%Solid-State Nuclear Magnetic Resonance - Analytical Chemistry

Shi Bai is manager of the NMR facility at the University of Delaware. He received ... and development of NMR laboratory management Web application sof...
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Anal. Chem. 2000, 72, 1R-7R

Reviews

Solid-State Nuclear Magnetic Resonance Cecil Dybowski* and Shi Bai

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716-2522 Review Contents Scope Reviews Instrumentation and Theory Tensors of Spins 1/2 Quadrupolar Nuclei Solid Materials Properties Porous Materials and Catalysis Synthetic Polymers Biological Solids 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 1997 and October 1999. Applications of NMR spectroscopy to liquids and solutions are not included. With each year, more and more papers on the magnetic resonance of solids from more and more research laboratories appear in the literature, so even covering the developments in that restricted field is difficult. A choice has to be made about what to include and what not to include, necessarily a subjective task. The comments and references below represent only a single view of a tremendously vibrant field. In particular, we may often give a reference to one of a number articles on similar systems as representative of a particular problem addressed with NMR spectroscopy. This review is meant to point the reader to areas and the kinds of research in those areas rather than enumerate all articles on a particular substance. Further literature research will usually be necessary to delve into a particular area or into uses of NMR on a particular system. REVIEWS It is important to remember that reviews in ongoing series are an important source of information on the NMR spectroscopy of solids. We do not point to specific articles, but rather urge the reader to seek information in long-standing review series such as the Specialist Periodical Reports, Progress in NMR Spectroscopy, NMR: Basic Principles and Progress, Magnetic Resonance Reviews, Advances in Magnetic Resonance, and Chemical Reviews. INSTRUMENTATION AND THEORY Development of instrumentation, pulse sequences, and experimental protocols is constantly occurring, sometimes tailoring the experiment to particular systems, sometimes focusing on general concerns. Recently, for example, a discussion of shimming a high10.1021/a1000002r CCC: $19.00 Published on Web 03/24/2000

© 2000 American Chemical Society

resolution MAS probe, taking into account the different symmetries of the coils and the MAS experiment, has been given (1). The measurement of temperature in MAS rotors is a process of some importance that has been the subject of articles in the past few years. Recently, a new approach using a quartz crystal temperature sensor has been demonstrated (2). Pulse sequences are constantly being reported for various experimental conditions and specific uses (3). The FIREMAT experiment for correlating isotropic and anisotropic chemical shifts has been described (4). Broad-band dipolar recoupling sequences (necessary for cross polarization at high spinning speeds) have been discussed and demonstrated (5, 6). Numerical comparisons of various schemes for recoupling have been reported (7). A review of dipolar recoupling and its use in determining torsion angles has appeared (8). Reintroduction of dipolar couplings through off-magic angle spinning to allow correlation of spectra has been shown for a 31P nucleus coupled to a quadrupolar nucleus (9). A theoretical evaluation of line narrowing under LeeGoldburg pulse sequences has been published (10). The efficiency of decoupling for strongly coupled carbons under continuous-wave proton irradiation, as exemplified by linear polyethylene, has been studied (11). The determination of structure is an important area of research with NMR of solids. Terao reviewed methods to determine structure from NMR experiments in a recent article (12). REDOR experiments continue to be important for analysis of local structure (13, 14), and closed-form numerical algorithms have been reported for calculating dephasing in three general cases (15, 16). The use of TPPM during the mixing period of a REDOR experiment is reported to improve decoupling efficiency (17). REAPDOR analyses of asparagine show how one may apply NMR to the determination of structure by interactions of 13C and 17O (18). An analysis of the MQ NMR of infinite one-dimensional chains of spin-1/2 nuclei having dipolar couplings is given in ref 19, and a general review of MQ dynamics has been presented (20). The selection rules for MQ NMR of coupled spin-1/2 systems have been recently given (21). Levitt’s group reported a doublequantum technique for determining torsional angles in peptides that does not rely on knowledge of the chemical shift tensor orientations (22). Griffin’s group presented a two-quantum experiment to determine the torsion angle, Ψ, and applies it to glycylglycine hydrochloride (23). They have also demonstrated a technique using 180° pulses inserted in the evolution period of a chemical-dipolar correlation experiment to increase dipolar effects for more efficient measurement of dipolar coupling, for example between 13C and 1H or between 15N and 1H (24). A twoAnalytical Chemistry, Vol. 72, No. 12, June 15, 2000 1R

