Multi-quantum Magic-angle Spinning Research Articles

High-Field One-Dimensional and Two-Dimensional 27Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of θ-, δ-, and γ-Al2O3 Dominated Aluminum Oxides: Toward Understanding the Al Sites in γ-Al2O3.

Herein, a detailed analysis was carried out using high-field (19.9 T) 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al2O3 phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS 27Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D 27Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-defined quadrupolar line shapes was observed in the θ-phase dominant sample and was unambiguously assigned to the θ-Al2O3 phase. The distinct line shapes related to the θ-Al2O3 phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al2O3 dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al2O3 phase to be extracted. The results show that the δ-Al2O3 phase contains three distinct AlO sites and three distinct AlT sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-Al2O3 spectrum. It is found that the γ-Al2O3 phase consists of Al sites with local structures similar to those found in the δ-Al2O3 and θ-Al2O3 phases albeit with less ordering. Spin–lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns 27Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.

Structural Origin of the Optical Properties of Ag-Doped Fluorophosphate and Sulfophosphate Glasses.

Given the ubiquity of glass formulations that are functionalized with silver compounds, the electronic interaction between isolated silver cations and the glass network deserves more attention. Here, we report the structural origin of the optical properties that result from silver doping in fluorophosphate (PF) and sulfophosphate (PS) glasses. To achieve this, solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) are combined with optical spectroscopic analysis and physical property measurements. Comparing the 31P NMR, 27Al 1d NMR, and 27Al multi-quantum magic-angle spinning NMR of doped glasses and glasses with large amounts of Ag+ added, we deduce silver's bonding preference in these mixed-anion aluminophosphate glasses. We show that such understanding provides an explanation for the large Stokes shift observed for Ag+ in PF and PS glasses, which is related to absorption by the ionic Ag+···-O-P species and transfer of the excitation energy within more covalently bonded Ag2O-like clusters. This is corroborated by DFT calculations, which show that the Ag+···-O-P and Ag+···-O-S bonds in corresponding crystals are mostly ionic. The introduction of more silver ions into the crystal structure results in more covalent bonding between Ag+ and the phosphate matrix.

A new look at the structural properties of trisodium uranate Na3UO4.

The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state (23)Na multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na3BiO4, has monoclinic symmetry, in space group P2/c. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition α-Na3(U1-x,Nax)O4 (0 < x < 0.18). The α phase adopts a mixed valence state with the presence of U(V) and U(VI). The two polymorphs of this compound described in the literature, m- and β-Na3(U1-x,Nax)O4, have also been investigated, and their relationship to the α phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature β phase is cubic, in space group Fd3̅m.

A 23Na Magic Angle Spinning Nuclear Magnetic Resonance, XANES, and High-Temperature X-ray Diffraction Study of NaUO3, Na4UO5, and Na2U2O7

The valence state of uranium has been confirmed for the three sodium uranates NaU(V)O3/[Rn](5f(1)), Na4U(VI)O5/[Rn](5f(0)), and Na2U(VI)2O7/[Rn](5f(0)), using X-ray absorption near-edge structure (XANES) spectroscopy. Solid-state (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) measurements have been performed for the first time, yielding chemical shifts at -29.1 (NaUO3), 15.1 (Na4UO5), and -14.1 and -19 ppm (Na1 8-fold coordinated and Na2 7-fold coordinated in Na2U2O7), respectively. The [Rn]5f(1) electronic structure of uranium in NaUO3 causes a paramagnetic shift in comparison to Na4UO5 and Na2U2O7, where the electronic structure is [Rn]5f(0). A (23)Na multi quantum magic angle spinning (MQMAS) study on Na2U2O7 has confirmed a monoclinic rather than rhombohedral structure with evidence for two distinct Na sites. DFT calculations of the NMR parameters on the nonmagnetic compounds Na4UO5 and Na2U2O7 have permitted the differentiation between the two Na sites of the Na2U2O7 structure. The linear thermal expansion coefficients of all three compounds have been determined using high-temperature X-ray diffraction: αa = 22.7 × 10(-6) K(-1), αb = 12.9 × 10(-6) K(-1), αc = 16.2 × 10(-6) K(-1), and αvol = 52.8 × 10(-6) K(-1) for NaUO3 in the range 298-1273 K; αa = 37.1 × 10(-6) K(-1), αc = 6.2 × 10(-6) K(-1), and αvol = 81.8 × 10(-6) K(-1) for Na4UO5 in the range 298-1073 K; αa = 6.7 × 10(-6) K(-1), αb = 14.4 × 10(-6) K(-1), αc = 26.8 × 10(-6) K(-1), αβ = -7.8 × 10(-6) K(-1), and αvol = -217.6 × 10(-6) K(-1) for Na2U2O7 in the range 298-573 K. The α to β phase transition reported for the last compound above about 600 K was not observed in the present studies, either by high-temperature X-ray diffraction or by differential scanning calorimetry.

