MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.     

13C CP MAS NMR and GIAO-CHF calculations of coumarins

13C cross-polarization magic-angle spinning NMR spectra were recorded for a series of solid coumarins. Ab initio calculations of shielding constants were performed with the use of GIAO-CHF method. The combined CPMAS NMR and theoretical approach was successful in characterizing solid-state conformations of coumarins; a relationship sigma (ppm) = -1.032 xdelta + 205.28 (R(2) = 0.9845) can be used to obtain structural information for coumarins, for which solid-state NMR or crystal structure data are not available.     

Investigation of the dipolar dephasing NMR on organic solids

We present a method based on integro-differential equations describing the¹³C?¹H dipolar dephasing behaviour of carbon magnetization which results from monoprotonated carbons, non-protonated carbons as well as rapidly rotating methyl groups. Good agreement with theoretical calculations and experiment is obtained in ammonium tartrate and durene. The frequently applied empirical methods for determination the ratio of protonated and non-protonated carbons are analyzed. The dipolar dephasing time constants of non-protonated carbons vary substantially as a result, of variation in their heteronuclear second moments and thus in structure. Two different methods are performed for determination heteronuclear second moments from dipolar dephasing data.     

Slow-down of 13C spin diffusion in organic solids by fast MAS: a CODEX NMR Study.

One- and two-dimensional 13C exchange nuclear magnetic resonance experiments under magic-angle spinning (MAS) can provide detailed information on slow segmental reorientations and chemical exchange in organic solids, including polymers and proteins. However, observations of dynamics on the time scale of seconds or longer are hampered by the competing process of dipolar 13C spin exchange (spin diffusion). In this Communication, we show that fast MAS can significantly slow down the dipolar spin exchange effect for unprotonated carbon sites. The exchange is measured quantitatively using the centerband-only detection of exchange technique, which enables the detection of exchange at any spinning speed, even in the absence of changes of isotropic chemical shifts. For chemically equivalent unprotonated 13C sites, the dipolar spin exchange rate is found to decrease slightly less than proportionally with the sample-rotation frequency, between 8 and 28 kHz. In the same range, the dipolar spin exchange rate for a glassy polymer with an inhomogeneously broadened MAS line decreases by a factor of 10. For methylene groups, no or only a minor slow-down of the exchange rate is found.     

Relationships between flow and NMR relaxation of fluids in porous solids.

Measurements are presented which correlate the displacements, X(Delta), determined by PGSE NMR, with the multi-mode transverse 1H NMR relaxation of water flowing through a glass bead pack, for which the dominant relaxation mechanism is diffusion through inhomogeneous internal magnetic fields. Analytical solution for the joint amplitude A[X(Delta), T(2k)] for the case of laminar flow in a circular pipe, with a diffusion-to-surface mechanism, shows that, for other than the lowest mode (k = 0), the contributions to the observed relaxation for a given X(Delta) may involve negative as well as positive amplitudes. The experimental measurements are shown to agree with this general conclusion, showing clear evidence of the presence of relaxation modes with negative amplitude at larger values of X(Delta). It is shown that in these, or similar measurements, which provide a spatially-resolved view of surface-mediated relaxation, allowance must be made for fitting with both positive and negative amplitudes.     

Temperature calibration for high-temperature MAS NMR to 913 K: 63Cu MAS NMR of CuBr and CuI, and 23Na MAS NMR of NaNbO3

The solid-state phase transitions of CuBr, CuI and NaNbO₃ can be readily observed using ⁶³Cu and ²³Na high-temperature magic-angle spinning nuclear magnetic resonance spectroscopy. Temperature has large, linear effects on the peak maximum of ⁶³Cu in each solid phase of CuBr and CuI, and there is large jump in shift across each phase transition. The ²³Na MAS NMR peak intensities and the line widths in NaNbO₃ also clearly show its high-temperature transition to the cubic phase. These data can be used to calibrate high-temperature MAS NMR probes up to 913 K, which is two hundred degrees higher than the commonly-used temperature calibration based on the chemical shift of ²⁰⁷Pb in Pb(NO₃)₂.     

