1H Magic Angle Spinning NMR Research Articles

Mechanism of protonic conduction in defect pyrochlore HNbWO 6·xH 2O using MAS NMR

We have carried out 1H Magic Angle Spinning (MAS) NMR measurements at various spinning speeds (1–12 kHz) on HNbWO 6· xH 2O ( x = 0 and1) defect pyrochlore systems. The variation of the line width with the spinning speed in the two systems points towards the presence of motions with different time scales. We conclude that the mechanism of conduction in both the compounds are similar except that the proton hopping in hydrated form is assisted by the water of hydration.

1H MAS NMR spectroscopy of chemically modified silica gels: a fast method to characterize stationary interphases for chromatography

Chemically modified silica gels used as stationary phases in chromatography have been investigated by means of solid-state 1H magic angle spinning (MAS) NMR spectroscopy. Since the organosilanes are bonded to the surface of the silica gel, their protons are diluted and possess a higher mobility in comparison to protons in pure organic solids. Thereby the usually strong homonuclear dipole-dipole interactions among the protons are reduced and it is possible to obtain well-resolved 1H NMR spectra of the organic interphases with MAS-only techniques. Effects of temperature and magnetic field strength on the resolution of the spectra are examined as well as the dependence of T 1, and T 1 ϱH relaxation times on temperature and spinning speed.

Laser-supported high-temperature MAS NMR for time-resolved in situ studies of reaction steps in heterogeneous catalysis

The temperature of zeolite samples containing various adsorbed molecules was rapidly changed (within 15 s) from room temperature to 600 K by means of a laser beam. The location of the sealed glass ampoule in a boron nitride container decreases the temperature gradient in the sample and avoids laser-induced reactions. The technique facilitates time-dependent magic-angle-spinning (MAS) NMR spectroscopy of high-temperature reactions which take place within 60 s. The H-D exchange in the hydrogen form of zeolites loaded with fully deuterated molecules, the methanol-to-gasoline conversion and the catalytic ethylbenzene disproportionation in zeolites were monitored by 13C and 1H MAS NMR by means of a “stop and go” method.

Use of Spin Echo Magic Angle Spinning 1H NMR in Reaction Monitoring in Combinatorial Organic Synthesis

Use of Spin Echo Magic Angle Spinning ¹H NMR in Reaction Monitoring in Combinatorial Organic Synthesis

X-Ray diffraction and solid-state nuclear magnetic resonance study of a silicate containing two different nitrogenous cations

Single-crystal X-ray diffraction has been used to determine the structure of a silicate hydrate containing both the tetraethylammonium cation and a cation containing three nitrogen atoms. The structure is monoclinic, space group C2/c, and may be regarded as a host–guest system, with the cations as guests in a hydrogen-bonded framework consisting of a cubic octameric silicate anion, [Si8O20]8–, and water molecules. Magic-angle spinning 13C, 29Si and 1H NMR spectra are consistent with the crystallographic asymmetric unit. The precursor solution, the mother-liquor after crystallisation, and the solution obtained by melting the crystals have also been examined by NMR spectroscopy. The implications for silicate solution equilibria and for zeolite synthesis mechanisms are discussed.

Oxidation sites in dealuminated mordenite. A combined EPR, NMR and product distribution study

Dealuminated mordenite zeolites have been prepared by different modification procedures and characterised by a variety of techniques such as 1H, 27Al and 29Si magic-angle spinning (MAS) NMR, nitrogen adsorption and elemental analysis with special emphasis on detecting irreversible changes of the framework and the presence of non-framework species. Radical cations were generated on the dealuminated H-mordenite zeolites following adsorption of alkenes, and were detected by EPR spectroscopy. The organic species present after 30 min reaction on the zeolites were distilled off and analysed using gas chromatography (GC). The generation of radical cations was found to be dependent on the pretreatment of the zeolite. The intensity of the EPR signal, and the reaction products could be correlated with the presence of extra-framework alumina in the zeolite pores. It is postulated that the formation of radical cations occurs on Lewis acid sites associated with the extra-framework alumina formed in the presence of oxygen during the activation process.

H NOESY NMR on Adsorbed Molecules

First 1H NOESY NMR investigations on adsorbed molecules are presented. Experiments have been carried out on allyl alcohol adsorbed on NaX zeolite with a loading of approximately four molecules per supercage. By application of magic angle spinning (MAS) on the sealed samples, all five magnetically inequivalent protons could be resolved spectroscopically. 1H MAS NMR spectra in the temperature range from 260 to 420 K are reported. Cross peak and diagonal peak intensities of 2D NOESY spectra at room temperature for different mixing times have been measured. Subsequently the matrix of cross relaxation rates has been obtained by a single least mean square deviation fit to all 2D peak intensities. Errors in the determination of cross relaxation rates are discussed. Selected cross relaxation rates could be interpreted by isotropic reorientation of the allyl group with a correlation time τc = (6.1 ± 0.4) × 10-9 s at a temperature of 300 K.

