Static 2D-exchange experiments in organic solids provide detailled information on the molecular geometry (bond angles, orientational correlation). These information can be extracted from the exchange signals ('ridges') in the 2D plane by comparison with simulated spectra. Using 2H experiments, very accurate investigations are possible due to the advantage of selective labeling and the favourable relation between quadrupolar splitting and intrinsic line width. However, this kind of experiments are very time-consuming as well as sensitive to experimental artefacts. Because the contained information is not only available from the complete ridges but also from carefully selected slices through the 2D spectrum, there is the possibility to reduce the experiment to 1D without loss of information. For that, a narrow frequency band in the 2H-powder pattern has to be excited, corresponding to a particular orientation of the deuterons. The desired molecular process changes the orientation and is in turn detected by exchange peaks off the excited frequency band. This method works for both dynamical investigations (molecular reorientation, chemical exchange) as well as for structural inverstigations (spin diffusion). We demonstrate this 1D-exchange method for the determination of the 2H-C-2H bond angle in glycine-d2. The narrowband excitation of the 250kHz wide powder pattern was performed by 1H->2H-cross polarization using 'time averaged precession frequency', followed by an additional filter sequence. This results in a just 5kHz wide line. The orientation of the two deuterons in the glycine molecule is correlated by spin diffusion. Since the latter is very unefficient in the deuteron system, we enhanced it mechanically by turning the sample during the mixing time by +-180deg around an axis perpenticular to the static magnetic field. This results in a considerable improvenment of the efficiency of the spin diffusion. We compare the results with simulated 2D-ridge plots calculated on the basis of the known crystal structure and tensor parameters of glycine.
Solid-phase MAS techniques have proven to be very useful in characterizing compounds bound to resin, however, until recently little has been reported on using NMR to characterize compounds attached to Chiron crowns. We have used proton, carbon, and COSY spectra obtained with a Varian Nano-nmr probe to characterize products from a published reaction sequence attached to MA/DMA (methacrylic acid/dimethylacrylamide copolymer) crowns. We have also performed solvent surveys to determine the best solvents for acquiring spectra of materials bound to both MA/DMA and polystyrene crowns.
Compounds of the type E(SiMe3)4 where E=C (1) or E=Si (2) are known to form plastically crystalline phases at ambient temperature. We will consider the molecular dynamic properties of solids 1 and 2 at temperatures below the phase transition temperature to the plastically crystalline state. While 1 and 2, from a chemical point of view, are closely related homologues, the low temperature dynamic properties of these two crystalline compounds differ considerably. Quantitative analysis of variable-temperature 13C and 29Si CP/MAS NMR spectra of 1 and 2 by means of spectral lineshape simulations in the exchange broadened regime, in conjunction with quantitative analysis of two-dimensional 13C and 29Si exchange (2D EXSY) experiments on 1 and 2, provides detailed insight. Both solids 1 and 2 display C3 molecular symmetry in their respective low-temperature phases. For 1, only thermally activated reorientation of the entire molecule is observed but no internal SiMe3 reorientation. For 2, both internal SiMe3 reorientation and reorientation of the entire molecule occur.
Numerically exact spectral lineshape simulations of straightforward MAS NMR spectra of polycrystalline powder samples containing isolated two- or three-spin systems yield unambiguous determination of magnitudes and orientations of all interaction tensors within the spin system. A necessary prerequisite is the evaluation of the spin dynamics under the complete, periodically time-dependent Hamiltonian. Experimental and simulated spectra for isolated three-spin systems ABX 31P2113Cd in polycrystalline solid compounds R3P2CdX2 are presented as representative examples. We will consider standard 31P and 113Cd MAS NMR spectra as well as various 1H-31P-113Cd triple resonance MAS NMR experiments, and we will discuss reliable strategies for complete characterization of three-spin systems from combining MAS NMR techniques with numerically exact spectral lineshape simulations.
Oxygen-17 multiple-quantum magic-angle spinning nuclear magnetic resonance studies were performed, for the first time, in the high field of 17.6 Tesla. A good resolution between two oxygen signals due to SiOSi and SiOAl fragments in the 17O enriched zeolite Na-ZSM-5 could be obtained. From the residual linewidth of the isotropic SiOSi signal, it is concluded that a change of the bridging SiOSi angle by 13.4 gives rise to a change in the isotropic chemical shift of less than 5.7 ppm. Values of the isotropic chemical shift delta and quadrupole parameters Cqcc and eta have been extracted for both species, directly from the 3QMAS spectrum. These parameters were introduced into the simulation of the usual 17O MAS spectrum in order to obtain the relative concentrations of the SiOSi and SiOAl fragments with concentrations=80 and 20 %, delta=40 and 30 ppm, Cqcc=5.3 and 3.5 MHz, eta=0.12 and 0.29, respectively. Relative actual concentrations are different from their corresponding MQMAS intensities which are 75 and 25 %. Indeed, experimental intensities are influenced by the different efficiencies of the multiple-quantum manipulations for the two species. Nevertheless, this high amount of SiOAl fragments is surprising. For an equilibrium isotopic exchange an amount of 9.5 % SiOAl should be determined from the Si/Al ratio. However, 29Si MAS NMR yields an framework Si/Al ratio of 19. The high relative intensity of the SiOAl signal could be explained by a favoured insertion of 17O into oxygen framework positions in the neighbourhood of aluminium atoms. The oxygen isotopic exchange degree was determined by 1H MAS NMR of the dehydrated 17O enriched zeolite NH4-ZSM-5. The sum of all centre and sideband intensities due to 17OH interactions divided by the total intensity of the 17O enriched zeolite gives an amount of 13% bridging 17OH groups with respect to the total number of bridging OH groups. This is only one half of the value that is expected for the equilibrium isotopic exchange with 24.7% 17O enriched water.
