ZSM-5 zeolites containing transition metal ions, in particular copper cations, have received considerable attention because of their combined catalytic and shape-selective properties and especially because of their high activity in the decomposition of nitrogen monoxide. In most work on these materials, the copper ions were introduced into the zeolite structure by conventional ion-exchange, either in aqueous solution or in solid state. In this study, we have used another approach that leads to the incorporation of copper cations into the intracrystalline pores without ion-exchange. This method involves the crystallization of a zeolite structure around molecular complexes. Using this technique we can introduce relatively large molecules such as metallo-phthalocyanine into the pore system. The Copper-ZSM-5 system is characterized by the combined application of ESR and 129Xe spectroscopies. These techniques allow us to study the location and the oxidation states of the copper ions after various thermal treatments. The 129Xe NMR gives evidence about the partial autoreduction of the Cu2+ ions during the dehydration process. The ESR spectra taken after adsorption of o-xylene or ammonia confirm that the Cu2+ are indeed located in the channels. However, NMR spectroscopy of adsorbed xenon, used as a probe, shows that when there is no chemisorbed phase, the Cu2+ are stabilized in the crystallites but outside the channels. They return to the channels upon chemisorption of ammonia.
Synroc is a titanate ceramic designed for the immobilization of Purex-type wastes from nuclear fuel reprocessing (E. R. Vance, MRS Bulletin, XIX, December 1994, pp. 28-32). Incorporation of Na and halides from process chemicals may affect the properties of Synroc, so we are studying their partitioning into the standard Synroc phases (perovskite, hollandite, rutile and zirconolite) and any new phases they may form. With high levels of Na in the waste, Synroc can be modified to allow the formation of a leach-resistant freudenbergite (NaAl2Ti6O16) phase. This phase has been studied in conjunction with model titanate phases such as Na2Ti3O7 and Na2Ti6O13 using 17O, 23Na and 27Al MAS NMR, as well as in relation to partitioning of Na into the other Synroc phases. 19F MAS NMR confirms that substantial quantities of fluoride can be retained as CaF2 in Synroc right through the process cycle. Unsuitable Synroc processing conditions can lead to the formation of water-soluble, readily leachable Cs compounds. The presence of even small quantities of these and other minor phases can be sensitively monitored by NMR.
Layered metal phosphonates have attracted attention because they may serve as intercalation hosts and can be derivatized for use as microporous catalysts. In addition, our interest in metal organophosphonates has been to take advantage of the mixed organic/inorganic layered structure to prepare materials with unique magnetic and transport properties. As part of these efforts, we have recently produced monolayer and mutlilayer films of both pure and doped metal octadecylphosphonates using Langmuir-Blodgett (LB) deposition methodologies. Here we demonstrate that the structural environment of the phosphonate group(s) in a series layered metal phosphonates can be determined using a combination of static, MAS and CP-MAS 31P NMR. Spectra are presented for materials with formulas MII(O3PR).H2O, MII(HO3PR)2, and MIII(HO3PR)(O3PR) where R is either an ethyl-, butyl-, octadecyl- or phenyl-substituent group, and examples of metals studied include Zn, Cd, Ca and La for the pure materials and Zn/Cd, Mn/Cd or Fe/La mixtures for the doped systems. The measured anisotropic chemical shifts can be directly related to the metal-oxygen-phosphorus bonding geometries, and the 1H-31P dipolar couplings can be used to assign the spectra of materials posessing more than one type of phosphonate bonding geometry. These NMR studies also provide information regarding the local geometry of the dopant sites, the number of different dopant sites and the relative percentages of each type present. Extensions of this methodology to studies of LB films of metal octadecylphosphonates can verify whether the structures formed by LB deposition posess the same metal-oxygen-phosphorus bonding geometry found in the powdered solid-state analogs.
We present a novel solid-state NMR probe design for the in situ study of catalytic reactions under flowing conditions. Until recently, it has been impossible to combine high resolution data acquisition with reactant flow due to the difficulty of sealing a rotor that is turning at the speeds typically used in magic angle spinning (MAS) (~180,000 RPM). Our design features gas-tight seals that isolate the flowing reactant mixture from ambient air for complete efflux capture, and utilizes a helical gear system to drive the sample rotor. We use the two dimensional magic angle hopping (MAH) pulse-sequence developed by Bax and coworkers to acquire the data 1. The MAH experiment allows us to acquire high-resolution data without fast sample spinning which is precluded by the friction inherent in our sealing system. In our preliminary experiments we followed the adsorption of methanol and ethanol on HY zeolite. The improved resolution provided by the MAH experiment allows us to resolve the individual methanol and ethanol resonances which could not be resolved under static (i.e. non-spinning) conditions. Future improvements, including reducing the experimental time and the studying reactions under high-pressure flowing conditions, will be discussed. 1. Bax, A. D., Szeverenyi, N. M., and Maciel, G. E., J. Magn. Reson. 52, 147 (1983).
In magic angle turning (MAT) 2D NMR, sample rotation has the effect of removing anisotropic interactions in one dimension of the spectrum. The purpose is usually to measure the chemical shift anisotropy (CSA), but MAT has been used to study other anisotropic interactions. In this work, the PHORMAT pulse sequence applied to 13C is modified by adding a pi pulse to invert 15N part way through the rotor cycle. This is closely analogous to REDOR, and it similarly restores the C-N dipolar coupling which would otherwise be canceled by the sample rotation. This results in a 2D CSA-dipolar correlation pattern. This pattern may be analyzed by comparison to simulations, to determine the orientation of the dipolar tensor relative to the CSA tensor. In comparison to non-spinning methods for CSA-dipolar correlation, this method has the advantage that it separates overlapping patterns according to their isotropic shifts.
Bulk samples with some degree of macroscopic orientational order show a dependence of the MAS sideband spectrum on the rotor phase. The application of this technique is used to analyze bulk calcium metaphosphate glass. We show the experimental setup in order to reach spinning speeds up to 4.5 kHz with the required asymmetric sample preparation in the MAS rotor. Affirmative experimental results and computer simulations are demonstrated.
The recently developed MQMAS pulse sequence has expanded the possibilities for the examination of quadrupolar nuclei. Particularly in materials in which severe overlap of second order quadrupolar lineshapes exists, the two dimensional nature of the experiment can allow one to deconvolute the different sites based on quadrupolar coupling and isotropic chemical shift. Taking advantage of this sequence, a low silica zeolite, Chabazite, with varying ratios of Na and Li cations was examined. Through 23Na MQMAS and MAS, 7Li MAS, and 6Li MAS, the different cation locations in Chabazite were characterized. Non-statistical filling of the alkali cation sites was observed and lithium and sodium appear to have different site perferences. Sodium sites with NQCC values up to 5 MHz were observed which are related to oxygen environments by point charge calculations. Additional details about the cation distribution and dynamics are reported on the basis static variable temperature and spin echo double resonance NMR data.