dimensional technique using windowless recoupling is reported and demonstrated (25). A 2D method for characterizing the principal values and relative orientations of the electric field gradient and chemical shift tensors of half-integer quadrupolar sites has been demonstrated (26). A technique for homonuclear polarization transfer (R2TR) under magic angle rotation has been tested on 13C-enriched L-alanine (27). A new version of the SEDOR experiment is proposed in which the flip angle is varied (28). Cross polarization from deuterium to carbon is used as means to provide correlation between a carbon and an attached deuteron in deuterated organic materials (29). An adiabatic polarization scheme for MAS spectroscopy is proposed and demonstrated that shows more complete transfer over a broader band (30). The use of optical pumping of InP as a means to increase nuclear polarization for solid-state measurements or for enhancing signals from overlayers of materials has been demonstrated (31). Increase of 13C polarization through optical nuclear polarization via excited triplet states allowed the detection of carbons in fluorene by greater than 3 orders of magnitude (32). The combination of temperature jumps and 2D spectroscopy has been demonstrated with an application to squaric acid (33). Developments in theoretical analysis of MAS spectra have also appeared. A general formalism for simulation of the NMR spectra of rotating solids has been published (34). A number of articles report on the use of the Floquet formalism for analyses to describe spin dynamics under magic angle spinning conditions (35-38) It has also been applied to the description of spin locking of the central transition of a spin-3/2 particle (39). An approach based on average Hamiltonian theory and the development of a superoperator formalism to calculate the MAS spectrum of a spin-1/2 nucleus coupled by the dipole-dipole interaction has also been proposed. (40) A simple perturbation approach has also been demonstrated for simulation of spectra (41). The nature of errors in chemical tensor determinations from MAS NMR side-band analysis has been discussed (42). An analytical expression for the time response under MAS of a spin undergoing two-site exchange has been reported (43). A new CRAMPS experiment based on the MSHOT-3 homonuclear decoupling sequence is reported to be more efficient at removing higher-order dipolar effects, have a wider spectral window, have more stability with respect to offset, and have fewer artifacts than the traditional CRAMPS (44). Vibrational effects on the NMR spectroscopy of solids have been investigated by Terao et al. (45). TENSORS OF SPINS 1/2 Principal pieces of information about the nuclear environment available from the NMR experiment are the elements of chemicalshielding and coupling tensors. These relate to the local electronic environment of the nucleus. The reliability of chemical shielding tensor element determination has been discussed, an important issue for anyone wishing to extract parameters from spectral analysis (46). A report of intrinsic isotope effects on chemical shifts of a variety of nuclei has been published (47). Reports of various 13C chemical shift tensor elements include, for example, trans-stilbene and some of its complexes (48), the carbonate carbon of diphenyl carbonate (49), the carbonyl of glycine in a dipeptide (50), caroyllophene oxide (51), solid 2R

