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In this paper, selective formation of noncentrosymmetric (Cc; α-crystal) and centrosymmetric (P21/c; β-crystal) 1,1,2,2-tetramethyl-1-(4-(N,N-dimethylamino)phenyl)-2-(2′-cyanophenyl)disilane (1) wa...

Hexagonal boron nitride (h-BN) was ball-milled at various rotation speeds (150–600 rpm) using a planetary ball-mill. Ball-milling disrupted the layered structure of the h-BN, resulting in significant increases of surface area. Ball-milling at 400 rpm gave the highest surface area of 412 m2 g−1 while higher rotation speeds decreased the surface areas due to agglomeration. Moreover, ball-milling resulted in the emergence of amino- and hydroxyl groups on the surface which were observed by Fourier transform infrared spectroscopy, and partial oxidation of the surface boron by the formation of B–OH groups was confirmed by X-ray photoelectron spectroscopy. The appearance of trigonal B–O and tetrahedral B–O was observed by boron-11 magic-angle spinning nuclear magnetic resonance spectroscopy. The number of base sites was increased with the increase of rotation speeds of milling, corresponding to the formation of amino groups. The ball-milled h-BN showed catalytic activity for the nitroaldol reaction between nitromethane and benzaldehyde in which the h-BN milled at 400 rpm exhibited the highest reaction rate and turnover frequency. In addition, the ball-milled h-BN could convert glucose with the formation of fructose at 40 °C whereas pristine h-BN showed no activity. The base sites were mainly responsible for the catalytic activity.

Hydrogenation of nonequilibrium alloys may form nanometer-sized metal hydride clusters, depending on the alloy compositions and hydrogenation conditions. Here in the Ti-rich compositions of the immiscible Mg-Ti system MgH2 clusters are embedded in a Ti-H matrix. Our previous works have indicated that the interface energy between the two metal hydrides reduces the stability of MgH2. The aim of our study is to obtain the structural information on the nanometer-sized clusters. Indeed, MgD2 clusters embedded in a face-centered-cubic (FCC) Ti-D matrix is found in Mg0.25Ti0.75D1.65 by means of 2H magic angle spinning nuclear magnetic resonance (MAS NMR). The atomic pair distribution function (PDF) analysis of neutron total scattering data suggests that the MgD2 clusters have an orthorhombic structure, which is different from a rutile-type body-centered-tetragonal (BCT) structure of α-MgD2 observed in the Mg-rich compositions. Our results suggest that we can tune the thermodynamics of hydrogen absorption and des...

Inorganic solid acid salts with hydrogen bond networks frequently show very long spin-lattice relaxation times even for 1H because the hydrogen bonds suppress motions. In the present work, the 1H spin-lattice relaxation in Cs2(HSO4)(H2PO4) containing a small amount of ammonium ions were studied in detail by use of the effect of magic angle spinning (MAS) on the relaxation. The 1H spin-lattice relaxation times of the acid protons decrease with increase in the content of ammonium ions. Reorientation of the NH4 group fluctuates the dipole-dipole interaction and relaxes the ammonium protons as well as the acid protons. The 1H relaxation times of the acid protons are a little bit longer than those of the ammonium protons at the MAS rate of 8 kHz. The spinning at 50 kHz makes the relaxation times of the acid protons longer and those of the ammonium protons shorter. Spin diffusion between the acid and the ammonium protons averages partially the 1H relaxation of the acid and the ammonium protons at the MAS rate of 8 kHz. The spin diffusion is suppressed completely at the MAS rate of 50 kHz. Spin diffusion between the acid protons is not suppressed at the MAS rate of 50 kHz. The acid protons always show the same relaxation times. The intrinsic relaxation times not affected by spin diffusion are evaluated quantitatively for both the acid and the ammonium protons. Those values are independent of the ammonium content. Contribution of the spin diffusion between the acid and the ammonium protons to the relaxation is estimated quantitatively. Using those parameters, the effect of ammonium ions on the 1H spin-lattice relaxation can be predicted. The 1H spin-lattice relaxation is a sensitive tool to study the distribution of ammonium ions in solids.