The nitrates of calcium, lead, strontium and barium are crystallographically isomorphous and form a continuous set of solid solutions. The crystal structures are cubic, with four magnetically inequivalent but chemically identical cations per unit cell. Each divalent cation is surrounded by a shell of six nitrate anions, and then a second coordination sphere of twelve cations at a distance of approximately 5.5 to the center cation. Of these twelve, six are in a distorted octahedral arrangement, and six are arrayed in a hexagon in the plane perpendicular to the threefold 111 axis of the crystal. The lead chemical shielding tensor is familiar to many solid-state NMR spectroscopists; the barium and strontium quadrupolar tensors have been measured in our laboratory; all three are axially symmetric. Crystals grown from aqueous solutions containing mixtures of these nitrates incorporate both of the divalent ions into the lattice: for example crystals grown from a 60:40 (molar) mixture of lead and strontium nitrates incorporate a few percent of strontium ions into the lead sites in the crystal. At this level of incorporation, 207Pb MAS spectra show in addition to the main lead resonance, a weaker upfield doublet which we assign to lead atoms which have an adjacent strontium either in the axial or the octahedral site. The shift due to a single strontium substitution is approximately 20 ppm. At higher levels of strontium incorporation, lines due to double substitution, triple substitution, etc. are observed; under the right conditions, all of the thirteen possible resonances, ranging from lead atoms surrounded by twelve lead atoms, all the way to lead atoms surrounded by twelve strontiums, can be observed in a single spectrum. The shifts are approximately additive at low strontium incorporations but tend to increase as the number of strontiums in the second coordination sphere increases. The origin of this shift is not understood; however, the intensities of these lines allow us to do a detailed analysis of the thermodynamics of solid solutions, and to analyse this category of substitutional disorder in far more detail than has previously been possible.
Lithium intercalation compounds of amorphous carbons prepared from polymeric precursors have been researched extensively for use as anode materials in rechargeable solid electrolyte batteries. We have applied variable-temperature high resolution 7Li solid state NMR techniques to characterize the lithium inventory in such materials. NMR is able to differentiate between electrochemically active lithium species, which are reversibly intercalated, and other lithium species which are lost to parasitic processes and are present in a passivation layer. Spatial proximity and chemical exchange between these species have been probed by homonuclear coherence transfer experiments. The 7Li chemical shift of the intercalated species changes with charging state in a characteristic manner. Detailed measurements of Knight shifts and of spin-lattice relaxation and spin-spin relaxation behavior will be reported for fully charged, partially charged, and fully discharged electrode materials. In addtion, variable-temperature static NMR studies provide insights about the diffusional mobility of the intercalated lithium species. These studies provide important data concerning the relationship between microstructure and performance characteristics of these materials.
Normally, paramagnets in a solid sample are considered a nuisance; producing broad lines and short relaxation times. We have been working with the zeolite TS-1, a titanium substituted analog of ZSM-5. Although there are extensive publications using various spectroscopies to characterize the titanium site in TS-1, none of them clearly proves that titanium is at tetrahedral sites in the framework. In the 29Si NMR of TS-1, there is no peak in the spectrum that can be associated with those Si nuclei with titanium as next nearest neighbor, as is seen in zeolites substituted with aluminum and boron. Peeters et al.*, have demonstrated that when cobalt is incorporated in the ALPO framework (aluminum - phosphate molecular sieve) a significant amount of the 31P NMR signal becomes "NMR Invisible". We applied this same idea to TS-1 which was synthesized following literature procedures and carefully characterized via x-ray diffraction, diffuse reflectance UV, and elemental analysis. The diamagnetic Ti4+ in the sample was reduced to paramagnetic Ti3+ with carbon monoxide. Much of the signal vanishes. On the basis of the 29Si spectra, we conclude that titanium is mainly present in tetrahedral coordination and probably dispersed throughout the lattice. We will present the data and interpretation in terms of dipolar and scalar coupling of 29Si to the unpaired electrons. * Peeters, M. P. J., Van de Ven, J. L. M., de Haan, J. W., and van Hooff, J. H. C., Colloids Surf. A: Physicochem. Eng. Aspects, 1993, 72, 87.
Zeolites are porous structural aluminosilicates with cations in the pores to maintain electroneutrality. The cations can be used to affect the adsorption and catalytic properties of the zeolite. In favorable cases the intracrystalline cation mobility can be measured by pulsed field gradient NMR methods, e.g., for protons1. However many cations are not amenable to this method because they have large quadrupole moments or are paramagnetic. Fyfe and coworkers demonstrated interparticle cation exchange in zeolite A using a combination of 29Si NMR and powder x-ray diffraction2. Again, this is a favorable case because the structure of the zeolite changes with different cations changing both the NMR shifts and the Bragg peaks in the diffraction pattern. We are exploiting the differences in 29Si relaxation times when different paramagnetic cations are ion exchanged into the zeolite. In particular we have prepared the zeolite ZSM-5 in the proton form and in the Cu+ form. If we are careful to exclude air (O2) the 29Si spectrum of H-ZSM-5 has a T1 of 10 s. In air, the T1 is about 4s. However, the Cu-ZSM-5 has a T1 of only 100 ms. If aliquots of the two samples are mixed in a dry and oxygen free atmosphere the 29Si saturation recovery is biexponential. After several days we observe the two relaxation times approaching each other. This indicates that the H+ and Cu+ are migrating from crystallite to crystallite. We will present more data on the affects of water on the interparticle contact exchange. The data will be presented and analyzed in terms of the dipole-dipole interaction between the paramagnetic electron and the 29Si nuclei. 1. Karger, J., Bar, N.-K., Heink, W., Pfeifer, H., and Seiffert, G. Z.Naturforsch. 1995 50a, 186. 2. Fyfe, C.A., Kokotailo, G.T., Graham, J.D., Browning, C., Gobbi, G.C., Hyland, M., Kennedy, G.J., and DeSchutter, C.T. J.Am.Chem.Soc. 1986 108, 522.