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parthenolide (52), and certain metal-olefin complexes (for which density functional calculations have been performed) (53). The 15N chemical shift tensor elements of a series of nitrosoarene complexes have been reported (54), as have the tensors of adenine, cytosine, guanine, and thymine (55) and [1-N-15]-2′deoxyguanosine (56). A review of 29Si chemical shifts of organosilicon compounds has been published (57). 77Se NMR parameters for a series of materials have also been collected in a review (58). The 31P and 77Se NMR parameters (both shifts and J couplings) of trimethylphosphine selenide and triphenylphosphine selenide have also been reported (59), as have similar trimethyl- and triphenylphosphine complexes of chromium, molybdenum, and tungsten (60). Selenium and carbon chemical shift tensors of selenomethionine have been analyzed recently (61). 125Te NMR shift parameters have been reported for some tellurium oxide species (62) and for some glasses (63). A report of the chemical shifts and relaxation times of some yttrium oxides has appeared (64), as has an extensive review of chemical shifts in solid mercury compounds (65). 207Pb NMR is becoming more popular as a nucleus to investigate. The NMR parameters of a series of inorganic and organic compounds have been reported (66), as have those of the aluminates and silicates (67, 68) and a series of barium lead phosphates (69). Anisotropic J couplings between 113Cd and 31P in Cd(NO3)2‚ 2PMe2Ph have also been analyzed and reported (70). The anisotropy of tin-tin J couplings in (benzyl-Sn)3O has been determined (71). A recent report indicates the scalar and anisotropic contributions to In-P J coupling in indium phosphide (72). The tensors for Cu-P J coupling in linear copper(I) phosphines have also been reported (73). QUADRUPOLAR NUCLEI For quadrupolar nuclei, the presence of a (often very large) coupling to an electric field gradient and the multistate manifold that exists for spins greater than 1/2 determine the kinds of experiments and information available from the NMR spectroscopy of these nuclei. A recent review addresses the NMR spectroscopy of nonintegral spin nuclei (74). The response of spin7/ nuclei has been studied theoretically (75). The use of very 2 high magnetic fields to improve resolution of quadrupolar nuclei is discussed, with experimental examples (76). Adiabatic demagnetization and adiabatic demagnetization in the rotating frame is demonstrated for exciting quadrupolar order of nuclei such as 2H and 23Na (77). Double-quantum cross polarization between 11B and 23Na in sodium diborate has been demonstrated (78). Double-resonance methods involving 11B and 27Al were also useful in analyzing aluminoborate glasses (79). A recent report of 9Be NMR in sodalite structures containing beryllium shows that the beryllium shift is correlated with bond angles (80). A report of 43Ca NMR at natural abundance has appeared (81). The lines are generally featureless, but the line position seems to be correlated with Ca-O bond length in the first coordination sphere. The application of 43Ca NMR to study of high-temperature superconducting oxides has also been examined (82). Some chemical shielding and quadrupolar coupling parameters of 51V in ortho- and metavanadates have been reported (83). 67Zn NMR spectroscopy of single crystals of Zn(CH3COO)2‚ H2O has been investigated (84).

Multiple-quantum NMR is a current area of exploding developments and applications. A review of MQ MAS methods has been published (85). An analysis of composite pulse excitation schemes has appeared (86). The origin of spinning side-band patterns in MQ MAS spectra has been explored (87). The effects of dipoledipole coupling in MQ MAS spectra have been probed in a recent article to yield structural data (88). The resolution of 17O MQ MAS spectra of solids has been shown to be sufficient to allow one to see crystallographically distinct oxygen sites (89). The use of pulsed field gradients to select coherence pathways in MQ MAS has been demonstrated (90). Optimization of higher-order multiplequantum pathways in MQ MAS has been described (91). Two techniques for resolution enhancement of MQ MAS spectra of solids were presented, one based on fast spinning, the second on semiindowless WHH-4 sequences (92). SOLID MATERIALS PROPERTIES Understanding the structure-property relationships of materials requires analysis of the structure and macroscopic properties. NMR contributes to this analysis through the dependence of the NMR parameters on the local structure. For example, a report shows how NMR can be used to measure the distribution of Al species in an organically functionalized alkoxysilane with differing amounts of aluminum present (93). Fire retardation by addition of various phosphine oxides to several polymers was investigated with solid-state NMR of the chars produced on burning (94). The results demonstrate that various kinds of additives lead to chars of differing aromaticity. The green phosphor Tb-doped Y3Al5O12 was investigated by a number of techniques, including 89Y NMR spectroscopy. Yttrium neighbors of Tb up to atom lengths away from the Tb center could be distinguished by the NMR shift (95). Glasses have become a major area of study with solid-state NMR techniques. 31P NMR studies have addressed the local structure of zinc and lead phosphate glasses that can be explained in terms of Van Wazer’s ionic modifying oxide model (96). 11B MAS NMR was applied to the investigation of borate glasses (97, 98), borosilicate glasses (99), lead bismuth borate glasses (100), and indium borate glasses (101). The sodium environments in albite that had varying amounts of water were examined with 23Na NMR (102). The dynamics of lithium ions in lithium niobatetunstate glasses have been investigated using 7Li NMR (104). The dynamics of fluoride motion has been probed in R-PbF2 by fastspinning fluorine NMR spectroscopy (104). The relation of structure in the solid state and macroscopic processes is something that is often desired in analysis. The structural nature of NMR information allows one to address these types of problems. Polymorphic forms of various materials have been characterized by NMR spectroscopy. For example, 13C, 19F, and 1H NMR were used to identify polymorphs of (3S-trans)-1[3,4-dihydro-3-hydroxy-2,2-dimethyl-6-(pentafluoroethyl)-2H-1bezoyran-4-yl]piperidin-2-one (105). The structure of trans-1,4dichlorocyclohexane in a phase exhibited below 260 °C was analyzed by the 13C NMR below this temperature (106). A solidsolid transition of a potent cholesterol absorption inhibition drug has been monitored by 13C MAS NMR spectroscopy, showing that a crystalline fraction correlated with incomplete dissolution of the drug (107).