An analysis of 17O nuclear quadrupole resonance data in various organic solids containing C-O-H...O=C hydrogen bonds shows that the principal value V33 of the electric field gradient tensor along the principal direction which is nearly perpendicular to either C-O-H or C=O...H plane reflects the strength of the hydrogen bond. A nearly linear correlation between V33 and the two other principal values of the electric field gradient tensor is observed through the whole range of weak and strong hydrogen bonds. A linear correlation between V33 at the C-O-H oxygen site and V33 at the C=O...H oxygen site in the same hydrogen bond is observed as well. These correlations indicate that a change in the length of the O-H...O hydrogen bond causes a redistribution of the pi-electrons around the two oxygen atoms. The distribution of the pi-electrons seems to be a continuous function of the O-H...O distance. A correlation between V33 at either the C-O-H or the C=O...H oxygen site and the length RO...O of the hydrogen bond is also observed. The observed correlations give a possibility of assigning complex 17O nuclear quadrupole resonance spectra. In addition structural data, such as for example RO...O, can be extracted from the 17O nuclear quadrupole interaction.
Lead-207 is an interesting nucleus for both practical and theoretical reasons. We report lead-207 NMR shifts of the ion in various solid materials that demonstrate the effects of coordination. The NMR shift parameters are compared to other parameters of the material and the effects of electronegativity and ligand substitution are discussed. We also discuss the spin-lattice relaxation behavior of lead in lead nitrate and compare this to other facets of the NMR spectroscopy of lead in this particular material.
The data presented concentrate on the parallel application of solid state MAS NMR and X-ray diffraction techniques to the study of phosphate speciation in calcium phosphate glasses and ceramics doped with a variety of structure forming and structure modifying ions (Mg2+, Na+, K+, Al3+, F-, etc.). 23Na, 27Al and 31P one- and two-dimensional spectra are interpreted, and show that in favourable circumstances the more detailed structural information from the ceramics may be used to model phosphate speciation in the glasses. Slow dissolution of the glasses enables the phosphate speciation to be investigated by solution state NMR methods. The presence of both acyclic and cyclic metaphosphate species is established. The solid state MAS NMR and X-ray crystallographic studies of selected model mixed-metal metaphosphates shows a dramatic cross-over between linear and cyclic metaphosphates with change in the metal ion.
In solid state NMR semi-integer spin nuclei (I > 1/2), which represent 70% of chemical elements, are subjected to the quadrupolar interaction. In the case of a strong quadrupolar interaction, only the central transition remains visible on a static powder spectrum. Even if this transition is broadened by the second order quadrupolar interaction, its lineshape exibits singularities which provide important information on the local surroundings of the studied nucleus. In such a case, the echo sequence is a first-class experiment to avoid the electronic dead-time of the probe and hence the resulting distortions. This poster deals with the optimization of the quadrupolar echo sequence and its experimental application on several compounds. The main purpose is to acquire the most efficient echo signal of the central transition with a minimum of distortions. Using an home-made powder spectrum simulation software package (PULSAR), taking into account all interactions and pulse effects during the entire sequence, we have determined the best experimental conditions for each semi-integer spin. The experimental static spectrum, calculated at the top of the echo is then compared to the simulated one obtained with a single perfect pulse. Finally, we show that optimal experimental conditions can be found, leading to a good determination of the characteristic interaction parameters for several compounds :23Na, 87Rb, 63/65Cu , 59Co and 93Nb.
In recent years, increasing gravity of polymorphism issues in the pharmaceutical industry has lead to the identification of solid-state NMR as a useful technique for the identification and characterisation of polymorphs and their mixtures. We report progress in the application of novel techniques to fluorinated, organic, pharmaceutical compounds including piperidinone-substituted benzopyran derivatives. Carbon-13 CP/MAS NMR spectra have been used to characterise polymorphic forms. They show variations in residual dipolar coupling characteristics and crystallographic splitting of resonances (indicating the presence of more than one molecule in the asymmetric unit). These studies involve triple-channel operation with simultaneous decoupling of proton and fluorine, including CP from fluorine as well as from protons. Fluorine-19 direct polarisation and CP/MAS measurements with and without proton decoupling were also carried out. Proton CRAMPS showed variation in H-bonding characteristics between polymorphic forms.
Molecular hydrogen complexes are inorganic complexes containing one or more hydrogen (H2) molecules as ligands. Much NMR work has been carried out to characterize these complexes in solution where it has been shown that the hydrogen molecules are quite mobile and the complexes are often quite fluxional in nature. Very few NMR studies of these types of complexes have been carried out in the solid state. Here we present a solid state 2H NMR study of Os(2H2)Cl2(CO)(P(iPr)3)2. Temperature dependent 2H NMR lineshapes have been used to determine that the coordinated hydrogen molecules are mobile even in the solid state. This motion was modelled as fast 180 degree flips about the axis between the Os atom and the bisector of the D-D vector. T1 measurements as a function of temperature have been used to evaluate the rotational barrier for this motion. With some knowledge of the molecular structure, we have shown that solid state 2H NMR can be used as a qualitative measure of the degree of molecular hydrogen character versus the degree of classical dihydride character.
Quantitative analysis of 27Al MAS spectra of zeolites is often a problem because of the distribution of chemical shifts and electric field gradients which strongly broaden the lines. Powder lineshapes can not be extracted properly from such spectra. MQMAS NMR technique partially solves the problem by increasing the resolution of the different species as a function of the coordination of each aluminium. However, their resonances are still broad and, moreover, their intensities are not directly comparable to the concentration of the various species. Fortunately, the analysis of such MQMAS spectra leads to a reliable information about the nature of the distribution, and quantitative information can be retrieved by using adequate simulation program for computation of the intensities. Details about the method used will be given and some applications to steam-dealuminated zeolite materials will be shown.
Inorganic-organic hybrid catalysts have recently attracted much interest, since these materials combine the advantages of homogeneous and heterogeneous cataylsis. The materials consist of an inorganic carrier matrix obtained by the sol-gel process of silyl functionalized complexes, a reactive center consisting of an organometallic transition metall complex, which is connected via a spacer unit to the matrix. The inorganic-organic hybrid materials are excellent candidates for multinuclear NMR investigation, since a variety of NMR active nuclei (e. g. 1H, 13C, 31P, 29Si) are available. By selecting the appropiate nucleus one is able to get insight into different parts of the complex material: The inert carrier matrix is well characterized with 29Si NMR. 13C solid state NMR yields information about spacer unit and the ligand backbone. 31P NMR characterizes the reactive center, allowing direct comparison with analogous homogeneous catalysts. Catalytic activity is directly dependend on the dynamic behavior of these materials. Dynamic NMR investigations are applied to determine mobility, which is not only observed by determination of the cross polarization parameters (e. g. T1rho; TXH etc.), but also by application of spin diffusion experiments. The application of the 2D WISE (wideline separation) experiment correlates the structure directly with mobility, thus allowing a site dependend investigation of the mobility.