Nitrogen is a very meaningful element in biochemistry. Particularly, since the I=1/2 and the chemical shifts are dispersed in a wide range, the nitrogen-15 seems a suitable nucleus for the NMR study. Unfortunately, its gyromagnetic ratio and the natural abundant are so small that the NMR sensitivity is quite low. Therefore, most NMR research works have to use the 15N-labeled samples, which are very expensive and difficult for preparation. Dynamic Nuclear Polarization (DNP) is an electron-nuclear double resonance technique which makes a polarization transfer from the excited electron to the nuclei and results in an enhancement of the NMR sensitivity. In an ideal condition, the enhancement factor might be as high as 660 and 6500 for the 1H and 15N, respectively. In this work, we report our most recent progress about the DNP studies of15N in solid state for both the 15N-labeled and the natural abundant 15N organic compounds in the presence of the free radicals (BDPA). The experiments were carried on a home-made DNP spectrometer with the resonance frequencies of 80.2 MHz , 8.1MHz and 53GHz for proton, 15N and electron, respectively. The results show that the best concentration of the dopped free radical is about 10% (w/w). The dominant enhancement mechanism is the solid state effect and the enhancement factor are several hundreds for the direct enhancement of15N and decades for the indirect enhancement (transfer the polarization to 1H first and then cross polarization). A 15N-DNP spectrum of the natural abundant sample was obtained. The options and results were discussed in detail. Acknowledgment: the15N labeled samples were granted by the professors D. M. Grand and R. J. Pugmire at the University of Utah (USA) and this work is partly supported by NEDO project of Japan.
MQMAS NMR allows high-resolution spectra of half-integer quadrupolar nuclei to be obtained, producing isotropic spectra in which the effects of quadrupole coupling are absent. The effects of dipole-dipole coupling however remain in the spectra, and indeed are multipled by N, the order of multiple-quantum coherence involved in the experiment. Spinning sidebands in MQMAS NMR spectra can be quantitatively analysed to reveal details of homonuclear and heteronuclear dipole-dipole coupling and hence yield valuable information on chemical structure. This work describes the theory behind the analysis of MQMAS spectra and applies the technique to a series of alkali metal silicate glasses, revealing details of the alkali metal ion distribution in these systems.
We present a simple pulse scheme for transferring coherence between 2H and 13C spins in solid-state NMR and some of its applications for studying the structure of polymers and peptides. The HMQC-type experiment exploits two-bond 2H-13C dipolar couplings and involves only two rf pulses on 2H. By standard heteronuclear recoupling pulses, the experiment is adapted to magic-angle-spinning (MAS) conditions. The technique can be used to determine molecular torsional angles by correlating the C-2H bond direction, which is the unique principal axis of the 2H quadrupolar interaction, with the 13CH2 chemical-shift anisotropy of an adjacent unit in a suitably labeled polymer. In polystyrene, different gauche states can thus be distinguished. Under MAS, the scheme is used to identify and characterize sites in peptides and proteins that have been deuterated by hydrogen/deuteron exchange between D2O and the amide protons. Applying this technique to a membrane protein, it will be possible to determine which amino-acid residues are accesible to water.
High-resolution 1H and 2H solid-state NMR has been used to study the intercalation of small molecules in fullerene C60. The octahedral interstitial void in C60 is ca. 4.2 across and ideally lends itself to the study of the behaviour of isolated incalated molecules and molecular clusters. As any diamagnetic anisotropy due to the host C60 is effectively averaged to zero by motion of the C60 cage, the host can be considered as NMR transparent. We discuss the behaviour of isolated molecular systems within C60 voids at different temperatures. The method can be used to study hydrogen bonding in small clusters also containing water, and opens up the possibility of examining the nature of hydrogen bonding and freezing mechanisms in isolated molecular clusters separated by hydrophobic C60 cages. The incalated species investigated include water, ammonia, formaldehyde and acetylene.
The lithium manganese oxide spinel, LiMn2O4, has shown promise for use as the cathode material in lightweight rechargeable lithium batteries due to its high capacity, and low toxicity and cost. However, these materials suffer from a capacity loss on repeated cycling through lithium insertion into and removal from LiMn2O4. This large capacity reduction during repeated charging/discharging has been attributed to the cubic to tetragonal distortion associated with the Jahn-Teller effect at the Mn3+ cations leading to the degradation of electrode particles at crystallite surfaces due to a large lattice mismatch and/or the degradation of oxide crystallites due to bulk volume expansion and contraction during the cubic to tetragonal phase transformation. Different cycling behavior has been observed in materials where manganese has been partially replaced by other cations such as lithium, nickel, or cobalt. In particular, nickel substituted lithium manganese oxide spinels (LiMn2-yNiyO4, 0 y 0.33) have shown superior cycling performance to LiMn2O4 and the absence of a Jahn-Teller distortion on lithium insertion into LiMn1.5 Ni0.5O4. In a continuing effort to comprehend the relationships between the solid state properties and electrochemical performance of these oxide spinels, variable temperature 7Li NMR spectroscopy was employed. The temperature dependence of the 7Li resonance shift may be used to examine the extent of electron delocalization in these materials, thereby giving insight into the complex electronic structure of these spinel oxides. Analysis of the temperature dependent shifts also provides a measure of the transferred hyperfine interaction at the 7Li nucleus which can be interpreted as a measure of covalency within these materials in the paramagnetic state. This information may prove useful in understanding the absence of the Jahn-Teller effect and substantial capacity fade in nickel substituted materials.
The study of quadrupolar nuclei under magic-angle spinning (MAS) and RF irradiation (Vega, A. J., J. Magn. Reson. V96, 1992, 50-68)) has recently resulted in new experimental schemes including the demonstration of rotation induced adiabatic coherence transfer (RIACT) (Wu, G., Rovnyak, D., Griffin, R. G., J. Am. Chem. Soc. V118, 1996, 9326), useful in the context of MQMAS experiments (Frydman, L., Harwood, J. S., J. Am. Chem. Soc. V117, 1995, 5367). The complexity of spin-locking and coherence transfers in half-integer quadrupolar nuclei under RF irradiation and MAS has led us to pursue the characterization and optimization of these effects considering high spinning rates, offset dependence, and amplitude-modulated pulse shapes. We find, for example, that the interconversion of the highest symmetric coherence and the central-transition coherence via RIACT has desirable performance characteristics at very high MAS rates. We also investigate alternative methods for homonuclear and heteronuclear correlations involving half-integer quadrupolar nuclei.
Flow MAS NMR has recently been applied to studies of optically pumped 129Xe and heterogeneous catalysis. While flow MAS NMR was used successfully to study these system, a drawback with these types of probes is that the flow stream is not isolated from the other gases used to heat, support, and spin the modifies rotors. This precludes recovery of the product stream for later analysis. Because the product stream analysis cannot be performed simultaneously with the NMR experiment, the results obtained from heterogeneous catalysis studies using this technique cannot be directly compared to results of an actual flow reactor. We are developing a flow MAS NMR probe with isolated flow gas to enable the simultaneous observation of events occurring on the catalyst with characterization of the product stream externally using an on-line gas chromatograph. Currently we have a working probe utilizing ceramic bearings that achieves isolation of gas streams at spin rates of 2 kHz. To test the flow aspects of the probe, we have observed methanol adsorption onto zeolite catalyst HZSM-5 with 13C flow MAS NMR, and observed structural changes via 13C flow CP MAS NMR in a vapochromic material when exposing it to volatile organic compounds.