A review of the use of solid-state NMR in cement chemistry has appeared (108). The hydration of tricalcium silicate to form calcium silicate and portlandite has been investigated with proton NMR, where it was necessary to use CRAMPS to achieve resolution sufficient to allow resolution of various species (109). Calcium aluminum hydrates have been examined with 27Al NMR and the various sites identified (110). Vanadium silicate spectra have also been reported (111). A recent report investigated whether ball milling and ultrasonic dispersion might alter the 13C NMR spectroscopy of solid lignin samples (112). The conclusion of the authors was that the structures were not changed by such processes to an extent that could be detected by NMR spectroscopy. Semiconducting alloys containing cadmium have been investigated with 113Cd NMR (113). Among other things, different structures with various numbers of next-nearest neighbors can be discerned from the NMR spectroscopy. Cadmium selenide quantum dots doped in organic-inorganic hybrid materials were studied with solid-state NMR (114). POROUS MATERIALS AND CATALYSIS Analysis of zeolites continues to be an important use of NMR spectroscopy in catalysis (115-118). Recent reviews discuss the analysis of Brønsted acid sites with NMR spectroscopy (119, 120). 15N NMR, as well as 2H NMR, has been applied to investigation of nanostructed mesoporous silicate materials imbibing pyridine (121). 129Xe NMR continues to be used to probe voids in microporous materials (122). Of course, the development of laserpolarized xenon for investigating surface processes is an area of active interest, for example, with the demonstration of SPINOE between the xenon and the protons of an aerosol (123). Exchange of tin in ZSM-5 has been studied with 119Sn NMR spectroscopy (124). Oxygen site exchange in stilbite was monitored with 17O NMR spectroscopy (125). Gallium NMR has been used to investigate a number of galliated materials (126-128). 133Cs NMR has been used to examine cesium-exchanged Y zeolites (129) and in ilite, kaolinite, boehmite, and silica gel (130). Both 23Na and 133Cs NMR are used to investigate cation migration in partially exchanged Y zeolite (131). Occluded sodium halides in Y zeolites have also been studied with 23Na NMR spectroscopy (132). Deuterated molecules in porous materials can be monitored with deuterium NMR, such as trimethylammonium ions in a tectosilicate nonasil (133). Indium phosphide in MCM-41 was examined with phosphorus NMR, the shift being indicative of quantum confinement effects (134). Monitoring of reactions over catalysts is an important area of use of NMR. The conversion of 2-propanol over zeolite LaNaY was investigated by proton and carbon MAS spectroscopy (135). Reactions of methanol and ammonia over H-RHO and H-SAPO34 have been reported (136). The interaction of methanol with H-ZSM-5 (137) shows two different kinds of methanol, the exchange of which can be monitored with NMR spectroscopy. The activation of propane over H-ZSM-5 has been studied with 13C NMR spectroscopy (138). The adsorption and reaction of acetone oxime on Cu-ZSM-5 and H-ZSM-5 have been probed with both 13C and 15N NMR spectroscopy (139). The reaction of acetylacetone with alumina to produce a variety of materials is another example of how 13C NMR can be used to elucidate the Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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processes happening during sorption of organics (140). An interesting report of the NMR CO on platinum particles in L zeolite compares the NMR spectroscopy to that of CO adsorbed on platinum particles supported on alumina or silica (141). Photcatalysis of material such as ethanol over titanium-containingmaterials is particularly amenable to study with solid-state NMR techniques (142). Surface contamination often plays a major role in environmental chemistry. Partitioning of materials such as naphthalene, phenanthrene, and pyrene from water to a range of soil and sediment samples has been investigated with 13C NMR spectroscopy (143). The co-contamination of motmorillonite with carbon tetrachloride and benzene was investigated and reactivity was seen in this system (144). SYNTHETIC POLYMERS Applications of NMR to the structure and function of synthetic polymers have a long history. One of the more interesting applications seen recently is the use of high-pressure CPMAS NMR to characterize plasticization of polystyrene by CO2 (145). Differences in mobility of chains as a function of the pressure of CO2 are discerned through the experiments. A similar investigation of poly(ethylene terephthalate) subjected to supercritical CO2 also indicated dramatic changes in the NMR spectroscopy (146). Antiplasticization of aryl-aliphatic epoxy resins was also investigated with NMR spectroscopy, as well as by dynamic mechanical studies (147). Solid-state NMR techniques to characterize segmental motions in polymer blends are reviewed in a recent article (148). The use of relaxation times to determine mobility continues to be exploited in the study of homopolymers and copolymers (149, 150). 2H NMR experiments were used to address the dynamics in poly(ethylene terephthalate) samples (151). Proton relaxation times and spin-diffusion measurements of liquid crystalline random copolyesters of 4-hydroxybenzoic acid gave information on domain size (152). A similar use is demonstrated in the application of solid-state NMR to the dynamics of amorphous polyaniline (153). The diffusion of xenon between domains in an ionomeric polymer blend was studied with 129Xe spectroscopy (154). The effects of mechanical deformation on NMR properties of poly(butylene terephthalate) were studied with 13C MAS NMR spectroscopy. Use of rotor synchronized experiments demonstrated that both hard and soft segments are well oriented (155). Studies of oriented polyamide fibers (156) and of orientation in films of isotractic poly(propylene) (157) have been reported. Similar experiments have also been used to study uniaxially drawn poly(ethylene terephthalate) (158), poly(ethylene isophthalate). and poly(ethylene 2,6-naphthalenedicarboxylate). The effect of radiation on fluoropolymers was investigated with high-speed MAS 19F NMR to reveal the nature of modifications in the materials (159), as has high-speed MAS 1H NMR spectroscopy (160). Differences in solid-state NMR of four different forms of isotactic poly(4-methyl-1-pentene) were explained by packing and conformational effects (161). Solid-state NMR was used to monitor thermochemistry of nylon-6,6 heated in the presence of labeled adipic acid and hexamethylenediamine (162). 4R