Experimental method for low-temperature reaction studies in sealed samples by 13C CP/MAS NMR

A new, simple, and inexpensive technique is presented for monitoring high-resolution solid-state NMR of ¹³C at temperatures ranging between 85 and 450 K. In this procedure, the reaction conditions are controlled by preparing samples at 77 K in 5 mm NMR tubes, while attached to a vacuum system. The NMR tubes are prefitted with a rotor for spinning. After preparation, the samples are sealed, transferred to the double-resonance MAS NMR probe, and analyzed, all while the sample temperature is maintained as low as 85 K. The spinning rates vary from 3.0 kHz at 85 K to 5.2 kHz at 300 K using nitrogen drive gas. Probe design and performance, sample-preparation procedure, and details of the low-temperature experiment are described. In general, the technique may be applied in studies of low-temperature reaction mechanisms and kinetics. ¹³C CP/MAS spectra of ethylene adsorbed on silica-supported ruthenium catalyst are presented to illustrate its performance and possible application.     

Indirectly detected through-bond chemical shift correlation NMR spectroscopy in solids under fast MAS: studies of organic-inorganic hybrid materials

Indirectly detected, through-bond NMR correlation spectra between (13)C and (1)H nuclei are reported for the first time in solid state. The capabilities of the new method are demonstrated using naturally abundant organic-inorganic mesoporous hybrid materials. The time performance is significantly better, almost by a factor of 10, than in the corresponding (13)C detected experiment. The proposed scheme represents a new analytical tool for studying other solid-state systems and the basis for the development of more advanced 2D and 3D correlation methods.     

The molecular basis for the behaviour of niobia species in oxidation reaction probed by theoretical calculations and experimental techniques

This paper describes the preparation and use of a new class of materials based on synthetic niobia as catalysts in the oxidation of organic compounds in aqueous medium. The chemical reactions were carried out in the presence of hydrogen peroxide (H₂O₂). The material was characterized with X-ray diffraction, XPS and H₂-TPR (temperature-programmed reduction) measurements. The organic molecule methylene-blue was used in the decomposition study as a probe contaminant. The analysis using the ESI-MS technique showed complete oxidation observed through different intermediates. This suggests the use of niobia species as an efficient Fenton-like catalyst in degradation reactions. Theoretical quantum DFT calculations were carried out in order to understand the degradation mechanism.     

Experimental and simulated propene isotherms on porous solids

The lack of treatment capacity of hydrocarbons by three-way catalysts during the “cold start” period creates an important environmental problem. During this period, the temperature of the three-way catalyst is too low for effective operation and cannot convert the hydrocarbons in the exhaust. 50-80% of the total hydrocarbon emissions are produced in this phase that accomplishes the first 60-120 s of the engine operation. In this study, the technology chosen to treat these emissions is the use of HC-traps, and molecular simulations are tested as a tool to reproduce the experimental adsorption behaviour of porous solids. Therefore, experimental and simulated adsorption isotherms of propene (model hydrocarbon) have been obtained for four different crystalline materials with distinctive framework structures (3D and 1D) and a variety of Si/Al ratios and cations (three zeolites: ZSM-5, BETA and Mordenite; and a silicoaluminophosphate molecular sieve: SAPO-5).     

Long-term stability of rotor-controlled MAS frequencies to 0.1 Hz proved by 14N MAS NMR experiments and simulations

Experimental and simulated 14N MAS NMR spectra of the NH4+ ions in the two polymorphs, mS60 and mP60, of (NH4)2MoO4 are used to illustrate that a long-term stability of rotor-controlled MAS frequencies to 0.1 Hz can be achieved using commercial instrumentation (MAS speed controller and 7.5 mm MAS probe with a single marked rotor) attached to a highly pressure-stabilized air supply. A new modification of the STARS simulation software employs a Gaussian distribution for the experimental spinning frequency around the frequency set for the MAS speed controller. A simulated spectrum is then obtained by summation of several calculated spectra for evenly spaced spinning frequencies around the set frequency with relative weight factors corresponding to the Gaussian distribution.     