The well known strategy of immobilizing transition metal complexes on organic/inorganic carrier matrices results in interesting polymeric materials which combine the advantages of the homogeneous with those of the heterogeneous catalysis. Optimization of the performance of heterogenized homogeneous catalysts suffers from the lack of detailed information about structural influences of the matrix imposed on the reactive centers due to the amorphous character of the materials. REDOR nmr spectroscopy seems to be a powerful tool to provide insight into the geometry of carrier fixed transition metal complexes since the MAS nature of the experiment does not require any cristallinity at all and the spatially separated reactive centers offer distinct spin systems. Beyond this, the presence of the easy observable nucleus 31P overcomes the unfavourable properties of metal solid state nmr spectroscopy. In this work 31P observed 195Pt dephased REDOR nmr studies of immobilized platinum phosphines complexes and related planar Pt(II)-complexes as model compounds are presented among others in which the performance of the REDOR experiment in the presence of extreme large chemical shift anisotropies is evaluated.
Ultraphosphate glasses show anomalous glass transition temperatures (Tg) effects with increasing concentration of alkali modifiers. These effects have been attributed to changes in the medium range order (MRO) within the glass. Solid state 31P, 7Li and 23Na MAS NMR provide a powerful tool to access these changes in the glass MRO. By combing information from a variety of nuclei, information about structural changes for different regions of the glass were obtained. Two dimensional 31P exchange experiments allowed the connectivity between different phosphate tetrahedra to be determined. These exchange experiments utilizing radio frequency dipolar recoupling (RFDR) techniques that re-introduce 31P-31P dipolar interactions in the presence of rapid spinning. For 7Li and 23Na a combination of MAS and z-filtered MQMAS experiments allowed the separation of chemical shifts, quadrupolar coupling constants (QCC) as a function of modifier concentration for a series of glasses. Static Hahn echo experiments also allowed the7Li and 23Na dipolar second moment (M2) to be evaluated, providing a measure of cation spatial distribution. These results are discussed in terms of proposed models for glass structure. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.
The hydration reactions of Portland cement are not well understood, due in part to the X-ray amorphous nature of the most important product, calcium silicate hydrate(CSH), which has been the material basis for much of human infrastructure in the last 2000 years. In order to gain more direct insight into the nature of water in these systems, we have used deuterium quadrupolar echo and relaxation measurements to study the hydration reactions in several D2O / cement phase and D2O / inorganic phase systems. Our findings indicate that the excess water undergoes a state change from liquid to solid during the hydration reaction that forms CSH. Before and during the hardening, solid Ca(OD)2 is also formed. The non-Ca(OD)2 deuterons in hardened CSH experience fast tetrahedral (or higher symmetry) jumps on the NMR time scale, similar to those observed in D2O ice.1 1. Wittebort, R. J.; Usha, M. G.; Ruben, D. J.; Wemmer, D. E.; Pines, A. J. Am. Chem. Soc., 1988, 110, 5668-5671.
In solid state 1H NMR spectroscopy, resonance lines are severly broadened by strong dipolar interactions. Magic-angle sample spinning (MAS) is conveniently used in order to reveal chemical shift resolution via dipolar decoupling. In most cases, spinning speeds of up to 35 kHz allow sufficient line narrowing. However, dipolar couplings contain detailed information on the topolgy of the coupled spin-system. Our technique, therefore, combines spectral resolution achieved by MAS at 10 to 35 kHz with dipolar recoupling or inefficient decoupling to investigate structure and dynamics of 1H systems. Different 1H sites are identified via their isotropic chemical shift, whereas the dipolar interaction is used to selectively prepare multiple-quantum (MQ) coherences. At appropriate MAS frequencies, residual dipolar couplings lead to spinning sideband patterns combining sufficient line narrowing with information on the structure of the coupled spin-system. In conventional single-quantum MAS spectra, these sidebands are induced by the rotor-modulation of the dipolar interaction acting on the spins during signal acquisition. The observation of MQ coherences requires two-dimensional techniques, in which both spectral dimensions are subjected to rotor modulation resulting in two-dimensional rotational sideband patterns. Moreover, selective partial decoupling or recoupling can be achieved by synchronization of the sample rotation to the pulse sequences used for MQ excitation. In our work presented here we show that distances and angles among protons in small spin-systems can be directly determined from MAS induced spinning sideband patterns of 1H double- (DQ) and triple-quantum (TQ) spectra. MQ coherences of orders two or three involve all protons of typical structural units like CH2 or CH3, respectively. Since interactions among protons inside this units do not contribute to the evolution of the system anymore, such total spin coherences and their MAS sideband patterns prove to be extremely sensitive to dipolar couplings to surrounding nuclei, thereby elucidating the topology of small dipolar coupled networks.
A multidimensional NMR experiment with magic angle sample spinning (MAS) is described for the determination of bond angles between 13C-1H spin pairs. It is based on the correlation of 13C-1H heteronuclear dipolar interaction via 13C spin diffusion. The experiment takes advantage of the high resolution and sensitivity obtained under MAS, whereby the dipolar interaction is recovered under MAS by proper matching of the radio-frequency fields during cross polarization (CP). Dipolar spectra are obtained from the CP transient oscillations, allowing in labelled compounds correlation between different 13C-1H spin pairs. The 4-dimensional experiment demonstrated here resolves dipolar correlation patterns with the corresponding 13C isotropic chemical shifts of the spin pairs, allowing the determination of bond angles among all resolved 13C-1H spin pairs in one experiment.
