Q-shear transformation for MQMAS and STMAS NMR spectra

The multiple-quantum magic-angle spinning (MQMAS) and satellite-transition magic-angle spinning (STMAS) experiments refocus second-order quadrupolar broadening of half-integer quadrupolar spins in the form of two-dimensional experiments. Isotropic shearing is usually applied along the indirect dimension of the 2D spectra such that an isotropic projection free of anisotropic quadrupolar broadening can be obtained. An alternative shear transformation by a factor equal to the coherence level (quantum number) selected during the evolution period is proposed. Such a transformation eliminates chemical shift along the indirect dimension leaving only the second-order quadrupolar-induced shift and anisotropic broadening, and is expected to be particularly useful for disordered systems. This transformation, dubbed Q-shearing, can help avoid aliasing problems due to large chemical shift ranges and spinning sidebands. It can also be used as an intermediate step to the isotropic representation for expanding the spectral window of rotor-synchronized experiments.     

Implementing SPAM into STMAS: a net sensitivity improvement in high-resolution NMR of quadrupolar nuclei

Gan and Kwak recently introduced two new tools for high-resolution 2D NMR methods applied to quadrupolar nuclei: double-quantum filtering in STMAS (DQF-STMAS) and the soft-pulse added mixing (SPAM) idea. Double-quantum filtering suppresses all undesired signals in the STMAS method with limited loss in sensitivity. With SPAM, all pathways are added constructively after the second hard-pulse instead of using a single pathway as previously. Here, the sensitivity, advantages and drawbacks of DQF-STMAS are compared to 3QMAS. Additionally, SPAM can be included into DQF-STMAS method, resulting in a net sensitivity gain with respect to 3QMAS of ca. 10-15.     

Al Chemical Shielding Anisotropy

Accurate values for the²⁷Al chemical shielding anisotropy (CSA) are reported for sapphire (α-Al₂O₃). The values (δ_σ= −17.3 ± 0.6 ppm, η_σ= 0.03 ± 0.06) are obtained from single-crystal²⁷Al NMR and appear to be the first convincing determination of an²⁷Al CSA.     

Structural characterisation of aluminium layered double hydroxides by Al solid-state NMR

²⁷Al solid-state NMR has been applied to study the local structure of pristine and chemically modified aluminium layered double hydroxides (LDH). The pristine LDH only shows six-fold coordinated, octahedral, aluminium, while the calcined and subsequently surfactant treated LDH sample shows a significant fraction of four-fold coordinated tetrahedral aluminium. The co-existence of two types of octahedral sites with different quadrupolar parameters is clearly observed in both samples. Quadrupolar coupling constants and isotropic chemical shifts have been measured from the ²⁷Al triple-quantum MAS NMR allowing to fit the ²⁷Al MAS spectra and quantify the different species in the samples. The quantitative analysis reveals that 30% of the aluminium is in four-fold coordination in the surfactant-modified LDH. We show that this chemical modification retains the two types of AlO₆ sites with a decreased intensity of the site showing the lowest quadrupolar coupling constant.     

Al and Cu NMR study in UCu5Al

We have studied the microscopic properties of the tetragonal UCu₅Al Kondo compound by ²⁷Al and ^63,65Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed χ>χ. Plots of K(T) against χ(T) at temperatures T≥100 K yield the transferred hyperfine fields of +5.9 kOe/μ_B for ²⁷Al nuclei, and +5.3 and −7.0 kOe/μ_B for ⁶⁵Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T<100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.     

A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBu^s)₂EAA or Al(OBu^s)₃/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the ²⁷Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with ²⁷Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the ²⁷A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (C_qcc = e²qQ/h). Hydration of the sample results in a decrease of the average C_qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C_qcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.     