1H and 2H NMR studies of benzene confined in porous solids: melting point depression and pore size distribution

The pore size distributions of four controlled pore glasses and three silica gels with nominal diameters in the range 4-24 nm were determined by measuring the 1H and 2H NMR signals from the non-frozen fraction of confined benzene and perdeuterated benzene as a function of temperature, in steps of ca. 0.1-1 K. The liquid and solid components of the adsorbate were distinguished, on the basis of the spin-spin relaxation time T2, by employing a spin-echo sequence. The experimental intensity curves of the liquid component are well represented by a sum of two error functions. The mean melting point depression of benzene and perdeuterated benzene confined in the four controlled pore glasses, with pore radius R, follows the simplified Gibbs-Thompson equation DeltaT=kp/R with a kp value of 44 K nm. As expected, the kp value mainly determines the position of the pore size distribution curve, i.e., the mean pore radius, while the transition width determines the shape of the pore size distribution curve. The excellent agreement between the results from the 1H and 2H measurements shows that the effect of the background absorption from protons in physisorbed water and silanol groups is negligible under the experimental conditions used. The overall pore size distributions determined by NMR are in reasonable agreement with the results specified by the manufacturer, or measured by us using the N2 sorption technique. The NMR method, which is complementary to the conventional gas sorption method, is particularly appropriate for studying pore sizes in the mesoporous range.     

The NMR microimaging studies of the interplay of mass transport and chemical reaction in porous media

PFG NMR is employed to perform a comparative study of the filtration of water and propane through model porous media. It is shown that the dispersion coefficients for water are dominated by the holdup effects even in a bed of nonporous glass beads. It is demonstrated that correlation experiments such as VEXSY are applicable to gas flow despite the large diffusivity values of gases. The PFG NMR technique is applied to study the gravity driven flow of liquid-containing fine solid particles through a porous bed. The NMR imaging technique is employed to visualize the propagation of autocatalytic waves for the Belousov-Zhabotinsky reaction which is carried out in a model porous medium. It is demonstrated that the wave propagation velocity decreases as the wave crosses the boundary between the bulk liquid and the flooded bead pack. The images detected during the catalytic hydrogenation of alpha-methylstyrene on a single catalyst pellet at elevated temperatures have revealed that the reaction and the accompanying phase transition alter the distribution of the liquid phase within the pellet.     

Heteronuclear X-Y double quantum MAS NMR in crystalline inorganic solids. Applications for indirect detection and spectral editing of rare-spin resonances.

Heteronuclear double quantum MAS NMR experiments in the solid state, involving pairs of highly abundant 31P spins and the rare spins 113Cd, 77Se, and 29Si, are described for the three crystalline compounds CdSiP2, CdGeP2, and Ag7PSe6. These experiments offer the opportunity of indirectly detecting the rare-spin resonance via chemical shift evolution of heteronuclear double quantum coherence. For suitable cases, this method results in significantly enhanced detection sensitivities. Criteria for favorable indirect detection experiments are established. Besides offering high sensitivity, the pulse sequence also acts as a heteronuclear double quantum filter, hence providing heteronuclear correlation and useful spectral editing for peak assignments.     

Monitoring mass transport in heterogeneously catalyzed reactions by field-gradient NMR for assessing reaction efficiency in a single pellet

An important aspect in assessing the performance of a catalytically active reactor is the accessibility of the reactive sites inside the individual pellets, and the mass transfer of reactants and products to and from these sites. Optimal design often requires a suitable combination of micro- and macropores in order to facilitate mass transport inside the pellet. In an exothermic reaction, fluid exchange between the pellet and the surrounding medium is enhanced by convection, and often by the occurrence of gas bubbles. Determining mass flow in the vicinity of a pellet thus represents a parameter for quantifying the reaction efficiency and its dependence on time or external reaction conditions. Field gradient Nuclear Magnetic Resonance (NMR) methods are suggested as a tool for providing parameters sensitive to this mass flow in a contact-free and non-invasive way. For the example of bubble-forming hydrogen peroxide decomposition in an alumina pellet, the dependence of the mean-squared displacement of fluid molecules on spatial direction, observation time and reaction time is presented, and multi-pulse techniques are employed in order to separate molecular displacements from coherent and incoherent motion on the timescale of the experiment. The reaction progress is followed until the complete decomposition of H2O2.     