Solid state NMR — a powerful tool for the investigation of surface hydroxyl groups in zeolites and their interactions with adsorbed probe molecules

Recent developments in the NMR spectroscopic investigation of surface hydroxyl groups in zeolites are reviewed in the present paper. The composition and porous structure of zeolites are briefly described. The structure and basic properties of surface hydroxyl groups (especially of bridging hydroxyl groups) in zeolites are explained. The influence of the magic angle spinning (MAS) technique on the 1H NMR spectrum of activated zeolites is described in detail. Different NMR spectroscopic methods which allow the measurement of the HAl distance r HAl between the protons and the 27Al nuclei in bridging hydroxyl groups are compared. The limitations of spectral resolution of the 1H MAS NMR spectra are discussed quantitatively on the basis of extended quantum chemical calculations. It is shown that the 1H NMR chemical shift δ H allows the measurement of the deprotonation energy of free surface hydroxyl groups in zeolites. The influence of electrostatic interactions upon δ H is discussed and a correlation between the 1H NMR chemical shift and the wavenumber of the stretching vibration of surface hydroxyl groups is stated. Further, 1H MAS NMR and IR spectroscopy are compared especially with respect to concentration measurements. Applications of low-temperature 1H MAS NMR spectroscopy to the investigation of the interaction between surface hydroxyl groups and adsorbed probe molecules are demonstrated. Finally, the present paper reviews recent NMR spectroscopic studies of probe molecules under the influence of bridging hydroxyl groups.

Multinuclear MAS NMR spectroscopic study of the zeolite, MCM-22

The sodium form of the novel silica-rich zeolite MCM-22 of moderate Si/Al ratio (44) has been synthesized hydrothermally and studied by multinuclear (1H, 27Al and 29Si) magic-angle spinning (MAS) NMR spectroscopy. 27Al spectra indicate that, in the as-synthesized (CN) and calcined materials, all the aluminium atoms are in the tetrahedral framework positions. The silicon resonance at δ=– 105.1 in the calcined material is found to correspond to Q4(1Al) and tetrahedral-sites (T-sites), T2+ T6[Q4(0AI)] of the orthorhombic structure. Silanol, Q4(1Al) and cryst allographically non-equivalent Q4(0Al) lattice sites are identified and information has been gathered on the interaction between the template protons and the lattice T-sites through 1H–29Si cross-polarization dynamics experiments. Our results also indicate that in the as-synthesized material, the 29Si resonances at δ=– 104.3 and – 120.9 correspond to distant T-sites that are located away from the template molecules in the zeolite pores.

ChemInform Abstract: Characterization of WO3/AlPO4 Hydrotreating Catalysts by 1H Magic Angle Spinning NMR Spectroscopy and Low‐Temperature Oxygen Chemisorption.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

New crown ether resins prepared by cationic template polymerization

AbstractThe synthesis of several dibenzo‐18‐crown‐6 derivatives, and their acid catalyzed incorporation into crosslinked styrene and 4‐methoxystyrene based copolymers is described. Some of these reactions were carried out in the presence of chiral ammonium cations acting as non‐covalent templates. The polymers have been characterized by IR, and by 1H and 13C solution state and magic angle spinning NMR.

A theoretical approach for the calculation of the Hartmann-Hahn matching under cross polarization and magic-angle spinning

A method for the calculation of the Hartmann-Hahn matching curve in 1H 13C cross polarization and magic-angle spinning NMR experiments is proposed. The theoretical results are compared to experimental data. The basis of the theoretical approach is a two-step autocorrelation function calculation using Mori's integrodifferential equation as a starting point for the introduction of suitable approximations (Anderson-Weiss-type approximation and short correlation limit) to find a description of the spin dynamics of the coupled 1H and 13C spin system under magic-angle-spinning conditions.

Detection of water in the internal aqueous phase of W/O/W multiple emulsions using magic angle spinning proton NMR

Increased resolution in proton ( 1H) magic angle spinning (MAS) NMR spectra of emulsions is demonstrated. Relative volumes of the internal and external water phases of water-in-oil-in-water (W/O/W) type multiple emulsions were estimated by integration of water resonances separated by use of a shift reagent, while broadening due to magnetic susceptibility mismatch among phases was removed by magic angle sample spinning. Strategies are presented for reducing effects on sample integrity due to spinning and due to addition of reagents. Internal water may also be uniquely detected in the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo spectrum of a sample containing added MnCl 2 as a relaxation reagent.