High magnetic field solid‐state NMR analyses by combining MAS, MQ‐MAS, homo‐nuclear and hetero‐nuclear correlation experiments

A general strategy of structural analysis of alumina silicate by combining various solid-state NMR measurements such as single pulse, multi-quantum magic angle spinning, double-quantum homo-nuclear correlation under magic angle spinning (DQ-MAS), and cross-polarization hetero-nuclear correlation (CP-HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for (1) H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of (27) Al in single pulse, DQ-MAS, and even in triple-quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ-MAS methods for homo-nuclear correlations between both (27) Al-(27) Al and (29) Si-(29) Si and by CP-HETCOR for hetero-nuclear correlations between (27) Al-(29) Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina-silicate minerals.

Characterization of local structures in amorphous and crystalline tris(8-hydroxyquinoline) aluminum(III) (Alq 3) by solid-state 27Al MQMAS NMR spectroscopy

The local structure of tris(8-hydroxyquinoline) aluminum(III) (Alq 3) in three different crystalline polymorphs, α-, γ-, and δ-Alq 3, and amorphous Alq 3, are investigated by 27Al NMR experiments, which include two-dimensional multi-quantum magic-angle spinning (MQMAS) experiments and one-dimensional variable B 0 field MAS experiments. The quadrupolar and chemical shift parameters of these Alq 3 samples are determined, including the distributions. These parameters reflect the local structure of respective samples and it is found that the γ- and δ-Alq 3 consist of the facial isomer with well-defined crystal structure, whereas the α- and amorphous Alq 3 consist of the meridional isomer with distributions of quadrupolar coupling constants. It is also found that two distinct components exist in α-Alq 3.

Hydration in Alkali-silicate Glasses Studied by Two Dimensional Multi-Quantum Magic Angle Spinning

The hydration in Na2O-3SiO2 glass was studied by 23Na Magic Angle Spinning (MAS) and 2D MQMAS NMR spectroscopy. It was found that one-dimensional MAS spectra for the hydrated glasses with more than 8.4 wt% water consist of two signals at around -10 and -40 ppm. On the other hand, anhydrous glass and glass with low water content (2.8 wt%) give a single resonance line at around -10 ppm. From 2D MQMAS spectra, the isotropic chemical shift ( δiso) and the quadrupole coupling frequency (VQ) for two sites (Na(1) and Na(2)) were estimated: δiso = 4.2 ppm and VQ = 0.6 MHz for Na(1) and δiso = 10.2 ppm and VQ = 1.3 MHz for Na(2). These results are discussed together with our previous results of 29Si and 1H NMR, and infrared spectra. It is speculated that Na(1) may exist in a similar environment as that in anhydrous sodium-silicate glasses, while Na(2) may be attached directly to water molecules

Formula omitted] Solid-state NMR studies of hydrated disodium adenosine triphosphate

