Anal. Chem. 2004, 76, 3263-3268
Solid-State Nuclear Magnetic Resonance Cecil Dybowski,*,† Shi Bai,† and Scott van Bramer†,‡
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716-2522, and Department of Chemistry, Widener University, Chester, Pennsylvania 19013 Review Contents Scope Reviews NMR Techniques Chemical Shielding and J-Coupling Measurements Porous Materials Semiconductors and Solid Ionic Conductors Glasses Polymers Biological Solids Quantum Computing Conclusions Literature Cited
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SCOPE This review focuses on developments and applications in the field of nuclear magnetic resonance of solids appearing in the literature between October 2001 and October 2003. During this period, a continuing trend is the use of solid-state NMR spectroscopy to study pure compounds, but also to study materials that are, either naturally or deliberately, combinations of chemicals whose properties are being probed. A search of the literature shows that in the period there were over 18 000 publications that mentioned solid-state NMR spectroscopy. Choosing papers to emphasize and to exclude is a difficult task. The choices of examples necessarily reflect the authors’ biases. We hope to point the interested reader to areas of research, rather than enumerate all articles on a particular substance or area of endeavor. Further literature research will be necessary to delve into a particular area or into uses of NMR on a particular system. REVIEWS Reviews are an important source of information. From time to time, journals that specialize in topics such as polymer chemistry or ionic conductors publish reviews on uses of NMR spectroscopy, which are helpful to the reader interested in those specific fields. For example, recent review articles include a general review (1), a review on the uses of solid-state NMR in analytical chemistry (2), a review on applications in macromolecular science (3), a review on use of uniformly labeled proteins and peptides (4), a review on application of double-resonance techniques to biological solids (5), a review on uses in pharmaceutical applications (6), a review on detection of explosives (7), and even a review on wood products (8). A particularly pertinent review describes the calculation of solid-state NMR parameters (9). Other reviews may be found in long-standing review series such as the Specialist † ‡
University of Delaware. Widener University.
10.1021/ac040048l CCC: $27.50 Published on Web 04/10/2004
© 2004 American Chemical Society
Periodical Reports, Progress in NMR Spectroscopy, NMR: Basic Principles and Progress, Magnetic Resonance Reviews, Advances in Magnetic Resonance, Chemical Reviews, and Accounts of Chemical Research. The range of reviews indicates the wide reach of this spectroscopy. Aside from reviews, it is sometimes useful to search the World Wide Web judiciously to discover projects being performed in various laboratories around the world. NMR TECHNIQUES The exploitation of symmetry often allows the design of schemes for obtaining specific information. A report demonstrates how rotor-synchronized pulse sequences can be used in fast magic-angle spinning to obtain high-resolution 1H NMR spectra of solids (10). In several reports, authors demonstrate techniques that improve or complement existing NMR techniques. Heteronuclear recoupling sequences are important in magic-angle-spinning NMR (11). A discussion of the design of such sequences has appeared (12). A simple heteronuclear decoupling scheme has been suggested and demonstrated that is claimed to have improved decoupling efficiency (13). A variant of the double-cross-polarization experiment for heteronuclear dipolar recoupling has been reported (14). The authors report more robust behavior for this sequence than that of earlier sequences. Symmetry-based recoupling sequences have also been proposed and demonstrated (15). Multiquantum magic-angle-spinning NMR has developed into a useful tool for analysis of half-integer quadrupolar nuclei. A new 2D method, labeled I-STMAS, has been reported that is complementary to the usual scheme for these sorts of experiments (16) and may be more useful than conventional sequences in certain cases. A discussion of the optimal strategies for isotopic enrichment in techniques for determining spatial distributions in proteins has been given, with a suggested framework for gauging the precision of the measurement of dipolar coupling constants and the potential for a set of such measurements to constrain structural calculations (17). Developments in instrumentation always have an effect on the way the instrumentation is used for analysis. The use of ultrahigh fields to improve the resolution of NMR of quadrupolar nuclei (18) has been demonstrated, with the result that spectra become more first order, as expected. One group has recently reported the design and construction of triple-resonance probes using variable transmission line segments as tunable reactances, which is claimed to control stray reactances and provide higher Q alternatives to ceramic capacitors (19). A recent report demonstrates a hitherto unknown mechanism of relaxation of 207Pb in Pb(NO3)2 (20). The authors interpret Analytical Chemistry, Vol. 76, No. 12, June 15, 2004 3263