Filtering the signals of dipolar-coupled spin pairs using a double-quantum filter under MAS has become important in the application of solid-state NMR for example to suppress natural-abundance background signal in selectively labeled materials. Due to the design of the multiple-pulse experiments, the maximum efficiency which can be reached by such a double-quantum filter is 73%. We propose a new homonuclear spin-pair filter which is based on an adiabatic amplitude sweep through the HORROR condition. We refer to this new experiment as Dipolar Recoupling Enhanced by Amplitude Modulation (DREAM). This new scheme is not based on the principles of the double-quantum filtration but on the different behavior of dipolar-coupled and isolated spins under an adiabatic amplitude sweep. In theory, such an experiment can reach an efficiency of 100%. Experimentally we have achieved efficiencies of up to 80% which is higher than the maximum theoretical efficiency in a double-quantum filtered experiment.
We have investigated the spin-diffusion process in high-abundant (1H and 19F) spin systems under fast magic-angle sample spinning (MAS). Spin diffusion can be indirectly observed through the line shape of a passively dipolar-coupled or scalar-coupled spin where the spin-diffusion process manifests itself as magnetization exchange between the multiplett lines. If the spin diffusion is fast enough to generate exchange narrowing between the multiplett lines, one often speaks of "self decoupling". By analyzing the line shape of the passive spins as a function of the spinning speed, we can extract information about the spinning-speed dependence of the spin-diffusion process.
13C NMR results on chemisorbed carbon monoxide (from the electrochemical oxidation of MeOH) on a commercial fuel cell grade graphite-supported platinum electrode will be presented. The effects of the potential at which the chemisorbed CO was produced will be discussed. To date, our results show that the 13C of CO ex MeOH has a frequency shift of ca. 400 ppm (with respect to TMS), much more deshielded than on a polycrystalline platinum electrode. Based on these results, and on the results of density functional calculation, and platinum NMR results of the same commercial fuel cell grade graphite-supported platinum electrode, we suggest a correlation between the local density of states at the Fermi level at clean platinum surfaces under different conditions, and the corresponding 13C Knight shifts. In addition, we have also been able to identify two 13CN- species adsorbed on a polycrystalline platinum electrode. One species resonates at ca. 170 ppm downfield from TMS while the other is at ca. 220 ppm. The spin-lattice relaxation times at room temperature as 0.69 s and 0.54 s respectively. Earlier experiments at lower field found only a single spin-lattice relaxation time, which may be attributed to more effective fast spin diffusion between the two species at low field.
Single crystal surfaces are out of the reach of NMR investigations when conventional techniques are used. A typical 1 cm^2 laboratory single crystal surface has about 10^15 adsorption sites. To be of interest to surface science a fraction of 10% or preferably 1% must be detectable. Here we show experimentally that optically pumped or hyperpolarized 129Xe can be used to reach such a sensitivity with conventional NMR, i.e. magnetic, detection. Using spin exchange optical pumping on a Rb, xenon (nat. composition), nitrogen mixture a 129Xe polarization of up to Pz=0.6 was reached. The latter was measured by comparison to a thermally polarized sample of identical geometry. To study single crytal surfaces the nitrogen must be removed from the gas mixture because of the competition between Xe and N2 for adsorption. In a single freeze-pump-thaw cycle the separation reached experimentally was 1:300 as shown with a mass spectrometer. At the same time a quantitative NMR measurement showed no additional polarization loss. For a sensitivity study laser polarized (Pz~0.2) 129Xe gas was condensed onto a liquid nitrogen cooled glass finger inside an NMR probehead. Various amounts of gas were released from the optical pumping manifold into the connecting plastic tube. Condensation of the gas was not well controlled. We show that with 0.8 10^15 129Xe atoms a signal to noise of 7 was reaced. The relatively large line width of 40 ppm is explained by susceptibility broadening of the glass/liquid nitrogen substrate. To overcome the ambiguity of the freezing location (outside the NMR coil) gas samples were analyzed. Here a low pressure gas (p=0.1 mbar, Pz=0.14) mixture was used with 0.9 10^15 129Xe atoms inside the NMR sensitive region. A noise optimized spectrum showed a S/N = 96 with a line width of 70 Hz in a single shot. From this we deduce that 1% of a monolayer (10^13 atoms) can be seen with a signal to noise of 5 and a line width of 300 Hz. For this the nuclear polarization must be around 0.5 an 30 averages must be taken. In summary, clear submonolayer sensitivity is shown. Experiments combining the ultra high vacuum surface preparation with NMR analysis are in preparation.
In order to test the surrogate probe strategy of substituting 113Cd for less spectroscopically favorable metals (ie. 67Zn) it is necessary to actually acquire data on the native metals. We have therefore begun an investigation into the comparison of the solid state NMR lineshapes of both cadmium and zinc systems. This comparison of the results obtained on the same complexes for each nuclide allows us to draw analogies between the patterns of the resulting quadrupole and shielding tensors. Discussed are the experimental details (i.e. overcoming the sensitivity issues of 67Zn NMR), the lineshape simulations and projections for future work in this area.
Recently we have developed a magnetic field gradient which geometry is compatible with a magic angle spinning stator(1). The field gradient is oriented along the magic angle spinner axis, resulting in a time independent gradient interaction as experienced by the rotating spins. This gradient has proven useful in high resolution MAS spectroscopy, in which liquid-like samples are spun along the magic angle in order to eliminate the line broadening resulting from susceptibility and dipolar interactions. Here we extend the application of magnetic field gradients to NMR spectroscopy of rotating solids. Although some of the main reasons for using gradients in liquid state spectroscopy, such as solvent suppression and time saving through elimination or reduction of phase cycling, do not generally apply to solid state NMR, gradients are useful for coherence selection and editing based on the selection of specific spin dynamics. (1) W.E. Maas, F.H. Laukien, D.G. Cory, J.Am.Chem.Soc. 118, 13085, 1996
Combinatorial chemistry is quickly becoming a very popular technique for organic synthesis in the pharmaceutical and biotechnology fields. In order to efficiently run the chemical assessment of solid-phase combinatorial libraries sophisticated analytical techniques are required. The use of high resolution Magic Angle Spinning technique ( Nano-probe) has been proven to be a valid tool in the identification of chemical transformation on solid phase. In this paper, some examples from our group will be presented and discussed with attention to the choice of solid support, of solvents, general experimental conditions and comparison with results by other techniques.
