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 ¹H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of ²⁷Al 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 ²⁷Al–²⁷Al and ²⁹Si–²⁹Si and by CP‐HETCOR for hetero‐nuclear correlations between ²⁷Al–²⁹Si 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. Copyright © 2012 John Wiley & Sons, Ltd.     

Precursor structure and hydrolysis-gelation process of Al(O-sec-Bu)3 modified with ethylacetoacetate

The precursor structure and hydrolysis-gelation process of aluminum-sec-butoxide, Al(O-sec-Bu)₃, modified with ethylacetoacetate (EAcAc) were investigated using IR, UV and high magnetic field (11.7 T) ²⁷Al NMR spectra. ²⁷Al NMR showed that the reaction of EAcAc with Al(O-sec-Bu)₃ led to the formation of six-coordinated structural units and most of the precursors were assumed to be present as linear trimer containing a four- and two six-coordinated Al atoms. It was found that EAcAc formed the strong chelating bonds with Al(O-sec-Bu)₃, which were hardly hydrolyzed by the attack of water molecules in the gelation process.     

Nuclear hyperfine splitting in the BX electronic band system of 127I2

The spacings between seven hyperfine components in the R(127) line of the 11-6 band of the B-X electronic transition of ¹²⁷I₂ are fitted, with a standard deviation of 17.3 kHz, by varying the nuclear quadrupole coupling constants eqQ′ and egQ′', the magnetic spin—rotation constant C_I and the tensor and scalar nuclear spin—spin coupling constants d′ and a′ in the hyperfine hamiltonian. The P(13) line of the 430 band is also analysed using an identical hamiltonian and a standard deviation of 6.25 kHz is obtained. No evidence for a magnetic octupole coupling is found to the precision of the data although this effect was invoked by Hackel et al. for the P(13) line.     

Chemical shift and quadrupole coupling of the 27Al NMR spectra of LiAIO2 polymorphs

²⁷Al chemical shift data of polycrystalline LiAlO₂ polymorphs, obtained from high field measurements in combination with magic-angle spinning techniques, have been used for the determination of the Al coordination in the α-, β-, and γ-phase. The β-phase was shown to have only tetrahedrally coordinated Al atoms. In addition quadrupole coupling data have been derived from analysis of low field NMR spectra of 27A1 in order to characterize the symmetry of the Al sites. The NMR results are discussed with regard to the crystal structure of the phases.     

Electronic effect on the rotational energy of a diatomic molecule

The rotational hamiltonian for a diatomic molecule has been rederived from the total classical hamiltonian. This procedure directly introduces the effect of electronic motion which is ordinarily neglected in zero-order approximation. Kronig's rotational hamiltonian is discussed and shown to be an approximation of our findings. Our general result is then specialized to ¹Σ states, and the theory tested by calculating the observed fractional discrepancy between the experimentally determined H³⁵Cl energy level constant Y₀₂ and its predicted value from Dunham's theory. When all corrections are summed, the results are in good agreement with experiment.     

Mössbauer spectra of the tetrakis-(1,8-naphthyridine) iron(II)perchlorate in external magnetic fields. Evidence of slow relaxation ln paramagnetic iron(II)

⁵⁷Fe Mössbauer spectra of [FeL₄] (ClO₄)₂ where L = 1,8-naphthyridine have been measured at 4.2°K in external magnetic fields up to 55 kG parallel to the direction of the γ-rays. The spectra have been fitted in the spin hamiltonian approximation assuming an orbital singlet ground state of the ⁵D multiplet of Fe²⁺. The fit of the spectra is not unique, yet the possible spin hamiltonian parameter sets found lead to a spin doublet ground state split by less than 1 cm^?1. The transition probabilities for spin-lattice relaxation have been calculated for those ground states. Orbach processes via excited spin hamiltonian states cannot be neglected. The results explain the fluctuations observed in the spectra in low external magnetic fields (10 kG).The spin hamiltonian parameters provide information on the orbital energy levels. Therefrom the reduction of the quadrupole splitting by spin—orbit coupling results to be small thus explaining the extremely large quadrupole splitting of 4.54 mm/sec.     

Hyperfine interactions at 57Fe in octaethylhaemin

Mössbauer measurements on octaethylhaemin have been made between 4.2 and 300 K, and (at the lower temperature) in applied magnetic fields of up to 50 kG. From a spin hamiltonian treatment the saturation value of the hyperfine field at the ⁵⁷Fe nucleus is found to be 495 kG, and the splitting parameter for the ferric ion levels by the axial ligand field is D = 8.0 cm^?1. The electric field gradient 1/2e²qQ = +0.93 mm s^?1.     

Heteronuclear NMR Spectroscopy as a Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina.