the results in terms of a phonon-modulated spin-rotation interaction. A more traditional study of dynamics is the analysis of motional anisotropy in (CH3)3AlND3 (21) through the deuterium relaxation rate. CHEMICAL SHIELDING AND J-COUPLING MEASUREMENTS Chemical shielding measurements provide a traditional measure of the electronic structure of a molecule or material. The calculation of chemical shielding tensor elements is becoming straightforward with modern computational techniques (22). Both experimental measurements and quantum calculations were used to investigate the chemical shielding of pure materials that include the study of the conformation of biphenyl in the solid state (23), the determination of the 13C chemical shielding tensors of a series of dialkyl carbonates and thiocarbonates (24), and the evaluation of phosphorus chemical shielding in phosphido ligands of ruthenium carbonyl compounds (25). The use of CRAMPS is exemplified by a study of the 19F chemical shielding in alkali metal fluorides and monovalent tetraalkylammonium fluorides (26). The chemical shielding of quadrupolar nuclei such as 59Co can also be determined, as shown in a recent report in which quadrupolar tensors were also analyzed (27). A report of 67Zn parameters for model compounds for metalloproteins has appeared that indicates some of the parameters for zinc NMR (28). Recently, interest in the use of anisotropic J coupling for analyzing solids has been exploited. NMR measurements combined with relativistic calculations allow one to explain the origins of couplings in simple molecular solids (29). A review has appeared on this subject (30). POROUS MATERIALS Two-dimensional double quantum 1H MAS NMR investigations of a surfactant-templated silicate thin film prepared from polyoxyethylene cetyl ether in a (tetraethoxysilane) silica sol-gel allowed the authors to define spatial contact between the surfactant and the silicate (31). The dynamics of ibuprofen encapsulated in mesoporous silica was studied with 1H, 13C, and 29Si NMR spectroscopy (32), and functionalization of the matrix was observed to have a significant effect on mobility. The study of infinite onedimensional water chains in two imidazole hydrates suggests that water molecules undergo different reorientational dynamics in the two cases (33). The studies of zeolites with NMR spectroscopy have been a traditional example of the applicability of solid state to problems in catalysis (34). Structural information has often been gleaned from 29Si or 27Al NMR spectra. Structure of synthesized materials such as an LTA-type zeolite (35) can be followed with NMR. A study of the 17O MQMAS of aluminosilicate xerogels provides yet a third way to determine structure in these systems (36). The effects of grinding on mixtures of kaolinite and calcium carbonate are addressed in a recent report using solid-state NMR (37). NMRactive counterions such as Cd are observed to study the effects of different treatments on the ions in a material; an example of this is seen in a recent article on heat treatment of Cd-exchanged zeolites (38). A study of the interaction of phenol with Na-X zeolite shows the interaction of the hydroxyl proton with the basic oxygen of the zeolite and the interaction of the aromatic ring with either the sodium ions or the framework oxygens (39). Zeolites 3264
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are used as substrates for reactions, and the resulting material is analyzed with NMR spectroscopy. This has been done for some (arene)tricarbonylchromium complexes (40). And the dynamics of hexane adsorbed in ZSM-5 have been probed with deuterium NMR and neutron diffraction (41). The observation of acetone oxime on Fe-ZSM-5 has been monitored with 13C NMR and reaction products identified (42). Adsorption on the surfaces of other materials such as a TiO2 catalyst are studied by NMR spectroscopy of the adsorbed phase (43). In these experiments, two types of surface ethanol species were formed and characterized upon adsorption of the ethanol. A quintessential porous material is chromatographic packing, and NMR spectroscopy has been used to study the surface phases on such packings. A new reversed-phase HPLC packing was investigated with 13C NMR to complement other traditional characterization techniques (44). SEMICONDUCTORS AND SOLID IONIC CONDUCTORS The nature of semiconducting materials is readily probed through NMR-active isotopes present in the material. Indium phosphide, for example, is conveniently analyzed with 31P NMR spectroscopy (45). In cases where the material contains several NMR-active isotopes (e.g., silicon, cadmium, or tin), the use of multiple spectroscopic measurements can give information in greater