Lactic acid based aliphatic polyesters are increasingly being explored for use in several applications including biodegradable packaging materials, food containers, bioresorbable medical implants and sutures, and drug delivery systems. A number of physical properties of poly(lactide) (PLA) are linked to its stereosequence distribution, which is influenced by many factors such as the starting lactide feed composition, polymerization kinetics, and extent of conversion. The polymerization kinetics in turn are influenced by the catalyst, temperature, impurities, batch vs. continuous process, etc. We have been using solution and solid-state NMR spectroscopy to understand important features of PLA such as polymerization kinetics, tacticity, and solid-state morphology. We have been using 1H and 13C NMR spectra of PLA in solution to determine the sequence of the lactic acid units from the relationship between the stereogenic centers and their influence on the chemical shift. In the solid state, 13C CP/MAS spectra provides information about polymer morphology and crystal structure. The solid-state NMR data suggest that there are five crystallographically inequivalent sites per unit cell. We are using two-dimensional exchange NMR to probe the microstructure of PLA to understand the conformation and arrangement of the polymer chains.
Polymorphism is defined as the ability of a substance to exist in two or more crystalline forms that differ in the arrangement and/or conformation of the molecules in the crystal lattice. The polymorphic form of a compound may significantly affect its physical and thermodynamic properties. Many compounds exist in multiple polymorphic forms for which the cyrstal structures are not known. Solid-state NMR spectroscopy can be used to identify the number of crystallographically inequivalent sites in the unit cell and to understand molecular structure. 13C CP/MAS NMR spectroscopy shows that aspartame (aspartyl-phenylalanine mehyl ester) exists in three distinct forms at room temperature, depending on preparation conditions. For two of the forms, there exists three resonances for each carbon, indicating three crystallographically inequivalent sites and therefore three distinct conformations and/or arrangements of aspartame molecules within that form. Assignment of all the solid-state NMR resonances for each form of aspartame is extremely difficult. We have been using two-dimensional exchange spectroscopy with high-speed MAS and TPPM decoupling to assign the solid-state NMR spectra of uniformly 13C labeled aspartame. Sufficient resolution has been obtained with these techniques to observe 13C-13C J-couplings in the 13C solid-state NMR spectrum. For one of the forms of aspartame that possesses multiple crystallographically inequivalent sites, we can readily identify connectivities between the nuclei of each conformation and/or arrangement of molecules.
Novel mesostructured aluminophosphates synthesized in the presence of tetraalkylammonium surfactants have been studied by 27Al, 31P and 13C solid-state NMR. 27Al and 31P MAS and CP/MAS spectra reveal the presence of several non-equivalent sites of phosphorus and aluminium in the structure. 31P NMR spectra show significant differences in the degree of condensation of the inorganic framework depending on the synthesis parameters and the mode of thermal treatment. 27Al and 31P NMR have been applied to optimise the synthesis conditions and to monitor the formation kinetics, while 13C NMR was used to characterise the organic template.
Several aspects of broadband double-quantum recoupling schemes are considered. First, the susceptibility of windowless recoupling sequences such as MELODRAMA to radiofrequency (rf) errors is examined by numerical simulations and average Hamiltonian theory. Compensated schemes based on composite rotation theory are demonstrated to remove the experimentally dominant error terms. With improved sequences, more thorough experimental investigations of multi-spin systems are possible for both chemical shift correlation and double-quantum filtration. Chemical shift correlation is demonstrated in U-13C-Erythromycin A. In addition, weak dipolar couplings (in two-spin systems) may be measured with improved precision by the use of echo sequences that refocus the dipolar evolution on long timescales, distinguishing signal loss due to the dipolar dephasing from loss due to double-quantum relaxation and residual rf errors.
Design of state-of-the-art instrumentation and software for acquisition and analysis single-crystal NMR spectra has been developed. The instrumentation involves highly accurate rotation of a goniometer, a rotation controlled by the host computer of the spectrometer via a homebuilt interface. The subsequent analysis of the single-crystal spectra is efficiently handled by the software package ASICS (Analysis of Single-Crystal Spectra) which has been developed and is currently undergoing optimisation in our laboratory. Using this instrumentation we have investigated the garnet YAG (Y3Al5O12) by 27Al single-crystal NMR spectroscopy. The 27Al study of YAG represents one of the most complex single-crystal NMR studies undertaken so far (i.e., there is a maximum of 35 heavily overlapping resonances from 7 distinct sites in each spectrum). However, employing ASICS it has been possible to make a complete analysis of the single-crystal spectra in terms of the 27Al quadrupole couplings and anisotropic chemical shieldings. Important information on the crystal structure of YAG has been obtained from the analysis. The instrumentation and software for single-crystal NMR spectroscopy along with the analysis and results of the 27Al single-crystal NMR study of YAG will be presented.
To develop 17O solid state NMR for structural studies in proteins, alpha-oxalic acid dihydrate was chosen as a model compound to examine the shielding and electric field gradient (EFG) tensors. All three oxygens in alpha-oxalic acid dihydrate were uniformly labeled by exchange with 20% 17O enriched water. The central transitions were observed by 1H-17O cross-polarization. The combined effects of quadrupole coupling and chemical shift on the NMR frequencies were measured as a function of crystal orientation and were separated based on their different angular dependence or opposite dependence on the static magnetic field strength. In fact, both 17O quadrupole coupling and chemical shielding tensors were over determined by single crystal rotation experiments at 6 and 12 T. EFG and shielding tensor orientations are closely related to molecular planes but not bond orientations. Unlike ice, the waters of hydration do not contain an EFG tensor component along the 2-fold axis, presumably due to the non-symmetric H-bonding environment. Also, different shielding tensors for the two oxygens in the carboxyl group were determined and the trace of the average tensor is equal to the chemical shift observed in solution. The systematic absence of expected lines in some single crystal orientations and distortions in powder patterns of Mg(17OH)2 are shown to be due to an orientation dependence of the Hartmann-Hahn condition. We verified experimentally that this is a 1st-order quadrupole effect on the central transition nutation frequency. A simulation procedure adding the effect of non-uniform excitation closely matches the experimental powder pattern of Mg(17OH)2. Also, a two level Hartmann-Hahn match recovers most systematic absences.