Magic-angle-spinning NMR studies of acid sites in zeolite H-ZSM-5

1H, 11C, 17Al, and 29Si magic-angle-spinning (MAS) NMR was used to elucidate the nature of the catalytic activity of zeolite H-ZSM-5. 1H MAS NMR of sealed samples after mild hydrothermal dealumination shows that the enhanced activity for n-hexane cracking is not due to an enhanced Brønsted acidity. The concentrations of the various OH groups and aluminous species suggest that the reason for the enhanced catalytic activity is the interaction of the n-hexane molecule with a bridging hydroxyl group and with extra-framework aluminium species. Loading the samples with HCOOH or HCl shows that those extra-framework aluminium species, which give rise to the enhanced activity, cannot be easily removed from their positions, and are therefore immobilized by the zeolitic framework.

The influence of homonuclear dipolar interaction on the residual linewidths of MAS NMR spectra

It is shown theoretically that at not too low spinning rates the residual linewidths of magic-angle-spinning (MAS) NMR spectra of solids with resonant spin I= 1 2 nuclei and dominant homonuclear dipolar interactions are proportional to the square of the static linewidhts and inversely proportional to the rotational frequency. The absolute values depend on the spatial arrangement of the spins and the calculated values are in reasonable agreement with experimental results of recent 1H MAS NMR experiments.

Solid state carbon-13 and proton NMR studies of carbonate-containing calcium phosphates and enamel

Carbonate ions which replace phosphates—type B—and hydroxide ions—type A—in bone and synthetic apatites were studied by 13C and 1H solid state NMR spectroscopy. Heated apatites and dipolar suppression experiments have been employed to assign the spectra lines. Two overlapping peaks around 170 ppm which behaved differently on heating were assigned to type B carbontes. Pure type A carbonate gave a sharp signal at 166.5 ppm, and we were able to identify this signal in type AB apatite and in a presumably pure type B carbonate apatite. Amorphous calcium phosphate carbonates gave a broad gaussian peak of about 3 ppm at full width half-maximum with a center of gravity at about 168 ppm. The same resonance line, but with lower intensity, was also observed for apatites containing type A and B carbonates. Magic angle spinning 1H NMR spectra of these apatites showed resolved OH − and H 2O signals but were inconclusive in identifying specific structural and adsorbed H 2O groups. Implications of the NMR data to the crystal structure of carbonate apatites is discussed.

Quantitative NMR studies of water in silicate glasses

The state of water in volcanic and synthetic silicate glasses is characterized by a number of different solid state NMR techniques. 1H solid echo NMR is used to quantitate total water contents reliably down to levels of 0.1 wt%. High-speed 1H magic-angle spinning NMR can be used to obtain relative species concentrations of OH and H 2O groups in glasses free from paramagnetic impurities. Similar results are obtained from deuterium quadrupolar echo NMR difference spectroscopy, utilizing the different spin-lattice relaxation times of OD and D 2O species. The results demonstrate that the percentage of water present in the form of molecular H 2O species increases with increasing total water content. The H 2O groups are anisotropically constrained, but undergo 180° flips about the bisector axis at rates greater than 10 5 s −1. The hydrous species are not clustered and are subjected to a distribution of hydrogen bonding strengths. A distribution function of O-D·O distances obtained from infrared results, shows good compatibility with the experimental 2H NMR lineshapes. The relative merits of the different experimental approaches are discussed in detail.

Magic-angle-spinning NMR studies of zeolite SAPO-5

SAPO-5 was synthesized using triethylamine as template. Magic-angle-spinning (MAS) NMR of 1H, 27Al, 29Si and 31P was used to study the silicon incorporation into the framework and the nature of the Brønsted sites. 1H MAS NMR shows two types of bridging hydroxyl groups. 29Si MAS NMR indicates that silicon substitutes mostly for phosphorus and that there is a small amount of crystalline SiO 2 in the zeolite powder.

NMR chemical shifts of dilute 1H in inorganic solids

Dilute 1H magic angle spinning NMR has been used to obtain 1H isotropic chemical shifts of solid inorganic species including H 2O, OH −, H 3O +. Ab initio calculations (6-31G and 6-31G ** basis sets) were carried out for comparison.

NMR study of propylene adsorption on γ-Al 2O 3

The conventional 13C and 1H magic angle spinning NMR of propylene on γ-Al 2O 3, has been studied. Based on the chemical shifts and spin-spin coupling constants in the adsorbed state, and MINDO/3 calculations of interactions of propylene with Al +3 ion, a model for the surface propylene complex is proposed. In this complex the bonding of olefinic π-electrons with Al +3 ions and of CH 3-protons with surface oxygen atoms takes place.