The biologically important compound hydrated disodium adenosine 5′-triphosphate (ATP) was studied focusing on the 23 Na nucleus (spin quantum number I=3/2) as probe in various solid-state NMR techniques including magic-angle spinning (MAS), cross-polarization MAS (CPMAS) and multi-quantum MAS (MQMAS) at 4.7 and 9.4 T. MQMAS experiments enabled the resolution of all the four crystallographically different 23 Na sites in the unit cell in the MQMAS spectrum at 4.7 T, but not at 9.4 T. The four sites were successfully assigned and the principal elements of the quadrupolar tensors and isotropic chemical shifts of the 23 Na nuclei at each lattice site determined by analyzing and computer simulating the above experimental spectra.

Radio-frequency-mediated dipolar recoupling among half-integer quadrupolar spins

It is demonstrated that through-space recoupling can be achieved in dipolar coupled quadrupolar spins in the presence of an appropriate radio-frequency field. Experimental and theoretical results for 23Na–23Na pairs are presented that elucidate the experimental conditions leading to homonuclear dipolar transfer. The effect of adiabatic amplitude modulation on spin-3/2 systems is compared to spin-1/2 cases and applications of this approach in the context of high resolution multiquantum magic angle spinning for dipolar filtering and correlation are discussed.

Quantification of MQMAS spectra of solids containing quadrupolar nuclei

This work presents a method that can yield quantitative multi- quantum magic-angle spinning (MQMAS) nuclear magnetic resonance (NMR) spectra by correcting experimental spectra with numerical simulations. The most important sources that cause non-quantitative MQMAS spectra are identified and can be calculated exactly. Therefore, the MQMAS spectra can be corrected with a number of intensity loss factors. Moreover, this method does not require intensive RF fields, extensive pulse optimization or high spinning speeds, etc. The effectiveness of this method is verified by treating the 23 Na MQMAS spectra of a series of sodium compounds to yield almost the X-ray site quantification.

Numerical and theoretical analysis of multiquantum magic-angle spinning experiments

Using a recent investigation of the Floquet’s theorem for magic-angle spinning nuclear magnetic resonance simulations (NMR), a procedure for computing multiquantum magic-angle spinning spectra is derived. The general formalism which is introduced here can be applied more generally to any solid-state NMR two-dimensional experiments. All interactions and their time dependency are considered during the pulses. Furthermore, for powder patterns, a formal average is possible on γ (the third component of the Euler angle describing the orientation of the crystallite) which leads to great simplifications and to an improved computing efficiency. As an application, the intensity of the spinning sidebands in the two-dimensional multiquantum magic-angle spinning spectrum is investigated. The recently reported appearance of numerous spinning sidebands in the multiquantum dimension is discussed. Such effects appear naturally in the present formalism which provides a theoretical framework for further investigations. Simulations of two-dimensional spectra are compared with experimental data.

Cation location in dehydrated zeolite NaY revisited: SI position is displaced from the center of the hexagonal prism

Recent multiquantum magic-angle spinning and double rotation 23Na-NMR experiments of dehydrated zeolite NaY have shown that sodium cations on site SI in the hexagonal prism are subject to appreciable quadrupolar interactions. This indicates that the SI cations are not located in the classical center position derived from X-ray and neutron diffraction studies but are displaced from the center by 0.6 Å along the [111] direction.

Shaped pulse excitation in multi-quantum magic-angle spinning spectroscopy of half-integer quadrupole spin systems

Coherence excitation by shaped pulses in the multi-quantum magic-angle spinning spectroscopy of half-integer quadrupole spin systems was explored. A theoretical analysis shows there exists a class of shaped pulses that are superior to the rectangular pulses normally used in MQMAS studies. Significant sensitivity enhancement can be achieved without loss of resolution and quantification. This is verified by numerical simulations based on decaying triangular and exponential shaped pulses. 23Na multi-quantum magic-angle spinning spectra of polycrystalline anhydrous di-sodium hydrogen phosphate (Na 2HPO 4) are shown to be well-explained by the theoretical analysis.