Several aspects of 1H-driven, 13C-13C spin diffusion (exchange of longitudinal 13C polarization in the presence of moderate to strong 13C-1H couplings) in spinning solids will be presented: (1) Accurate Two-Spin Measurements: Previous studies (e.g. Suter and Ernst, Phys. Rev. B v32 p5608) have shown that in single crystals where chemical shift differences can be controlled through crystal orientation, measurement of exchange rates as a function of shift difference yields data that unambiguously relates to zero-quantum lineshape and internuclear distance. In spinning solids the connection between spinning speed and shift difference has been discussed (e.g. Levitt et. al. J. Chem. Phys. v96 p6347 and Kubo and McDowell J. Chem. Soc. Faraday Trans. 1,v84 p3713). We exploit this similarity in spinning solids by measuring exchange rates as a function of spinning speed near rotational resonance, to gather data that directly relates to zero quantum lineshape and internuclear distance under conditions where decoupling is not applied during exchange. We show accurate (+/- 0.1 angstrom) measurements of 4-5 angstroms in model compounds and demonstrate that 13C-13C distance measurements over 10 angstroms should be possible using this techniques. (2) More Uniform Exchange Rates: Polarization transfer among numerous 13C nuclei in multply 13C-labeled samples under spin diffusion conditions (i.e. no 1H decoupling) proceeds at a variety of rates between different spin pairs, depending on sample spinning speed, chemical shift difference, and strength of coupling to 1H nuclei. At moderate to high spinning speeds, exchange rates are significantly attenuated by MAS-modulation of 1H couplings. We demonstrate that a weak rf field applied to the 1H channel (to satisfy a HORROR recoupling condition) during 13C-13C exchange blocks this attenuation and yields more broadband spin diffusion. At high magnetic fields, sufficient dispersion in the 1H spectrum may exist to allow selective modulation of different 1H-13C couplings and hence allow selective control over 13C-13C spin diffusion exchange rates. (3) Application to biomolecular structure determination will be discussed.
The use of adiabatic coherence transfer in NMR is well established. Methods have included adiabatic frequency sweeps and amplitude sweeps. Transformations carried out have included homo- and heteronuclear cross polarization, population inversion and the creation of J-order. We will present an examination of the application of adiabatic coherence transfer to systems of isolated quadrupolar nuclei. In particular, we will look at the creation of quadrupolar order, by use of an adiabatic demagnetization in the rotating frame, and the excitation of double-quantum coherence, by the use of a multiple-pulse train applied with an offset which is swept adiabatically. Both are expected, theoretically, to generate the maximum efficiency of transformation to these states from equilibrium magnetization. These novel experiments will be examined theoretically in order to establish the optimum rate of sweep and the nature of the final states produced. Computer simulations and experimental results, on biological samples, will also be presented.
Recently, there has been much interest in the realisation that inhomogeneous second-order quadrupolar broadening can be removed from NMR spectra of half-integer quadrupolar nuclei by correlation of multiple- and single-quantum coherences in the presence of MAS. We have used a novel "split-t1" multiple-quantum (MQ) MAS experiment to record high-resolution spectra of a wide range of nuclei, including 11B, 17O, 23Na, 27Al, 71Ga and 87Rb. In particular, we have shown that a quintuple-quantum split-t1 experiment can often yield additional resolution in comparison with the triple-quantum experiment, allowing sites with similar crystallographic parameters to be distinguished. These experiments provide a useful and important structural probe for many materials, such as the industrially important microporous AlPO, AlMePO and GaPO compounds, or the geologically important silicate minerals. We are interested in several potential developments of the original MQMAS experiment that aim to enhance sensitivity or provide further structural information by editing the spectrum. For example, the addition of a cross-polarisation sequence to the experiment offers, under certain circumstances, the possibility of signal enhancement and spectral editing.
The value of the high-resolution solid state technique 13C CP-MAS and CRAMPS for the observation of different type of fullerenes, fullerenes chemisorbed or physisorbed on silica and hydrides fullerenes of C60 and C70. The resolution sufficient to resolve resonance signals of the different molecular symmetry groups. Fullerene hydrides are prepared by transfer hydrogenation of fullerenes C60 by 9,10 - dihydroanthracene. The main product of C60 hydrogenation is C60H36, which is a sufficiently stable molecule. The comparison of IR and solid state 1H and 13C NMR data for C60H36 with the theoretical ones allows the suggestion that fullerene hydride has a T symmetric structure and contains 4 isolated benzenoid rings located at tetrahedral position on the surface of a closed skeleton of molecule. In the frame of systematic search of fullerenes investigated high-pressure modified fullerenes by means of solid state NMR.
High resolution solid state NMR is rapidly becoming an important technique for probing the molecular mechanism of cooperative phenomena such as ferroelectric and antiferroeletric phase transitions. There is, however, an ever-lasting quest for additional enhancement in the spectral resolution, since the transition mechanisms generally involve only minute changes in ionic sizes and orientations, and lattice and molecular dynamics. In this presentation, we describe a novel two-dimensional CPMAS temperature-jump NOESY technique, abbreviated TOESY, and its application in detecting slow dynamic processes in the close vicinity of the structural phase transitions. Squaric acid (H2C4O4) was chosen as a model compound and the measurements were made in the vicinity of its antiferroelectric transition at 100C. It is demonstrated that TOESY enhances the spectral resolution significantly, which is crucial for distinguishing between several different models of proton exchange in squaric acid. With the TOESY technique, a multiple structure of the so-called "central peak" in the vicinity of the phase transition (TC = 373 K) in squaric acid has been observed for the first time. It indicates that one of two protons is effectively localized in the middle of its relevant O ... O bond while the other remains in the O-H ... O bond as in the low-temperature phase. TOESY thus provides a new avenue for detecting very slow exchange precessses.
It has been demonstrated recently that high-resolution solid state NMR spectroscopy is a superior techniques for the in situ studies of the mechanisms of heterogeneous reactions. In this contribution, solid state NMR has been used for the investigation of alkanes transformations over microporous solid catalysts like zeolites.13C MAS NMR study of the early stages of propane 2-13 and isobutane 2-13C activation was performed over H-ZSM-5 catalysts with various content of protonic and aprotonic sites. The technique used to produce samples, sealed under controlled atmospheres in precision glass ampoules, suitable for MAS in the region 2 to 4 kHz is described. Solid state NMR spectroscopy such as CP-MAS, CRAMPS, 2D solid-state spin diffusion 13C NMR experiment in situ apply for the investigation of alkylation reaction of benzene with alkanes. These techniques can give a useful information on the mobility of the adsorbed molecules and on the distribution of reactants and products in microporous solids.