Identification of fluorine sites at the surface of fluorinated gamma-alumina by two-dimensional MAS NMR

By means of 27Al triple quantum Magic-Angle Spinning Nuclear Magnetic Resonance (3QMAS NMR) and 27Al[19F] WISE MAS NMR, we were able to detect three different Al-F sites on the surface of fluorinated gamma-alumina. Three 19F resonances at 9, 20, and 33 ppm (from C6F6) correlated to 27Al resonances in the octahedral range. While the positions of the maxima in the 27Al dimension were ill-defined due to the inherently low efficiency of the 27Al[19F] CPMAS process, the center of gravity of the lines shifted significantly upfield in that dimension with increasing wt.% F. Tentatively, these three resonances were assigned to (VI)Al(O(6-n)Fn) (n = 1, 2, 3) environments on the F/gamma-Al2O3 surface. At F contents above levels corresponding to the full fluorination of the gamma-Al2O3 surface, neoformation of an AlF3 x 3H2O phase was also evidenced with an 19F resonance at -8 ppm and with an 27Al resonance at -17 ppm.     

Double-quantum excitation in the NMR of spinning solids by pulse-assisted rotational resonance

We describe a new technique for double-quantum excitation in magic-angle-spinning NMR of powdered solids. The technique is designed to efficiently excite double-quantum coherence in the vicinity of a rotational resonance condition. The offset from rotational resonance allows the double-quantum filtered signals to be observed with high resolution and sensitivity. The method uses rotational excitation of zero-quantum coherence, assisted by radiofrequency pulse cycles. The zero-quantum coherence is converted into double-quantum coherence by a frequency-selective inversion sequence. Experiments on [(13)C(2), (15)N]-glycine demonstrate a double-quantum filtering efficiency of approximately 41% at a sample rotation frequency of 8.300 kHz, which is 1.600 kHz away from the n = 1 rotational resonance. We achieve 32% double-quantum filtering efficiency at a spinning frequency of 9.250 kHz, which is 2.550 kHz away from rotational resonance.     

Zirconia‐Alumina Nanoparticles Prepared by Laser Evaporation: Powder Characterisation by TEM and 27Al MAS NMR

Contrary to predictions of the ZrO₂‐Al₂O₃ phase diagram powders of solid solutions of alumina in zirconia with up to 40 mol% Al₂O₃ are described in the recent literature. Up to now the interpretations of the microstructure of these materials are still inconsistent. Therefore we reinvestigated nanopowders prepared by laser evaporation starting from a mixture of 55 mol% ZrO₂ and 45 mol% Al₂O₃ by analytical high resolution transmission electron microscopy (TEM) and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The results reveal that in case of this preparation route a heterogeneous powder system is formed consisting dominantly of nanocrystallites of zirconia with an incorporation of, obviously, only a very small amount of alumina, and that the crystallites are surrounded by an amorphous layer of predominantly alumina. No indication was found that Al³⁺ ions occupy seven‐ or eight‐fold co‐ordinated sites of Zr⁴⁺ ions. A simple kinetic model for the formation of the nanoparticles' microstructure is given.     

Probing Membrane Surfaces and the Location of Membrane-Embedded Peptides by C MAS NMR Using Lanthanide Ions

A simple but efficient ¹³C MAS NMR method is presented for the determination of the location of embedded molecules such as peptides relative to biological membrane surfaces by exploiting the interaction with paramagnetic lanthanide ions. Using various aqueous Dy³⁺ concentrations a distance-dependent differential paramagnetic quenching of NMR lipid resonance intensities for specific carbon sites was observed, with residues at the bilayer surface quenched effectively and hydrophobic sites unaffected by Dy³⁺. Tested on the membrane-embedded 50 residue long M13 coat protein, ¹³C labeled at its Val-29 and Val-31 residues, no paramagnetic quenching was observed for the peptide resonances by Dy³⁺, suggesting that Val-29 and Val-31 are not in close proximity to the bilayer interface, but buried deeply inside the hydrophobic region of the lipid bilayer.