detail (46). The vanadium oxide-based phases show various behaviors that can be monitored with vanadium NMR (47, 48). In manganese-containing phases, the 55Mn resonance can be used to study physical changes, for example through measurement of Knight shifts (49, 50). Optically detected NMR spectroscopy can provide useful information on photoinduced properties of materials such as gallium arsenide (51) or indium phosphide (52). Organic semiconductors are also readily investigated with NMR (53, 54). The connectivity in the fast solid ion conductor Cu2P3I2 has been investigated through dipolar recoupling experiments (55). The couplings suggest a relationship between the 31P chemical shifts and the bonding structure of this solid. Lithium NMR has been used to monitor capacity fade in lithium manganese oxide spinels (56), indicating the potential utility of NMR to characterize processes that degrade the performance of certain high-performance electronic materials. GLASSES The amorphous structure of glasses often makes interpretation of the NMR spectra difficult. Nevertheless, studies have yielded structural information that can be obtained in no other way. A recent study of the 77Se NMR of AsxSe1-x glasses allowed the evaluation of the various three-atom linkages in the structure (57) and indicated that selenium clustering was probably occurring. The same group found a similar effect in GexSe1-x glasses (58). NMR has been used to study the preparation of silica aerogels with GeO2 included (59). 31P NMR of sulfate-phosphate glasses accounts for the different types of phosphorus-oxygen structural units present (60). 109Ag NMR spectroscopy of silver-based borate glasses shows that only part of the silver ions participate in conduction (61).
POLYMERS NMR spectroscopy is widely used to investigate the chemical structure of polymeric materials. An example of such a use is a study of 1H and 13C NMR of dendritic polymers, to determine chemical and supramolecular structural information from the NMR parameters (62). Blends of nylon-6 with poly(propylene oxide) have been studied with 15N NMR spectroscopy of the naturalabundance nitrogen (63). The spectroscopy is sensitive to the presence of the poly(propylene oxide), and various crystalline forms are identified with NMR spectroscopy. Fluorine NMR has been used to determine the structures formed when perfluoroalkoxy resins are irradiated (64). Modest thermal cycling of poly(propylene) samples has been monitored using NMR spectroscopy (65). Polymeric materials may be used in combination with refractory compounds to form materials. NMR has been shown to give information on the dynamics of poly(styrene-ethylene oxide) in clays (66). The data are consistent with a model in which the poly(ethylene oxide) blocks intercalate sideways into 50-nm stacks of hectorite. The products of vulcanization of natural rubber could be enumerated by examination with carbon NMR spectroscopy, including information on the structural linkages formed on vulcanization (67). In a similar study, changes in styrene-butadiene rubbers upon vulcanization were explored (68). NMR was one of several techniques used in a comparison of thermal and microwave curing of resins (69). BIOLOGICAL SOLIDS The analysis of melanin and hair with 13C and 15N NMR has been reported (70). Another unusual application of NMR has been the study of diatoms, in which silicon NMR is used to probe differences in the amounts of various silicate structures in different species, which suggests the existence of extracellular pools of less condensed silica (71). The structures of peptides (72) and moderately sized proteins (73) are being analyzed using various constraints from the values of NMR parameters, which provides a means of defining structure in these systems. The exploration of structure with the 43Ca nucleus has led to a report of the chemical shift tensors of calcium ions in a helical peptide (74). The recent focus on Alzheimer’s disease has led to an interesting solid-state NMR study that places constraints on the supramolecular structure of the amyloid fibrils. The study shows that the constraints of NMR, when used in conjunction with electron microscopy results, provide a more detailed structural model of the fibrils (75). The structure of vancomycin bound to Staphylococcus aureus has been reported from NMR analysis (76), providing a look at the structure of the complex of an antibiotic with a substrate. NMR has been applied to a wide variety of biological materials that are foodstuffs. For example, aging of bread has been monitored with 13C NMR spectroscopy (77). Skim milk powders having undergone different heat treatments are distinguishable by 31P NMR spectra (78), showing the utility of the technique for analysis of changes under these treatments. Green and black teas have been probed with 13C NMR (79). The mobility and distribution of water in cassava and potato starches has been investigated (80). Structural changes in maize and sorghum upon cooking have been studied with 13C NMR spectroscopy (81). The structural features of the polymorphs of neotame have been addressed with