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BIOLOGICAL SOLIDS There is a vast literature on biological materials studied by NMR. We do not cover the area in great detail. Instead, we point to some interesting uses of NMR in the investigation of biological materials, often whole biological materials. For example, gradient, high-resolution, MAS NMR spectroscopy was used to probe ex vivo lipoma and liposarcoma tissues (163, 164). The method permitted the detection of a metabolite that could not be detected by the usual nonspinning methods. Examination of kidney tumors and normal kidney tissue with 1H MAS NMR spectroscopy allowed the authors to suggest a means of classifying different types of tumor tissues by the observed NMR properties (165). Surgically removed breast implant tissue has been examined with 29Si MAS NMR spectroscopy to determine where the silicon-containing material resides (166). Recent articles demonstrate the use of solid-state NMR in determining information about membrane proteins (167, 168). Structural information on ordered peptides may be studied efficiently with 2D and 3D polarization inversion spin exchange to obtain structural information (169). The use of NMR to address in situ properties of molecules is evidenced by NMR relaxation measurements of onion cell walls (170). Similarly, NMR has been applied to study changes in cell walls of ripening strawberries (171). A serious issue in analysis of prefrozen biological samples is the recent observation that freezing of rat kidney tissue produced differences in spectra before and after freezing (172). NMR spectroscopy of major ampullate gland silk, minor ampullate gland silk, and genetically engineered material (173) showed certain alanine resonance similarities, but differences were also noted. From the results, the authors concluded that genetically engineered protein is a good starting material for biofiber synthesis but that it is not an exact duplicate of the gland fibroin. Aligned silk fibroin was examined with NMR spectroscopy to determine orientation of the glycine carbonyl (174). An interesting use of NMR spectroscopy is the application to understanding the change of cellulose, wood, and peat to what the authors call “artificial coals” (175). A review of factors controlling humification and mineralization of solid organic matter in the tropics discusses the use 13C NMR in these systems (176). Studies have also been reported of soil organic matter taken from different depths in Antarctica (177). Carbon NMR has been used to follow the decomposition of enriched D-glucose or glycine in soil (178) and the decomposition of municipal solid waste (179, 180). It has also been applied to a variety of other materials: foliar litter (181), wheat straw (182), the stems and leaves of barley (183), wood composites (184), wood tar pitch (185), maize starches (186), and mustard in soils (187). It has been used to evaluate the amount of cellulose in tobacco (188) and to study natural fibers obtained from sugarcane (189). An unusual use of 13C NMR spectroscopy is the study of coalified remains of nonvascular Lower Devonian plants (190). A review of the use of 15N NMR to study soils has also recently appeared (191). CONCLUSIONS One is continually amazed at the wide variety of applications of NMR spectroscopy to problems of interest to chemists, physicists, geologists, biologists, engineers, and other scientists. The focus of an experiment may be the structure of a particular site in a biological material, the products of a reaction, the solid-