Deep inelastic collisions of32S +27Al reaction at 130 MeV bombarding energy

The³²S +²⁷Al reaction was studied to investigate the deep inelastic collisions at a bombarding energy of 130 MeV which is well above the Coulomb barrier. The energy distributions of the binary decay products of 6⩽Z⩽10 were determined using a large area position sensitive ionization chamber. The average kinetic energies of the reaction products indicate that the exit shapes correspond to highly stretched scission configurations in the deep-inelastic processes.     

High-field 19.6T 27Al solid-state MAS NMR of in vitro aluminated brain tissue

The combination of (27)Al high-field solid-state NMR (19.6T) with rapid spinning speeds (17.8 kHz) is used to acquire (27)Al NMR spectra of total RNA human brain temporal lobe tissues exposed to 0.10 mM Al(3+) (as AlCl(3)) and of human retinal pigment epithelial cells (ARPE-19), grown in 0.10 mM AlCl(3). The spectra of these model systems show multiple Al(3+) binding sites, good signal/noise ratios and apparent chemical shift dispersions. A single broad peak (-3 to 11 ppm) is seen for the aluminated ARPE-19 cells, consistent with reported solution-state NMR chemical shifts of Al-transferrin. The aluminated brain tissue has a considerably different (27)Al MAS NMR spectrum. In addition to the transferrin-type resonance, additional peaks are seen. Tentative assignments include: -9 to -3 ppm, octahedral AlO(6) (phosphate and water); 9 ppm, condensed AlO(6) units (Al-O-Al bridges); 24 ppm, tetrahedral AlO(3)N and/or octahedral Al-carbonate; and 35 ppm, more N-substituted aluminum and /or tetrahedral AlO(4). Thus, brain tissue is susceptible to a broad range of coordination by aluminum. Furthermore, the moderate (27)Al C(Q) values (all less than 10 MHz) suggest future NMR studies may be performed at 9.4T and a spin rate of 20 kHz.     

Al and Si MAS NMR investigations of cordierite glass, μ- and α-cordierite

²⁷Al and ²⁹Si Magic-Angle Spinning NMR results are reported for conventionally prepared glass of cordierite stoichiometry (2MgO · 2Al₂O₃ · 5SiO₂), the metastable high-quartz solid solution (μ-cordierite) and the high-temperature polymorph of cordierite (α-cordierite). Both, ²⁷Al two-dimensional (2D) quadrupole nutation experiments and ²⁷Al satellite transition spectroscopy (SATRAS) have been applied to identify two different tetrahedrally-coordinated aluminium sites (AlO₄). SATRAS has been used to extract the quadrupole interaction parameters and their distribution, the isotropic chemical shifts and the relative populations of the different Al sites. Both, the ²⁷Al and ²⁹Si NMR results, lead to the conclusion that a perfect Si/Al disorder does not exist in these investigated cordierite samples.     

Mössbauer and SEM study of Fe–Al film

Fe–Al alloy with Fe/Al ratio of 3:1 was first prepared by argon arc melting. It was subsequently coated on glass slide and cellophane tape using an electron beam gun system to have a thickness of 2,000 Å. X-ray diffraction spectrum of the coated sample indicates a definite texture for the film with a preferential growth along the Fe(110) plane. SEM micrographs of the film showed the presence of nano islands of nearly 3?×?10¹²/m² surface density. Composition of different parts of the film was determined using EDAX. Room temperature Fe-57 Mössbauer spectrum of coated sample showed the presence a quadrupole doublet with a splitting of 0.46 mm/s, which is typical of Al-rich iron compounds. MOKE study shows an in-plane magnetic moment.     

Soft-Pulsed Aluminum-27 Quadrupolar Central Transition NMR Studies of Ovotransferrin

We have employed soft (Gaussian) pulses to examine²⁷Al NMR signals arising from Al³⁺bound to ovotransferrin. In addition to enhancing the general detectability of²⁷Al signals from a practical standpoint, this technique makes it possible to invert the central component of the protein-bound²⁷Al signals, providing a route for, to our knowledge, the first inversion-recovery measurements on such systems. The suitability of this approach for gaining insights into the nature of metal ion binding sites in large metalloproteins is critically assessed.     