A new technique is presented that adds to the group of solid state NMR methods which utilize heteronuclear recoupling for measurements of the dipolar interactions in rotating solids. The experiment combines the capabilities of multiple-quantum magic angle spinning (MQMAS) with the rotational-echo double resonance (REDOR). We present a detailed description of the experiment and demonstrate its usefulness in a study of 19F-27Al spin pairs in chabazite-like AlPO4 aluminophosphate. The advantage of the MQ-REDOR technique is that it provides direct inference of the connectivities between spin-1/2 (e.g. 1H, 19F, 31P) and quadrupolar (e.g. 11B, 17O, 23Na, 27Al) nuclei in solids under high resolution conditions. In addition, we have also shown that analysis of the MQ-REDOR data yielded accurate rAl-F distances for the three distinct aluminum sites in fluorinated AlPO4.
It is known that the patterns of spinning sidebands observed in the multiple-quantum dimension of MQMAS spectra are often significantly wider than expected from the anisotropies of relevant interactions. It has been recently shown by others that these sidebands are generated due to the rotor-driven reorientations that the quadrupole tensors of the crystallites undergo during the evolution period between the multiple- and single-quantum conversion processes. We present an experimental and theoretical study of the effects of the spinning speed and rf field strength on the development of these sideband patterns. The theoretical analysis relies upon numerical simulations and includes propagation of the density matrix during the entire MQMAS experiment. The possibility of additional rotational encoding during the rf pulses is discussed. Both the theoretical and experimental results show the benefits of using the highest available nR and nrf.
By combining optical nuclear polarization, Larmor-beat optical detection, and light-synchronized multiple-pulse line-narrowing, we have spectrally resolved the radial distribution of Knight shifts surrounding optically relevant point defects in an epitaxially grown GaAs/AlGaAs heterostructure. Multiple-pulse line-narrowing reduces dipolar, quadrupolar, and static Zeeman contributions to the linewidth to below 25 Hz, while pulses of circularly polarized light near the band gap supply spin-polarized electron density during appropriate intervals in the nuclear spin trajectory. The resulting asymmetric lineshape is consistent with identification of the defect sites observed as shallow donors, apparently present at concentrations below the limits of electrical detection. In contrast to earlier treatments, we find that the Knight shift at the maximum of the lineshape is not a reliable indicator of the spin-polarized electron density at the Bohr radius, but is sensitive to the underlying linewidth and the polarization dynamics, which together with the wavefunction determine the global fit.
The adduct of bis(diethyldithiocarbamato)zinc(II) with pyridine, Zn(EDtc)2 Py, and its clathrates with pyridine, Zn(EDtc)2 Py Py, and benzene, Zn(EDtc)2 Py C6H6, were prepared and studied by means of ESR (63Cu+2 and 65Cu+2 were used as spin labels), solid state natural abundance 13C and 15N CP/MAS NMR spectroscopy and the single crystal X-ray diffraction analysis [1, 2]. Two rotation isomers of the adduct, Zn(EDtc)2 Py, were unambiguously determined by these three independent methods. It was found that these two isomers have different orientations of the pyridine ring about the bisectorial axis which is divided by two four-membered chelate rings. Adduct molecules adopt a geometry which is intermediate between the square pyramidal, C4v and the trigonal bipyramidal, D3h. It was also found that in the clathrate, Zn(EDtc)2 Py Py, two pyridine molecules have different structural functions: One pyridine molecule is coordinated to the Zn atom of the Zn(EDtc)2 complex while the other molecule has only a hydrogen bond with a sulphur atom of a ligand. Apart from X-ray data this is also clearly seen from Solid State NMR measurements: The two structurally inequavalent pyridine molecules have different 13C and 15N isotropic chemical shifts. Both clathrates Zn(EDtc)2 Py Py and Zn(EDtc)2 Py C6H6 adopt a geometry close to the trigonal-bipyramidal (D3h). All ESR and NMR resonances of Zn(EDtc)2 Py, Zn(EDtc)2 Py Py and Zn(EDtc)2 Py C6H6 were assigned. [1] A. V. Ivanov, M. Kritikos, O. N. Antzutkin, and T. A. Rodina, submitted [2] A. V. Ivanov, M. Kritikos, O. N. Antzutkin, and V. I. Mitrofanova, submitted
Based on high-order error term analysis, we introduce an improved variant of the seven-fold symmetric C7 pulse sequence for efficient recoupling of homonuclear spin-1/2 pair dipolar interactions in magic-angle-spinning solid-state NMR spectroscopy. The robustness of C7 towards isotropic as well as anisotropic chemical shift offsets and rf inhomogeneity is improved dramatically by replacing the original pulse sequence element (2)x (2)-x with the cyclically permuted element (/2)x (2)-x(3/2)x. This permutation transforms the largest error terms (third- and fourth order offset and second-order cross term between offset and rf inhomogeneity) into a form that allows their removal by the z rotations of the seven-fold supercycle. The improved performance of this Permutationally Offset Stabilized variant of C7 (abreviated POST-C7) is analyzed by average Hamiltonian theory to fifth order, numerical simulations, and demonstrated by experiments on doubly-13C-enriched organic solids.
The combination of cross-polarization (CP), magic angle spinning (MAS) and high power proton decoupling has made possible the acquisition of high resolution solid-state NMR spectra of dilute spins such as carbon-13 for powder samples. However, the problem of spectral assignment remains a challenge. A variety of 2D correlation experiments have recently been devised for assignment of the carbon spectrum which involve recoupling of the homonuclear dipolar interaction. An alternative to correlations via the (through-space) dipolar interaction, is to make use of the through-bond scalar coupling interaction, yielding a less ambiguous basis for assignment. We thus describe the application of the INADEQUATE (1) experiment to solid-state NMR of ordinary organic samples. Apart from a proton-carbon cross-polarization period, the pulse sequence is identical to its familiar liquid-state counterpart; double quantum coherences are created by the homonuclear scalar J coupling (of the order of 50 Hz for two bonded carbons) during t1 and give antiphase doublets in the second dimension. Example spectra will be shown for natural abundance compounds as well as 13C enriched samples. The INADEQUATE experiment is shown to be well adapted to the identification of 13C-13C bonds and hence the determination of the carbon backbone of solid-state organic compounds. Additionally we show that the INADEQUATE experiment can be used to provide a quantitative determination of heteronuclear scalar couplings in the solid state. Finally, we present the results of experiments which make use of scalar couplings to provide techniques for spectral editing in powdered solids. (1) A. Bax, R. Freeman and T. A. Frenkiel, J. Am. Chem. Soc. 103, 2102, (1981).