Inorganic solid acid salt Cs 2 (HSO 4 )(H 2 PO 4 ) showed high proton conductivity in the superprotonic phase, and the phase was retained on cooling even at room temperature for a long period. In the present work, we attempt partial replacement of Cs ions by ammonium ions. Incorporation of ammonium ions up to 2.3% of the total cations is succeeded. Successful incorporation of ammonium ions is confirmed by the crystal structure measured by X-ray powder diffraction, the phase transition measured by thermal analyses and phosphorus environments measured by 31 P magic-angle-spinning (MAS) NMR. Furthermore, we present 1 H and 133 Cs MAS NMR spectra and the result of 1 H{ 31 P} rotational-echo double resonance (REDOR) experiments. The 1 H and 133 Cs MAS NMR spectra show that incorporation of ammonium ions leads to increase of disordered structures.

This work explores the use of hexagonal boron nitride (h-BN), a graphite-like compound, as a novel catalyst with base and acid functionalities. For use as a solid catalyst, the layered structure of h-BN was disrupted by ball-milling, exposing boron and nitrogen edge sites as well as increasing the surface area from 3 to ca. 400 m 2 g −1 . Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and proton magic-angle spinning nuclear magnetic resonance spectroscopy ( 1 H MAS NMR) indicated simultaneous and adjacent formation of amino and hydroxyl groups by milling, which function as Bronsted base and acid sites, respectively. Analysis using color indicator reagents and pyrrole-adsorbed 1 H MAS NMR results revealed that the ball-milled h-BN had basic sites of strength +9.3 > H − ≥ +7.2, comparable to those of KY zeolite. Measurements of 31 P MAS NMR of adsorbed trimethylphosphine oxide indicated that the ball-milled h-BN had weak acid sites, comparable to those in HY zeolite. Despite its weak basicity, the ball-milled h-BN showed high activity and selectivity toward β-nitroalkenes for the nitroaldol reaction (Henry reaction) and the Knoevenagel condensation, whereas nontreated h-BN did not show activity. The nitroaldol reaction was considered to proceed in two steps: the abstraction of a proton from nitromethane by the amino group and the formation of an imine followed by a nucleophilic attack of the deprotonated nitromethane. Kinetic isotope effect experiments using D -substituted nitromethane revealed that the first step was the rate-determining step. Several nitroaldol reactions using a variety of monosubstituted benzaldehydes indicated that electron-donating groups enhanced the activity, suggesting that the formation of adjacent base and acid sites is responsible for it. This study shows the high catalytic activity of BN, a solid catalyst with moderate basicity and weak acidity.

Mechanisms of the 1 H spin-lattice relaxation in NH 4 H 2 PO 4 were studied in detail by use of the effect of magic angle spinning on the relaxation. The acid and the ammonium protons have different relaxation times at the spinning rates higher than 10 kHz due to suppression of spin diffusion between the two kinds of protons. The intrinsic relaxation times not affected by the spin diffusion and the spin-diffusion assisted relaxation times were evaluated separately, taking into consideration temperature dependence. Both mechanisms contribute to the 1 H relaxation of the acid protons comparatively. The spin-diffusion assisted relaxation mechanism was suppressed to the level lower than the experimental errors at the spinning rate of 30 kHz.

Both symmetrical and unsymmetrical cyclophanes containing disilane units, tetrasila[2.2]cyclophanes 1–9, were synthesized. The syn and anti conformations and the kinetics of inversion between two anti-isomers were investigated by X-ray diffraction and variable-temperature NMR analysis, respectively. The flipping motion of two aromatic rings was affected by the bulkiness of the aromatic moiety (1 vs 6), the phase (solid vs solution), and the inclusion by host molecules (1 vs 1⊂[Ag2L]2+). The photophysical, electrochemical, and structural properties of the compounds were thoroughly investigated. Unsymmetrical tetrasila[2.2]cyclophanes 5–8 displayed blue–green emission arising from intramolecular charge transfer. Compound 6 emitted a brilliant green light in the solid state under 365 nm irradiation and showed a higher fluorescence quantum yield in the solid state (Φ = 0.49) than in solution (Φ = 0.05). We also obtained planar chiral tetrasila[2.2]cyclophane 9, which showed interesting chiroptical properties,...