state structures of compounds, the dynamics of molecules in restricted environments, or the connection of microscopic properties to technologically important uses of materials. NMR spectroscopic analyses give information readily used by the chemist to answer fundamental questions about each of these environments. In addition, even though it was first discovered over half a century ago, intellectual challenges in designing and using experiments to separate and/or correlate the components of spectra still occupy the thoughts and efforts of many spectroscopists. The examples above attest to the continual development of this technique. ACKNOWLEDGMENT

C.D. acknowledges the support of the Petroleum Research Fund of the American Chemical Society under Grant 33633-AC. Cecil Dybowski is professor of physical chemistry at the University of Delaware. He received B.S. and Ph.D. degrees from the University of Texas at Austin. His research interests include all aspects of multinulcear solidstate NMR spectroscopy. He has over 130 publications covering a variety of topics from NMR to mass spectrometry and inelastic electron tunneling. He is author of two books on NMR spectroscopy and is currently on editorial boards of Solid State Nuclear Magnetic Resonance, Magnetic Resonance Reviews, and Applied Spectroscopy. He is currently an Associate Editor of Applied Spectroscopy. Shi Bai is manager of the NMR facility at the University of Delaware. He received a B.S. degree from Lanzhow University in the People’s Republic of China and a Ph.D. degree from Brigham Young University. He has over 15 publications on NMR spectroscopy. His research interests include chemical shift tensor measurements, NMR under high pressure, structure elucidation of organic and biomolecules, and development of NMR laboratory management Web application software.

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