B(1)(+)/actual flip angle and reception sensitivity mapping methods: simulation and comparison

Knowledge of the spatial distribution of transmission field B₁⁺ and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B₁⁺ and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo.Several B₁⁺ mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B₁⁺ mapping techniques.This study introduces a quantitative comparison between two known methods for B₁⁺/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.     

Structural characterisation of the silica and alumina Zener pinning phases in nanocrystalline CeO2 by 29Si and 27Al nuclear magnetic resonance

Nanocrystalline CeO₂ samples have been manufactured using sol-gel techniques, containing either 15 % silica or 10 % alumina by weight to restrict growth of the ceria nanocrystals during annealing by Zener pinning. ²⁹Si and ²⁷Al MAS NMR have been used to investigate the structure of these pinning phases over a range of annealing temperatures up to 1000 °C, and their effect on the CeO₂ morphology has been studied using electron microscopy. The silica pinning phase resulted in CeO₂ nanocrystals of average diameter 19 nm after annealing at 1000 °C, whereas the alumina pinned nanocrystals grew to 88 nm at the same temperature. The silica pinning phase was found to contain a significant amount of inherent disorder indicated by the presence of lower n Qⁿ species even after annealing at 1000 °C. The alumina phase was less successful at restricting the growth of the ceria nanocrystals, and tended to separate into larger agglomerations of amorphous alumina, which crystallised to a transition alumina phase at higher temperatures.     

14N NMR relaxation times of several protein amino acids in aqueous solution--comparison with 17O NMR data and estimation of the relative hydration numbers in the cationic and zwitterionic forms

The 14N nuclear magnetic resonance (NMR) linewidths of the alpha-amino groups of several protein amino acids were measured in aqueous solution, with and without composite proton decoupling, to estimate the effect of proton exchange and molecular weight on the linewidths. It is shown that, contrary to earlier claims, the increase in the linewidth at low pH is not exclusively due to the effect of proton exchange broadening. The 14N linewidths, under composite proton decoupling, increase with the bulk of the amino acid, and increase at low pH. Statistical treatment of the experimental 14N and literature 17O NMR data was performed assuming two models: (i) an isotropic molecular reorientation of a rigid sphere in a medium of viscosity eta, (ii) a stochastic diffusion of the amino and carboxyl groups comprising contributions from internal (tauint) and overall (taumol) motions. Assuming a single correlation time from overall molecular reorientation (taumol), then, a linear correlation was found between the linewidths and the molecular weights of the protein amino acids at the pH values 0.5 and 6.0, which are characteristic of the cationic and zwitterionic forms, respectively. The slopes of the straight-lines were found to be dependent of pH for 14N, contrary to the 17O linear correlations whose slopes were found to be independent of pH. Assuming effective correlation times of the amino and carboxyl groups, which comprise contributions from the internal (tauint) and overall (taumol) motions, then, a significant improvement of the statistics of the regression analysis was observed. The 14N relaxation data, in conjunction with 17O NMR linewidths, can be interpreted by assuming that the 14N quadrupole coupling constants (NQCCs) are influenced by the protonation state of the carboxyl group, the 17O NQCCs remain constant, and the cationic form of the amino acids is hydrated by an excess of 1-3 molecules of water relative to the zwitterionic state.     

26 Al beam production by a solid state laser ion source at TRIUMF

Many experiments carried out at radioactive beam facilities require the production of intense, isotopically clean and isobar free beams of a particular isotope. At TRIUMF the addition of a resonant ionization laser ion source (TRILIS) enables a multitude of new beams and therefore new experiments to be carried out. ²⁶Al was one of the first radioactive ion beams delivered to an experiment using TRILIS. This paper outlines the development of the ²⁶Al ion beam for nuclear astrophysics.      