Cross-polarization between nuclei of spin 3/2 and 5/2 (Li-7, B-11, Na-23, Al-27) under magic-angle spinning at 10.0 kHz has been examined in three samples: sodium diborate (Na2O-2B2O3), aluminum boride (AlB2), and lithium aluminum oxide (LiAlO2). In addition to the difference Hartmann-Hahn match conditions well known for CP-MAS involving at least one spin 1/2, sum match conditions have been observed having the general form {I+1/2}f1I + {S+1/2}f1S = nfR, n=1,2. A simple theoretical explanation can be made for the 180 degree phase difference between signals obtained at sum and difference match conditions. It is probable that sum matches have not been observed previously because CP-MAS experiments have been performed with relatively low spinning speeds and high radiofrequency field strengths compared to those that are favorable for spin locking of half-integer quadrupolar nuclei in the experiments described here. Sodium diboride presents a simple case in which the quadrupole frequencies are small relative to the spin-lock radiofrequencies, and the terms in brackets above are dropped from the match conditions; all six physically reasonable match conditions (four difference, two sum) are observed. For aluminum boride, in which the boron quadrupole frequency appears to be relatively large compared to the spin-lock radiofrequency used, the matches cover a wider frequency range. Lithium aluminum oxide shows a curious pattern of match conditions, with some of the expected conditions as well as others at both lower and higher frequencies.
59Co solid state NMR methods were applied to the study of natural cobalamins and of structurally related derivatives including cobaloximes, phthalocyanines and porphyrins. Standard magic angle spinning (MAS) could be employed when the line broadening contribution from quadrupolar anisotropies was sufficiently small, but for systems with large quadrupolar constants MAS failed to present any significant advantages with respect to static acquisitions. For these cases, a protocol was developed that allowed us to retreive undistorted powder lineshapes even when quadrupole and Zeeman interactions were of comparable magnitudes. Experimental traces were then fitted at several magnetic field strengths with numerical algorithms to retrieve valuable information on the nuclear coupling parameters. The observed trends for the cobalamins could be rationalized in terms of standard ligand field theories and resembled those displayed by the cobaloximes. By contrast, porphyrins and phthalocyanines possessed substantially different ratios between their shielding and quadrupole anisotropies. Temperature- and crystallization-dependencies were observed for the 59Co cobalamin spectra, whose nature was elucidated with the aid of other solid state techniques and whose possible significance will be discussed.
Lithium is a widely used element in inorganic and organic chemistry. It has two NMR active nuclei, of which 7Li is present in much higher natural abundance than 6Li. The chemical shift range for 6,7Li is small (10 to -5 ppm), but the quadrupolar moment, though small compared to other nuclei, is measurable and different for each isotope in the approximate ratio 50 (7Li) to 1 (6Li). These differences between the isotopes allow both interactions to be measured: Lithium-6 has a greater relative influence of shielding anisotropy than 7Li because quadrupolar interactions dominate for the latter. Using a combined fit of the data for both nuclei gives more accurate results and has the potential to yield orientational information for the two tensors.
The tris(o-phenylendioxy)cyclotriphosphazene (TPP) and tris(2,3-naphthalendioxy)cyclotriphosphazene (TNP) form inclusion compounds (ICs) with several molecules and macromolecules and present aromatic channel-type structure with 0.5 and 1 nm diameters. The multiplicity of nuclei makes them interesting for NMR characterization. TPP/n-alkane ICs with different chain lengths were prepared and compard to TPP/PE IC. The linear alkanes are wrapped up in the rigid paddle-wheel channel matrix and the PE carbon-13 chemical shifts are diagnostic of the particular aromatic environment. The TPP IC with perdeuterated-PE has been selectively formed in order to determine the average distance between the host and the guest, following the intermolecular cross-polarization from the host matrix protons to the guest carbons. The dynamic of the polymer motions inside the channel has been explored by carbon-13 relaxation times and by deuterium NMR. 1H T(1rho) relaxation measurements of single PE chain in perdeuterated matrix will be presented. Carbon-13 and phosphorous-31 CP MAS spectra of the guest-free TPP and TNP matrix and of the ICs with benzene and p-xylene were performed for the first time. The multiplicity of the carbon-13 peaks associated with each aromatic carbon atom is compared to the non-equivalent carbon atoms in the crystal cell. A dramatic reduction of carbon-13 CSA of benzene guest molecule is present in TNP IC with respect to TPP/ benzene IC, confirming the larger dimensions of the TNP channels. Phosphorous-31 CP MAS spectra are informative about the symmetry of the phosphazenic ring. A residual dipolar coupling between phosphorous-31 and nitrogen-14 is also observed.
14N is the most abundant isotope of Nitrogen, which is in turn one of the most important constituents of natural and synthetic materials. Owing to its spin -1 character and relatively large quadrupolar moment, this isotope has so far eluded most attempts to the coherent narrowing of its powder lineshapes. An important exception to this rule has been the overtone NMR experiment 1,2 , which avoids first order quadrupolar effects by focussing on -1<->+1 Zeeman transitions. The present poster discusses this experiment as viewed from the stand-point of the density matrix formalism. It is shown that unlike double-quantum excitation at the Larmor frequency, the overtone experiment can be fully described in terms of a 2x2 space (i.e. as a fictitious spin 1/2). The nutation Hamiltonian in this space is unusual, but enables the efficient excitation and storage of coherences in powdered samples under realistic conditions. Upon sample spinning this formalism predicts the appearance of multiple spinning sideband patterns and spectral shifts, yet the removal of second order quadrupolar anisotropies by bidimensional dynamic -angle-spinning strategies involving certain angle combinations appears feasible. 1) M.Bloom and M.A. LeGros, Can. J. Phys. 64, 1522 (1986) 2) R. Tycko and S.J.Opella,J.Chem.Phys 86, 1761 (1987)
Nanoporous silicas are potential candidates for catalyst supports, gas separation membranes, and sensor materials. A detailed characterization of the porous structure is important for the understanding of the gelation and drying processes and, more importantly, the properties of the new material in order to establish processing-structure-property relationships. The porous structure of three silicas is probed with conventional 129Xe NMR spectroscopy at high xenon loadings and with optically polarized xenon at lower loadings. A correlation of the dependence of the chemical shift on temperature and pressure with pore size is sought. Connectivities between the pores are studied by exchange NMR. Previous BET isotherm characterization yielded pore diameters in the range of 2.2 nm to 3.1 nm for these samples. The xenon NMR results indicate that one of the samples has a distinctly different pore structure that the other two, a result which is obscured by conventional isotherm analysis. Whereas the isotherm characterization is derived from the integral xenon uptake, 129Xe NMR can differentiate between physical adsorption on the outer and inner surface of the silica material.