Silver ionic conductor of crystalline Ag7NbS6 and its family show large ionic conductivity of ~1Sm-1 at room temperature. To study mobility of silver ions of these compounds, temperature dependence of relaxation times of T1 of mobile silver, 109Ag and 107Ag, and immobile 93Nb are measured at 9.4 Tesla and compared to the result of the measurement of ionic conductivity. To observe magnetic field dependence of T1(Ag), T1(Ag) at 7.0 Tesla is also measured and compared to T1(Ag) at 9.4 Tesla. Also chemical shifts of Ag NMR are measured for Ag7NbS6 and its family compounds. Among activational energies, the next relation of E(T1(Ag))<E(T1(Nb))~E(conductivity) are given. To explain short T1(Ag) minimum of 150ms, both of mechanisms of relaxation concerned to anisotropic chemical shift and relaxation by scalar coupling are considerable. Since the result of T1(Ag) at fast motional limit does not depend on the strength of magnetic field, this relaxation should be explained by scalar relaxation. These compounds show quite large Ag NMR chemical shifts of 1300~1100ppm, and are compared to other silver ionic compounds.
In Nuclear Quadruploar Resonance (NQR), compounds with long spin-lattice relaxation times can be detected efficiently using the Carr-Purcell-Meiboom-Gill sequence. After the initial excitation pulse, a train of refocussing pulses is applied which leads to a spin-lock. The echoes formed after each refocussing pulse are added. This method makes it feasible to achieve a sufficient signal-to-noise ratio in one scan, hence avoiding time consuming multiple scan experiments. It was found that the CPMG sequence is very sensitive to frequency offsets, and the detectability can suffer severely depending on how far the resonance is away from the on-resonance condition. Our experimental data show that the detected signal amplitude oscillates as a function of frequency offset. The maxima of the signal amplitude are separated by the inverse of tau where tau equals the time between the center of one refocussing pulse and the center of the following refoccussing pulse. A simplistic model that calculates the spin-locked component as a function of frequency offset gives good agreement with the observed data. The line width of the observed resonance is found to be another important factor that determines the ease or difficulty of signal detection using the CPMG sequence. The loss of signal amplitude as a function of frequency offset is less severe for compounds that give rise to an overall broad line width compared to those that exhibit narrow line widths.
Host-guest interactions in the dihalohexane/urea inclusion compounds are governed by a fine balance of forces. Slight temperature changes have a severe influence on the asymmetry of guest motion. We showed previously the difficulty of interpreting X-ray data and 2H NMR data of the very disordered systems around room temperature. Towards lower temperatures the deuterium NMR spectra of selectively deuterated guests show lineshapes characteristic of motion in the intermediate regime for correlation times. These spectra allow us to characterize the guest motion and thereby understand the underlying geometries. Above room temperature X-ray and calorimetric studies show a phase transition to a hexagonal lattice. We use deuterium NMR to follow the development of the guest motion into this new crystal form.
An investigation on the effect of shaping the excitation or conversion pulses in multiple quantum magic- angle spinning (MQMAS) experiments was carried out on polycrystalline samples containing spin-3/2 quadrupolar nuclear spins. Both theoretical analysis and numerical simulations show [1,2] that there exists a class of shaped pulses that are more advantageous than the usual rectangular pulses in exciting multiple-quantum coherences (MQCs) and converting these into single quantum coherences (1QCs) or zero-quantum coherences (0QCs) (in the z-filter cases). The sensitivities are enhanced without loss of resolution or increasing the RF offset dependence. Moreover, properly shaped pulses can moderate the requirements on RF power and sample spinning speeds. Especially important for application purposes, is the finding that lattice site quantification can also be improved by the use of certain shaped pulses. This technique combined with a gradient field MAS (GFMAS) probe [3], incorporating a Doty 5-mm high-speed stator, is under active study. Up to date results will be presented. [1] S. Ding and C. A. McDowell, Chem. Phys. Lett., 270, 81(1997). [2] S. Ding and C. A. McDowell, J. Magn. Reson., submitted. [3] W. E. Maas, F. H. Laukien, and D. G. Cory, J. Am. Chem. Soc. 118, 13085(1996).
NMR Knight shift and spin lattice relaxation measurements have recently provided new insight into the formation of charged spin texture excitations (i.e. Skyrmions) in the context of the quantum Hall effect in GaAs/AlGaAs quantum wells. Although the observable nuclei in these NMR experiments (69Ga, 71Ga and 75As) all posess a nuclear quadrupole moment, no quadrupole splitting is observed due to the cubic symmetry of the lattice. We demonstrate that uniform and controlled strain can be useful to identify optically pumped NMR signal contributions from different layers in the sample and to determine the nuclear spin polarization. A uniform biaxial strain in the plane of the wells can be induced by epoxy bonding of the GaAs/AlGaAs multilayer structure to a Si (100) support. The result is a quadrupole splitting of 55 kHz at 1.5-4.2 K. Differences in the spin lattice relaxation of the central versus satellite transitions of the 71Ga signal are attributed to quenching of spin diffusion at the GaAs/AlGaAs boundary. Spin diffusion into the barrier regions influences the apparent nuclear spin lattice relaxation time in the wells, an effect we have modelled with the relaxation-diffusion equation. These calculations are directly relavent to NMR studies of the QHE.
One- and two-dimensional Solid-State NMR of pyroelectric PbTiO3 materials and cation-exchanged sodalites is being performed in order to characterize these interesting materials. PbTiO3 is a ferroelectric, pyroelectric, and dielectric material, which has possible applications in heat sensor and non-volatile memory (FRAM) devices. These materials are being made as thin films from aqueous solution in the presence of an MOH base, where M is an alkali metal or tetramethyl ammonium. The morphology of the crystallites depends on the radius of the M species. One dimensional 207Pb, 23Na, 133Cs, 87Rb, and 13C NMR reveals that the local ordering of the material is perturbed by the presence of the cation and that the local ordering changes upon heat treatment. Cation-exchanged sodalites are being studied because they probe framework-guest ion interactions which can lead to the determination of accurate framework charges.In addition, they are precursors to producing order-arrays of F-centers, which have unique electronic, magnetic, and optical properties. 23Na, 29Si, 27Al, 87Rb, 205Tl one- and two-dimensional NMR probes the local environments in the sodalites. Two-dimensional 27Al MQMAS indicates the presence of multiple aluminum sites in the sodalites. The number of these sites depends on type of cation in the sodalite. These spectra are providing vital structural information about these systems.