Structural changes of piezoelectric La3Ga5SiO14 induced by paramagnetic ions revealed by Ga multiple quantum magic angle spinning

Experimental evidence that nuclear magnetic resonance (NMR) can detect structural changes of piezoelectric La₃Ga₅SiO₁₄ induced by dilute paramagnetic ions is presented. Gd³⁺ and Eu³⁺ cations have been incorporated into La₃Ga₅SiO₁₄ monocrystals. As expected, the line-width of the tetrahedral ²⁹Si magic angle spinning (MAS) NMR spectra as well as the inverse of the T₂ relaxation time of ⁷¹Ga increases with the concentration of the paramagnetic ions. A surprising result is shown by ⁷¹Ga multiple quantum (MQ) MAS NMR spectrum, which changes with the concentration of paramagnetic ions. The changes in the ⁷¹Ga MQMAS spectra can be explained by a more ordonated distribution of Ga ions inside the oxygen tetrahedra. The ⁷¹Ga MQMAS NMR spectra allow identification of the one octahedral and two tetrahedral Ga sites.     

Silane overpressure post-implant annealing of Al dopants in SiC: Cold wall CVD apparatus

We report on the successful post-implantation annealing of 4H-SiC samples that were implanted at elevated temperature with high-dose Al implants. Anneals at temperatures up to 1700 °C were conducted in a silane ambient at a process pressure of 150 Torr using a cold wall CVD reactor. A series of 30 min anneals were performed in 3% silane premixed in 97% UHP argon (Ar), which was further diluted in a 6 slm Ar carrier gas. The surface morphology of the samples was studied via plan-view secondary electron microscopy (SEM) and atomic force microscopy (AFM). The resulting surface morphology showed no evidence of step bunching or any other surface degradation. We also report the electrical characterization of Al⁺ implanted p⁺/n 4H-SiC diodes realized on samples that were annealed at 1600 °C. The current-voltage characteristics of the p⁺/n diodes and the resistivity of the implanted layer were measured at room temperature. Most of the diodes had a turn-on voltage of 1.75 V, with ideality factors of up to 1.2 and very low reverse leakage current at −100 V, corresponding to an average reverse leakage current density of (9.7 ± 0.4) × 10⁻⁹ A/cm². The breakdown voltage was near the theoretical value for an epitaxial 4H-SiC layer. However, the resistivity value of the implanted Al⁺ layer was 11 Ω cm, which was an order magnitude higher than the expected value.     

Observation of "hidden" magnesium: first-principles calculations and 25Mg solid-state NMR of enstatite

²⁵Mg NMR parameters have been determined for two polymorphs of enstatite (MgSiO₃), an important magnesium silicate phase present as a major component of the Earth's upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites; however, only a single resonance is observed in ²⁵Mg MAS NMR spectra recorded at 14.1 and 20.0 T. First-principles calculations performed on geometry-optimised crystal structures reveal that the quadrupolar interaction for the second site is expected to be very large, resulting in extensive broadening of the spectral resonance, explaining its apparent absence in the NMR spectrum. ²⁵Mg QCPMG NMR experiments employing variable offset cumulative spectroscopy (VOCS) are used to observe the broadened site and enable measurement of NMR parameters. The large difference in quadrupolar interaction between the two crystallographic magnesium sites is rationalised qualitatively in terms of the distortion of the local coordination environment as well as longer-range effects using a simple point charge model.     

Sensitivities of competitions between different upconversion mechanisms to exciting wavelength in Er:YAG with different concentrations

The sensitivities of UC mechanisms to exciting wavelength have been studied in Er³⁺:YAG crystals with 8.4 and 10.5 at.% dopant concentrations. The impacts of both the dopant concentration and the excitation wavelength on UC fluorescence have been taken into account together and it has been found that in samples with different concentrations though the UC fluorescence concerning the ²P_3/2 and the ²G_9/2 levels are obviously distinct, their sensitivities to exciting wavelength are very similar.     

High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.