13C MAS and static spectra of potassium carbonate, K2CO3, 13C-enriched potassium trithiocarbonate, K2CS3, potassium monomethylcarbonate, KO2COCH3, and the complete series of monomethyl substituted potassium thiocarbonates, KXYCZCH3, X,Y,Z = O or S, were taken. Earlier solution work showed an approximately linear trend from lower to higher chemical shifts of the central carbon nuclei when the oxygen atoms are replaced by sulfur atoms. This trend was reproduced by the MAS data. A standard CP pulse sequence was used to obtain the isotropic chemical shifts as well as the anisotropic chemical shift tensor values for all carbon atoms in the methyl substituted compounds. A Bloch decay sequence including a flipback pulse was used to obtain the NMR data for potassium trithiocarbonate and potassium carbonate. In the case of potassium dithio-O-methylcarbonate, X-ray crystallography indicates the presence of three distinct anions per unit cell. An MAS spectrum recorded at a carbon frequency of 100 MHz resolved the three different central carbons and two different methyl carbons in a 2:1 ratio. A 2-D MAT experiment was used to obtain the principal values for all of the distinct carbons. In addition to the experimental NMR work, theoretical GIAO calculations of the chemical shielding tensor values were performed allowing for the assignment of the orientations of the shielding tensor axes in the molecular frame and a discussion of the trends observed in the principal values.
Measurement of the principal values of the 13C chemical shift tensors are reported for coronene (C24H12) and corannulene (C20H10) at both room temperature and low temperature (100 K). At room temperature both molecules are spinning about their symmetry axis, but at the lower temperature the motion is stopped. From a series of normal contact time, short contact time, and dipolar dephasing powder patterns the assignment of the principal values of all carbons are made. A comparison of principal values obtained at the two temperatures allows the determination of the magnitude of the angle between the axis of rotation and the 33 component. Quantum chemical calculations of the chemical shift tensor are in good agreement with the experimental results in both the principal values and the orientation of the principal axes systems. The results of these calculations provide the sign of the angle between the rotation axes and the 33 component.
The solution of phosphorus in silicate and aluminosilicate melts is of considerable interest in the material and geological sciences. To elucidate the modes of incorporation of phosphorus in such melts a series of aluminosilicate glasses with variable sodium/aluminum ratio and phosphorus contents in the range of 1 to 6 mol% were investigated using solid-state NMR techniques. 31P MAS and two-dimensional spin diffusion experiments provided constraints about the number of distinguishable phosphate units and showed the absence of extended metaphosphate chains. The short-range dipolar couplings of phosphorus atoms to aluminum and sodium were used to deduce the connectivities of these phosphate units to AlO4 tetrahedra and/or sodium by performing 31P {27Al} and 31P {23Na} TRAPDOR NMR experiments. In this way, different phosphorus environments (Na3PO4 and Na4P2O7 units, PO4 units attached to 1 to 3 Al atoms, but also AlPO4-like species) were unambiguously identified. Based on the abundance of these species as a function of glass composition and the results obtained with TRAPDOR experiments we present a new model for the incorporation of phosphorus within the aluminosilicate network.
Water in silicates is an important problem for both geochemistry and industry, but the water dissolution mehanism for aluminosilicate melts is still not well known. In this study, we used 17O NMR to determine how water affects the structure of the aluminosillicate melt. The spectra for anhydrous and hydrous Na- and K- tetrasilicate glasses and crystalline analcime were used as a basis for understanding anhydrous and hydrous albite glass spectra. All samples were enriched in 17O, and the spectra were collected using static, MAS, 3Q MAS and CP NMR tehniques. The bridging oxygen (BO), non-bridging oxygen (NBO) and SiOH sites are clearly resolved in the 3Q MAS spectra of the hydrous NTS glass. The NBO sites have a wider distribution of chemical shifts compared to the anhydrous NTS glass, which may be due to a random distribution of protons and sodiums around the NBO. The oxygens in water molecules are clearly observed in the MAS spectra for analcime crystal, with a quadrupolar coupling constant of 7.6 MHz and a chemical shift of 18 ppm. The oxygens in water have a much faster spin-lattice relaxation time than the framework oxygens. The oxygen water peak was observed in the hydrous KTS, NTS and albite glass spectra, this is the first time the water oxygen has been seen in a silicate glass. In the short contact-time 1H- 17O cross-polarization experiments a similar triangular peak was seen in the silica gel, NTS glass and albite glass spectra. At long contact times the NBO peak is enhanced relative to the BO peak in the hydrous NTS spectra, and the Al-O-Si peak is enhanced relative to the Si-O-Si peak in the hydrous albite spectra. The Al-O-Si and Si-O-Si peaks were clearly resolved in the anhydrous and hydrous glass 3Q MAS spectra. Despite the resemblances between the anhydrous and hydrous albite glass spectra, the hydrous albite Al-O-Si peak has extra contours at the position of the SiOH peak in the hydrous NTS glass spectrum. It is worth noting that in both the anhydrous and hydrous glasses there are extra intensities (overlapping with the Al-O-Si peak) at the position of the anhydrous NTS NBO peak.