a combination of techniques that include solid-state NMR spectroscopy (82). Crystallization of polymorphs of sufamerazine has been characterized by several techniques including NMR (83). The investigation of soils and substances derived from soils has become an active area of study with its own special problems (84). Recent reports have addressed various materials, including humid subalpine soils (85), soils from unmanaged hardwood forests (86, 87), arctic tundra (88), and subtropical Australia (59, 89) as well as peat (90) and sandy loam (91). NMR analysis has been used to study the transformation of TNT under anaerobic/ aerobic composting (92). The study of binding of TNT to soil through analysis with deuterium NMR shows binding similar to that of TNT to a montmorillonite sample (93). An interesting application to archaeology is to the study of Neolithic soils from a site in Bavaria () (94). The study of woods with NMR is a well-developed discipline. In one recent study, solid-state NMR spectroscopy was used to determine the degree of cellulose crystallinity of pine and birch pulps (95). Gingko wood has been analyzed after specific enrichment with 13C through administration of selectively enriched coniferin (96). Cork samples have been analyzed with 13C NMR, and the spectra have been correlated with FT-IR spectra of the same samples (97). Wood replicas have been studied with NMR to probe structure and order (98). Recently, the interactions between wood and phenolic resin have been evaluated by comparing relaxation times in the rotating frame for the interphase with the neat resin (99). In some cases, experimenters have focused on measurement of relaxation times to determine dynamics in biologically derived materials. Examples are studies of water dynamics in wool fibers (100) and in cellulose (101). Relaxation studies show that pectin in the walls of kiwifruit become “softened” in the early stages of ripening (102). QUANTUM COMPUTING The proposal to use spin systems as models for quantum computers has been a vogue among persons interested in spin dynamics. The early demonstrations used solution-state spin systems that were manipulated to produce operations. In the last two years, a trend to use of solid-state spin systems can be seen in the literature. For example, a solid-state quantum computer composed of semiconducting silicon has been proposed (103), as has a decoupling-free NMR quantum computer (104). Yet another proposal involves CdTe, the atoms of which both have spins of 1/2, with optical pumping as an integral part of the computing operation (105). It is obvious from the papers that this area of work is still in its infancy, with a variety of factors yet to be considered and optimized. CONCLUSIONS The developments in solid-state NMR spectroscopy in the last period continue to emphasize its use as a probe of identity, structure, and dynamics in fields as diverse as biological membranes and superconducting solids. We find interesting the large number of situations in a wide array of disciplines in which the identity or physical properties of a material of relatively complex structure may be analyzed with NMR spectroscopy. Whether it is a component in a catalyst, a foodstuff, or some contaminant in the environment, the application of NMR to the analysis provides Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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a direct, and often novel, means of identifying properties. Intellectual challenges in designing and using NMR experiments to simplify analysis still are seen in some efforts reported in this period. The studies of theoretical constructs such as quantum computing show yet another sophisticated technology that can be addressed with experiments in NMR spectroscopy. Cecil Dybowski is a professor of chemistry at the University of Delaware. He received B.S. and Ph.D. degrees from the University of Texas at Austin and was a research fellow in chemical engineering at the California Institute of Technology before coming to Delaware. He has over 150 publications on various aspects of spectroscopy, including two books on NMR spectroscopy. Shi Bai is manager of the NMR facility at the University of Delaware. He received a B.S. degree from Lanzhou University in the People’s Republic of China and a Ph.D. degree from Brigham Young University. He has over 20 publications on NMR spectroscopy. His research interests include structure elucidation of small organic and biomolecules and chemical shift tensor measurements. Scott E. Van Bramer is a professor of chemistry at Widener University. He received his B.A. in chemistry from The Colorado College in 1987 and his Ph.D. in chemistry from the University of Colorado in 1992. His research interests include solid-state NMR and the use of technology in chemical education.
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