NMR spectroscopy has a unique advantage that nuclear spin Hamiltonians can be easily manipulated and modified to serve the special needs by applying external perturbations such as rf-fields or sample spinning. So far, manipulations of Hamiltonians in the spin space have usually been carried out by applying continuous or pulsed rf-fields without modulations. On the other hand, a very limited number of Hamiltonian manipulations by modulated rf-fields have been reported. In such methods, however, modes of modulation are still limited to simple ones such as switching between a small number of amplitudes [1] , phases [2] or frequencies [3] , or a sinusoidal modulation of amplitude [4] and/or frequency [5,6]. In this study, we discuss a general method to design the rf-field in which the amplitude, phase, and frequency can be arbitrarily dependent on time. As a useful example, we apply this method to restoration of anisotropic interactions under MAS (magic angle spinning). Using the average Hamiltonian approach, a group of pulse sequences is produced to reintroduce 13C-chemical shift anisotropy and/or the 13C-15N dipolar interactions. Their validity is confirmed by simulations and experiments [1] S. Hediger, B. H. Meier, and R. R. Ernst, J. Chem. Phys. 102 (1995) 4000. [2] A. E. Bennett, C. M. Rienstra, M. Auger, K. V. Lakshmi, and R. G. Griffin, J. Chem. Phys. 103 (1995) 6951. [3] T. Fujiwara, T. Anai, N. Kurihara, and K. Nagayama, J. Magn. Reson. A 104 (1993) 103. [4] K. Takegoshi, K. Takeda, and T. Terao, Chem. Phys. Letters 260 (1996) 331. [5] C. S. Yannoni and H. M. Vieth, Phys. Rev. Lett. 37 (1976) 1230. [6] R. Fu, P. Pelupessy, and G. Bodenhaousen, Chem. Phys. Letters 264 (1997) 63.
13C {1H} CPMAS {27Al} TRAPDOR and REDOR NMR experiment are ported with the aim of detecting 13C 2 27 Al proximities and distances in solids. The 13C and 27 Al pulses are applied to the same probe channel, because their resonance frequencies lie extremely close to each other. The study of the heteronuclear dipolar interaction between these two nuclei, which ar of fundamental importance in solid state science, is not possible with standard double resonance approaches. Results are presented for the model compounds aluminum lactate, aluminum acetylacetonate and aluminum acetate. The TRAPDOR results for the different carbon positions are in excellent agreement with the mean Al-C distances, calcualted from crystal structure data. The dependency of the TRAPDOR efficiency on the rotational frequency and 27 Al RF field is studied with the aim of extracting quantitative distance information. The results illustrate the feasibility of this method for the study of systems where the interaction of organic and inorganic fractions is directing the structure (template/zeolite) or controlling the catalytic efficiency (organic reactant/catalytically active sites in zeolites or clays)
The structural analysis in the solid state is often complicated in systems with many equivalent nuclei and/or in the presence of structural disorder. Examples include peptides or membrane proteins for which homo- and heteronuclear filtering methods that lead to spectral simplification are desirable. We investigate coherence transfer techniques that are sensitive to the dipolar and chemical shift parameters of a heteronuclear (e.g. 13C, 15N) spin pair. We show that by appropriate modification of the conventional Hartmann Hahn cross polarization approach (CP) selectivity can be achieved employing either zerio or double quantum transfer. Experimental results will be presented on 13C, 15N labeled model compounds and a membrane protein that demonstrate the efficiency and selectivity of these methods.
UV initiated photooxidations of trichloroethylene, ethanol, and acetone over Degussa P-25 powder and a monolayer TiO2 catalyst dispersed on porous Vycor glass (TiO2/PVG) have been investigated using solid-state NMR methodologies. 13C MAS NMR spectra obtained using an in situ optical MAS probe allow us to identify long-lived reaction intermediates and final products during the photo-reactions. The formation of surface-bound reaction intermediates was observed and identified via 13C CP/MAS experiments. Formation of acetates (CH3COO-, Cl2CHCOO-, Cl3CCOO- depending on the presence of chlorines in the starting reactants) were found to takes place via reaction of mobile intermediates and surface hydroxyl groups. The 13C NMR shifts of carbonyl carbons in these acetates were used to distinguish between surface bound species at titanium and silicon sites of the TiO2/PVG catalyst. Degradation studies of these surface-bound species indicate that the photooxidation of acetate is slow and results in their conversion to the final product CO2, while trichloroacetate remains resistive to further degradation on the TiO2/PVG catalyst. The influence of surface species on the catalytic activity will also be reported.
Many organic solids exhibit conformational polymorphism, compounds that have the same chemical structure, but differ in molecular conformation. The study of conformational polymorphism, therefore, requires an analytical technique that is sensitive to these structural differences. Since conformational differences can result in variations in local electron density, NMR is an ideal probe for this type of behavior, via the chemical shielding tensor. Unfortunately, even in moderately complex organic solids, overlapping spectral features make chemical shift analysis of 1D MAS spectra nearly impossible. 2D solid-state NMR techniques, such as the 2DTOSS pulse sequence (de Lacroix, S. F.; Titman, J. J.; Hagemeyer, A.; Spiess, H. W. J. Mag. Res. 1992, 97, 435-443), can allow the separation of isotropic and anisotropic chemical shift information over two dimensions, making it possible to distinguish individual carbon atoms under moderate magic angle spinning (MAS) conditions. We applied the 2DTOSS pulse sequence to the study of three conformational polymorphs of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbontirile. C-3 of the thiophene ring, alpha to the nitrile carbon, was chosen to probe the differences in chemical environment between polymorphs. Spinning sideband intensities obtained from the 2D spectra were used to determine the three principal values of the chemical shielding tensor. The three polymorphs exhibit large variations in both isotropic and anisotropic chemical shifts for this site. Density functional calculations support the observed trends in chemical shift, which can most likely be attributed to charge transfer between the phenyl and thiophene rings. The results from our study will be presented and the utility of 2D SSNMR for studying conformational polymorphism in crystalline compounds will be assessed.
The enhancement of NMR signals arising from a polarization transfer from electrons to nuclei is performed by irradiating at the sum or difference of the electron and nuclear Larmor frequencies [1]. We investigate how this enhancement is changed using (msec) bursts of high power (40 W) microwaves. Dynamic nuclear polarization (DNP) enhancements with pulsed excitations are observed in polystyrene doped with the stable free radical bisdiphenylene phenyiallyl (BDPA). The DNP effect is used to investigate structure in shell cross-linked knedels. Knedels are 100 A particles synthesized with a diblock copolymer of styrene and acrylic acid. In aqueous solution, the hydrophobic polystyrene core is covered by a cross-linked, web-like hydrophilic poly(acrylic acid) surface [2]. Knedel particles can complex DNA and have potential as transfection vehicles in gene therapy [3]. DNP enhancements from lyophilized knedels with BDPA doped polystyrene cores are compared to those of doped polystryrene itself. [1] Abragam, A. and Proctor, W. G., C. R. Acad. Sci., 1958, 246, 1258 [2] Thurmond, K. B., Kowalewski, T., Wooley, K. L., JACS, 1996, 118, 7239-7241 [3] van der Woude et al., Biochim, Biophys, Acta. 1995, 1240, 34-40