We report the first detailed 195Pt NMR data on commercial fuel cell grade graphite-supported platinum electrocatalysts. Samples were cleaned electrochemically by holding the reduction potential at a value within the double layer region and the NMR data were taken at low temperatures. The characteristic particle-size dependence of the 195Pt NMR spectra, obtained at 80 K from electrocatalysts having average particle sizes of 2.0 nm, 2.5 nm, and 8.8 nm, permitted the low-field signals observed to be assigned to surface platinum atoms, as in the gas phase case. The high-field signals are assigned to the atoms inside the particles. In contrast to oxide-supported or zeolite-encapsulated platinum catalysts, The T1 measurements reveal the existence, at both surface-like and bulk-like spectral positions, of two well-separated components having differing relaxation behavior. The rapidly relaxing component follows the Korringa relationship, while the slower one appears to be less temperature dependent, i.e., its T1T is smaller at lower temperatures than at higher temperatures. The exact mechanism responsible is still unclear, but one possibility is the appearance of a sharp feature in the local density of states around the Fermi energy, due to strong metal-support or/and metal-electrolyte interaction.
Sucrose octapalmitate is an important component of the commercial non-caloric fat substitute marketed as "Olestra" . This study focusses on the interaction of this octaester with the fat soluble vitamins A and E, when materials are adsorbed on to uniform sized silica beads. Sucrose octapalmitate with all eight sidechains deuterated -ie. 248 deuteriums has been prepared. Its solid state 2H NMR spectrum has been recorded at a series of temperatures with and without coadsorbed vitamins. The nature of the moleular motions in the sidechains are compared to those found in perdeutero palmitic acid and potassium palmitate. In addition, an octapalmitate has been prepared with only one sidechain deuterated. Spectra of this material are compared to those for the perdeuterated counterpart. Results of Differential Scanning Calorimetry will be included.
By appropriate combination of the Baker-Campbell-Hausdorff (BCH) expansion and the Magnus expansion, we derive a semi-continuous BCH expansion (scBCH) capable of handling high-order terms in the effective Hamiltonian describing the evolution under a time-dependent Hamiltonian exhibiting piecewise changing functionality. Using the scBCH expansion along with new relations for high-order terms in permuted pulse sequences as well as z-rotational decoupling, we devise a way for systematic design of multiple-pulse sequences eliminating undesired interactions to high order. The applicability of the method is demonstrated by the design of high-order homonuclear dipolar decoupling and recoupling sequences. Among these range the high-order, broadband MSHOT-3 homonuclear decoupling sequence, which through low sensitivity towards offset and rf inhomogeneity produces 1H CRAMPS spectra of high quality. These properties enables accurate determination of anisotropic 1H shielding tensors from MSHOT-3 CRAMPS spectra through standard spinning sideband analysis. The method is demonstrated on a range of organic and inorganic compounds and found to be in excellent agreement with single crystal studies. Furthemore, using the scBCH expansion, a new 48-pulse homonuclear decoupling sequence with unprecedented decoupling capabilities is derived and demonstrated by numerical simulations.In dipolar homonuclear recoupling, the scBCH approach proves highly efficient as a tool in high-order error term analysis. An improved C7 sequence (POST-C7, Permutationally Offset STabilized C7) which eliminates offset and rf-inhomogeneity terms to high order is constructed based on these principles. The methods are demonstrated numerically and experimentally.
We present results obtained on Phosphorus and Lead based glasses by high resolution 207Pb and 31P NMR. Dipolar recoupling and double quantum experiments are used to describe precisely the long range order in the Phosphate network of these glasses. Following the previoulsy established correlation between isotropic/anisotropic chemical shift and binding state of lead in crystalline structure(1), we show that lead is mostly ionic and plays a modifier structural role in phosphate glasses. This is in contrast with silicate systems in which lead appears as a network former. From 1D high speed MAS spectra we describe the evolution of the phosphate network former units versus composition for these glasses and derive the dissociation equilibrium constant. The obtained results confirm the lead NMR approach. To go further in describing the long range order of the phosphate network we use dipolar recoupling and double quantum two dimensional techniques. We show that it is possible to build a common representation of these two types of experiments. These results clearly show that, in the case or lead phosphate, it is possible, even in amorphous systems, to separate Qn units depending on the nature of their second neighbors (Qn, Qn-1, Qn+1). We thus propose an overall scheme of connectivity of the phosphate groups and its evolution with composition. (1) F. Fayon, I. Farnan, C. Bessada, D. Massiot, and J.P. Coutures, J. Am. Chem. Soc., 119, 6837 (1997).
Solid imidazole is known to be a protonic conductor. In the crystal imidazole molecules form inifinite hydrogen-bonded chains along the crystallographic c axis. Conductivity in imidazole is highly anisotropic with the largest value (of the order of 10-13 Scm-1 at room temperature) measured along the chains. Possible mechanisms for conduction include the formation and migration of a pair of charged solitons and a Grotthuss-type process. All such mechanisms involve a subsequent reorientation of the imidazole molecules during which they return to their original orientation in the chain. This is necessary if current is to flow in a continuous fashion and is believed to be the rate limiting step. In this poster we present the results of a solid-state NMR study of molecular reorientation in imidazole using rotor-synchronized magic angle spinning exchange experiments on mono nitrogen-15 labelled samples in an attempt to shed some light on the conduction mechanism.
Half integer quadrupolar nuclei undergoing severe second order broadening suffer difficulties in getting high resolution NMR spectra : under magic angle spinning the numerous overlapping spinning sidebands precludes resolution furthermore it becomes impossible to excite multiple quantum coherences and obtain MQ-MAS spectra. To address these difficulties we explored two possible directions : * As the second order broadening is proportional to the inverse of the principal field a gain in resolution and in sensitivity, proportionally to the square of the field can be obtained (800MHz) and combined with very high spinning rates (35kHz). Under these conditions it becomes possible to obtain resolved spectra in complex multisited crystalline or disordered solids. * The QPASS(1) (Quadrupolar Phase Adjusted Spinning Sidebands) experiment, extension to second order of the PASS experiment used for I=1/2 nuclei. It aims at reconstructing, through a two dimensional experiment, a spinning sideband free "infinite spinning rate spectrum" with an increased resolution, even at conventional fields. We describe the methods and discuss experimental results obtained at very high field(800MHz)/ very high spinning rates (35kHz) and with QPASS at 400MHz with exemples of 71Ga NMR in oxide crystalline and disordered solids. (1) D.Massiot, V. Montouillout, F. Fayon, P. Florian, C. Bessada; Chem.Phys.Letters, 272, 295-300 (1997).
