistry (1990,28) is devoted exclusively to the NMR of solids, although there are no specific articles on solid-state fluorine NMR... 

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear. /-resolved 2D NMR (423). 

In general the solid-state spectra in Figures 1-11 have considerably broader lines, and in some cases a related decrease in fine structure. These differences in line widths are typical of the difference in resolution limits that one finds between the spectra taken on solids by CP/MAS and on solutions. In a few cases one actually sees additional fine structure (or apparently higher multiplicities) in the solid-state spectra in comparison with the corresponding compounds in solution. Such situations clearly correspond to cases in which there is greater structural diversity in the solid state than in solution, and they clearly show the great structural sensitivity of CP/MAS P-31 NMR of solids. 

Kaiser, E., Simpson, A. J., Dria, K. J., Sulzberger, B., and Hatcher, P. G. (2003). Solid-state and multidimensional solution-state NMR of solid phase extracted and ultrafiltered riverine dissolved organic matter. Environ. Sci. Technol. 37,2929-2935. 

Multiple-Pulse NMR of Solid Polymers Dynamics of Polytetrafluoroethylene... 

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. 

Gerstein BC, Dybowski CR (1985) Transient techniques in NMR of solids an introduction to theory and practice Academic Press, Orlando, 295 pp Hatcher PG (1987) Chemical structural studies of natural lignin by dipolar dephased solid-state nC nuclear magnetic resonance Org Geochem 11 31-39 Hatfield GR, Maciel GE, Erbatur O, Erbatur G (1987) Qualitative and quantitative analysis of solid lignin samples by carbon-13 nuclear magnetic resonance spectrometry Anal Chem 59 172-179... 

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