The surface hydroxyl groups of γ‐alumina dehydroxylated at 500 °C were studied by a combination of one‐ and two‐dimensional homo‐ and heteronuclear ¹H and ²⁷Al NMR spectroscopy at high magnetic field. In particular, by harnessing ¹H–²⁷Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the ¹H magic‐angle spinning (MAS) NMR spectrum was demonstrated thanks to ¹H–²⁷Al RESPDOR (resonance‐echo saturation‐pulse double‐resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {¹H}‐²⁷Al dipolar heteronuclear multiple quantum correlation (D‐HMQC), which was used to establish a first coordination map. Then, in combination with ¹H–¹H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO₂ reactivity with alumina.     

Synthesis and properties of magnetic fluids produced on the basis of magnetite particles

Magnetic fluids based on magnetite synthesized by the chemical condensation method at temperatures of 25, 40, 60, and 80°C were obtained and studied. Magnetite particles were examined by X-ray phase and X-ray fluorescence analyses and electron microscopy. The average size of the coherent scattering region of magnetite particles was 13–17 nm, depending on the synthesis temperature. Magnetic fluids were synthesized from magnetite particles obtained at 25 and 80°C, with water and octane serving as carrier fluids. The NMR method was used to determine the saturation magnetization and average magnetic moment of the particles: for water-based magnetic fluids, 2100 A m^–1 and 5.7 × 10^–19 A m² at magnetite particle synthesis temperature of 25°C and 3670 A m^–1 and 4.6 × 10^–19 A m² at magnetite particle synthesis temperature of 80°C; for octane-based magnetic fluids, 2250 A m^–1 and 4.1 × 10^–19 A m² at magnetite particle synthesis temperature of 25°C.     

Preparation and properties of magnetic alumina microspheres with a γ-Fe_2O_3/SiO_2 core and Al_2O_3 shell

Magnetic alumina composite microspheres with γ-Fe 2 O 3 core/Al 2 O 3 shell structure were prepared by the oil column method. A dense silica layer was deposited on the surface of γ-Fe 2 O 3 particles (denoted as γ-Fe 2 O 3 /SiO 2 ) with a desired thickness to protect the iron oxide core against acidic or high temperature conditions. γ-Fe 2 O 3 /SiO 2 /Al 2 O 3 particles with about 85 wt% Al 2 O 3 were obtained and showed to be suitable for practical applications as a magnetic catalyst or catalyst support due to their magnetic properties and pore structure. The products were characterized with scanning electron microscope (SEM) and transmission electron microscope (TEM), nitrogen adsorption-desorption, and vibrating sample magnetometer (VSM). The specific surface area and pore volume of the γ-Fe 2 O 3 /SiO 2 /Al 2 O 3 composite microspheres calcined at 500 ? C were 200 m 2 /g and 0.77 cm 3 /g, respectively.     

Solid-state 27Al NMR studies on polycrystalline aluminates of the system CaO-Al2O3

²⁷Al NMR isotropic chemical-shift and quadrupole-coupling data have been determined for a series of polycrystalline calcium aluminates with varying CaO/Al₂O₃ ratios. The data were derived from static and magic-angle spinning NMR spectra recorded at different magnetic field strengths up to 11.7 T. The chemical-shift and quadrupole-coupling data are discussed with regard to the crystal structure of the compounds studied. It is shown that structural effects are reflected more sensitively in the quadrupole-coupling data than in the chemical shifts. In favourable cases crystallographically different Al sites could be resolved in the NMR spectra.     

Electronic structure and quadrupole interactions in triple molybdates Li<Subscript>2</Subscript>M<Subscript>3</Subscript>Al(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>, M = Cs,Rb

Within the density functional theory the electronic structure of triple molybdates Li₂M₃Al(MoO₄)₄, where M = Cs, Rb, is studied for the first time. It is found that all molybdates studied belong to wide band insulators with a band gap of ~4 eV. Quadrupole frequencies and asymmetry parameters of the electric field gradient near magnetic ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are calculated and experimental NMR spectra are interpreted.     

Extra-framework aluminium in thermally treated zeolite CaA

Nuclear magnetic resonance spectra of aluminium including magic-angle spinning (²⁷Al MAS NMR) have been measured in order to study the dealumination which is caused by a heat treatment of zeolite CaA. The extra-lattice aluminium exists partly in the form of Al(OH)₄⁻ anions, corresponding to at most about 17% of lattice dealumination.     

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect ²⁷Al-³¹P spin-spin coupling constants, and ²⁷Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the ³¹P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The ²⁷Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well-reproduced by the static ²⁷Al wideband uniform rate smooth truncation (WURST) Carr-Purcell-Meiboom-Gill (WCPMG) NMR experiment. ²⁷Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three- and four-coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance-echo saturation-pulse double-resonance (RESPDOR) experiment was developed by using efficient saturation via frequency-swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.     