Pulsed field gradients (PFGs) have been applied for the first time to select coherence transfer pathways in multiple-quantum (MQ) MAS NMR spectra of half-integer spin quadrupolar nuclei in rigid solids. For this purpose a gradient MAS probe was constructed with a single set of gradient coils, mounted cylindrically around the spinner axis, to give an effective B0 field that increases linearly along the spinner axis. 27Al triple-quantum (3Q) MAS NMR spectra of the aluminophosphate molecular sieves, hydrated VPI-5 and as-synthesized AlPO4-18, and others have been used to demonstrate the selection of the (0) -> (3) -> (-1) coherence transfer pathway using PFGs with no phase cycling of the r.f. pulses and the receiver. The experiments employed two gradient pulses for selecting the coherence transfer pathway. Either the basic two r.f.-pulse scheme or a Rotation-Induced Adiabatic Coherence Transfer (RIACT) pulse sequence were used for the excitation and conversion of the 3Q coherences. The PFG-MQMAS experiments were found to give almost identical absolute intensities and signal-to-noise ratios as MQMAS spectra that employed phase cycling schemes. The main advantage of the PFG-MQMAS technique is its simplicity. The absence of phase cycling of the r.f. pulse and the receiver has facilitated the development of extended experiments that combine MQMAS with other pulse sequences, particularly heteronuclear spectroscopy. Here are presented the results of heteronuclear correlation spectroscopy. This makes it possible to deduce the 3-D connectivity patterns in complex framework structures such as AlPO4 molecular sieves. Preliminary results on coherence selection using PFGs in homonuclear connectivity experiments will also be presented. All the results presented here share a common isotropic F1 projection arising from the basic PFG-MQMAS foundation.
The typical Faraday detectors used in magnetic resonance are relatively insensitive to low frequencies of precession, making it difficult to do NMR or MRI in low fields. To address this issue, we use a Superconducting QUantum Interference Device, commonly called a SQUID, as a detector. Its advantage is its excellent sensitivity that does not scale with frequency. We present briefly some of our recent experiments, including low-field ( <100 Gauss) NMR, zero-field NQR, and imaging of optically polarized noble gases in static fields below 20 Gauss.
In nanocrystalline solids, high resolution spectroscopic data is difficult to obtain due to the presence of a heterogeneous surface. Nanocrystallite faceting can be observed by TEM, but TEM does not easily offer a way to correlate structural data with chemical information. Solid state nuclear magnetic resonance studies are directed at the resolution and assignment of multiple chemical sites on passivated InP nanocrystal surfaces. Through double and triple resonance experiments, local chemical and structural environments are investigated. In addition, we have studied effects of dipole and pseudo-dipole tensor cancellation in InP by means of single crystal rotation and 115In/113In - 31P cross-polarization experiments.
59Co NMR is useful as a sensitive method for understanding the influence of the immediate environment of the cobalt atom. Recent improvement in NMR pulse methodology and instrumentation make the study of solid state 59Co NMR more accessible, especially in the context of the measurement of chemical shift anisotropy and quadrupolar coupling. When more than one cobalt site/environment is present and more than one interaction contribute to the spectral frequency and intensity, measurements at different magnetic field strengths are required for a satisfactory characterization of the cobalt environments. In this presentation, a run down of the methodology to record distortionless 59Co line shapes and their analysis thereof, using the simulation program QUASAR, will be presented. 59Co NMR experiments conducted at different magnetic field strengths (2.35T, 7.05T, 9.4T and 14.09T) on the inorganic complex K3Co(CN)6 and the low dimensional oxide LiCoO2 and their spectral simulations are illustrative of this approach in the estimation of accurate values of the quadrupole and chemical shielding tensor parameters.
Advancements in the field of crystal engineering requires the identification and understanding of the interactions present between the molecules in the solid state that effect their packing and crystal properties. Recently, there has been renewed interest by organic chemists in utilising the stacking interactions between phenyl perfluorophenyl rings to construct supramolecular crystals. We have used solid state multiple pulse techniques to measure the chemical shift anisotropy of the fluorines present in several simple systems, such as benzene/hexafluorobenzene, napthalene/octafluoronapthalene and biphenyl/decafluorobiphenyl in order to assess the similarites and differences between such adducts and their single species equivalents.
Pure and highly crystalline ETS-10 and ETAS-10 titano-silicate molecular sieves, devoid of the impurity ETS-4 phase, were synthesised and fully studied by MQ-MAS NMR. Triple quantum 23Na MAS experiments point to the presence of three distinct cation environments in ETS-10. Similar experiments in ETAS-10 additionally confirm that sodium environments are in tact in the aluminium substituted material, indicating thereby an isomorphic aluminium substitution in the ETS-10 lattice. The MQ-MAS results are further rationalised in terms of sodium ion locations as determined by cation modeling of the two ordered polymorphs, which are known to exist for the structure of ETS-10.
In zeolites both 1H and 29Si nuclei offer good detection sensitivity. Besides, being spin 1/2 nuclei, their spin manipulations by hard r.f pulses can be readily accomplished in heteronuclear dipolar recoupling experiments. From the experimental point of view also, a classical CP-MAS probehead would suffice for an easy implementation of the desired experiment and the high speed sample spinning (ca. 15 kHz) allows one to measure the REDOR/REAPDOR behavior at short dephasing times. This latter feature allows us to estimate shorter internuclear distances. We have carried out 29Si-1H REDOR and 1H-27Al REAPDOR experiments in the zeolites H-Y, H-mordenite and H-ZSM-12. Highly resolved proton resonances due to the bridging hydroxyl groups of the zeolite framework (3.9 to 5.6 ppm), terminal SiOH groups at crystal surface and crystal defects (1.8 ppm) and residual NH4 ions (7.0-7.5 ppm) allow us to measure the individual REDOR/REAPDOR behavior. In each case, the observed dephasing curves could be calculated by considering either an isolated pairwise interaction (Si(OH)Al) or a sum over N equivalent interactions (for the ammonium ions), using PULSAR.