27Al NMR diffusometry of Al13 Keggin nanoclusters

Although nanometer-sized aluminum hydroxide clusters (i.e., ϵ-Al₁₃, [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺) command a central role in aluminum ion speciation and transformations between minerals, measurement of their translational diffusion is often limited to indirect methods. Here, ²⁷Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO₄ core of the ϵ-Al₁₃ cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO₄ center of the ϵ-Al₁₃ cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T₁ and T₂ ²⁷Al NMR relaxation coefficients for ²⁷Al PFGSTE NMR studies. Stokes–Einstein relationship was used to relate the ²⁷Al diffusion coefficient of the ϵ-Al₁₃ cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published ¹H PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that ²⁷Al PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.     

Speciation and complexation in aqueous Al(III)–quinolinate solutions: a spectroscopic study

Aqueous solutions containing quinolinic acid (2,3-pyridinedicarboxylic acid) and aluminum chloride were investigated using attenuated total reflection Fourier transform infrared spectroscopy, ²⁷Al nuclear magnetic resonance spectroscopy (²⁷Al NMR), and potentiometry. Conditional equilibrium constants were acquired at an apparent ionic strength of 1.33 M for the formation of quinolinic acid (H₂L), the HL⁻ species, and each of the Al(III)–quinolinate complexes (AlL⁺ and AlL₂ ⁻). The resolved infrared spectra of the quinolinate ion (L²⁻), the HL⁻ species, and quinolinic acid were assigned and interpreted with respect to the proton binding properties of the ligand. The assignments of the ²⁷Al NMR and infrared spectra of the Al(III)–quinolinate complexes provide evidence that quinolinate preferentially chelates to Al(III) through both the nitrogen of the pyridine ring and one of the oxygens of a carboxylate substituent.     

Hochfeld-27Al-NMR-Untersuchungen zur Aluminiumkoordination in kristallinen Aluminiumphosphaten

High-field ²⁷Al-NMR Investigations on the Aluminium Coordination in Crystalline Aluminium Phosphates The ²⁷Al NMR spectra of crystalline aluminium phosphates obtained at high-magnetic fields under magic-angle spinning (MAS) conditions allow a direct determination of the aluminium coordination. So for five new Al phosphates it is shown that, as it is almost exclusively the case for Al phosphates of known structures, the Al is octahedrally coordinated, too. Chemical shifts between ?13 and ?21 ppm for isolated AlO₆ octahedra, and 39 ppm for isolated AlO₄ tetrahedra (in crystalline AlPO₄), were determined. In comparison to the results on aluminates and aluminum oxides these values are generally upfield-shifted about 30 ppm. The stronger magnetic shielding is attributed to the influence of the phosphorus atoms present in the second coordination sphere of the aluminum.     

27Al NMR MAS Spectral Studies Inferring the Initiation of Geopolymerization Reaction on Together Mechanochemical Grinding of Raw Materials

Chemical shifts and intensities of the ²⁷Al NMR signals provide structural information about the environment of Al nuclei in presence of an external magnetic field. This paper analyzes the structural information of the aluminum nuclei present in the precursor material after mechanochemical co‐grinding of the raw materials, namely fly ash, NaOH, and amorphous tricalcium phosphate [Ca₃(PO₄)₂], with the help of ²⁷Al MAS NMR spectral studies. The results indicate transformation of the sixfold coordination Al ions with oxygen AlQ6 present in aluminosilicate material fly ash to fourfold AlQ4 and fivefold AlQ5 in the precursor material. The variation in chemical shift is between δ 64 and 65 ppm. This indicates that, in addition to direct bonding to the oxygen atom, the Al tetrahedron is also bonded to Si as [AlQ4(4Si)]. Thus, the mechanochemical co‐grinding of the raw materials initiates a solid‐state chemical reaction among them. The addition of water alone to this precursor material results in the formation of the geopolymeric material unlike the conventional geopolymeric system which requires the addition of a highly alkaline aqueous solution to fly ash. This study helps in the determination of the reaction mechanism during the mechanochemical transformation of raw materials into the geopolymeric product by a novel process.     

Exchange interaction and orbital degeneracy in the Cu(II)-Co(II) heterobinuclear complex

The temperature dependence of the magnetic susceptibility of the title compound was studied in the range 3.6–300 K. This heterobinuclear complex may be considered as the simplest polynuclear system in which the problem of the orbital degeneracy occurs. The experimental data were interpreted with a hamiltonian taking into account the distorsion and the spin-orbit coupling around the Co(II) ion on one hand, and the Cu(II)-Co(II) interaction on the other. From this hamiltonian, a quite satisfying simulation of the experimental magnetic curve was obtained. The effective exchange interaction parameter J of the ?J?_Cu?_Co term of the hamiltonian was found equal to =62 ± 2 cm^?1. This value was compared to those obtained with the [CuCu], [CuNi] and [CuMn] complexes prepared with the same bichelating ligand. This comparison was carried out in a framework of an orbital model, previously established in the case of interacting ions without orbital momentum and here extended to the case where one of the interacting ions has an orbital degeneracy.