The inorganic solid acid salt Cs2(HSO4)(H2PO4) showed high proton conductivity in the superprotonic phase above 370 K, and the phase was retained on cooling even at room temperature for a long period. These characteristic properties should correlate to the hydrogen bond network connecting the SO4 and PO4 groups. In the present work, the structures of the hydrogen bond network were studied by solid-state NMR. We present 31P, 1H, and 133Cs magic-angle-spinning (MAS) NMR spectra, 31P static NMR spectra to derive chemical shift anisotropy, a 31P{1H} dipolar dephasing experiment, and 1H{31P} rotational-echo double resonance (REDOR) experiments. The 31P NMR spectra indicate that all the P sites are crystallographically equivalent, and the negative value of the chemical shift anisotropy suggests that four hydrogen bonds are formed around the PO4 tetrahedron. The 31P{1H} dipolar dephasing experiment strongly supports that the number of hydrogen bonds around the PO4 tetrahedron is four. The 1H and 133Cs MAS NMR sp...

The interfacing carbon materials can improve the visible light absorption of titanium dioxide (TiO2). Here TiO2 was anchored on carbon spheres (CSs) obtained by the hydrothermal polymerization of sucrose to bathochromically expand its light-response region. Nano-TiO2 is condensed on the CS surface upon hydrothermal treatment to generate a core–shell structure (TiO2@CS). Because of interface formation between the two materials, TiO2@CS achieved an enhanced visible light absorption compared to pure TiO2. In addition, it degraded organic pollutants, such as methyl orange, bisphenol A, and Oseltamivir, more efficiently than pure TiO2 and the well-known graphene–P25 TiO2 nanocomposite under visible light irradiation. This promoted visible light photoactivity was evidenced by the enhanced photocurrent responses and structure-dependent changes of electron spin resonance spectra that disclosed the critical role of an interfacial structure containing a doping level formed by tuning electrons from CS to TiO2. Therefore, the facile hydrothermal formation of TiO2@CS reveals new avenues for cost-effective ultraviolet-free photocatalysts exhibiting high efficiency.

Hydrogen storage is a key technology for the advancement of hydrogen and fuel cell power technologies in stationary and portable applications. MgH2, an example of a high-capacity hydrogen storage material, has two major material challenges for practical applications: slow hydrogen desorption kinetics and high hydrogen desorption temperature. Numerous studies have reported enhancements in kinetics but only a few in thermodynamics. Here, we present a simple but effective way to improve upon both the kinetic and thermodynamic aspects of desorption by utilizing the immiscibility of Mn, a non-hydrogen absorbing metal, with Mg. Mg0.25Mn0.75, prepared through ball milling MgH2 and Mn powders, is a nanocomposite where the nanometer-sized MgH2 domains are randomly embedded in a Mn matrix. This sample readily and reversibly absorbs and desorbs deuterium even at a temperature of 200 °C without the addition of any catalysts. This is nearly 180 °C lower than the typical operating temperature of conventional bulk Mg. Furthermore, at a given temperature, its deuterium desorption pressure is clearly elevated compared to that of pure Mg, indicating the destabilization of MgD2. The average crystallite size of MgD2 in deuterated Mg0.25Mn0.75 determined from X-ray diffraction data is around 9 nm. Nuclear magnetic resonance spectroscopy studies show that MgD2 domains are heavily strained and some of the D atoms are coordinated by a few Mn atoms, suggesting that a large number of lattice defects, including the partial substitution of Mg with Mn, are introduced during ball milling. Furthermore, the Mn matrix firmly locks nanosized MgD2, preventing the agglomeration of MgD2 below 250 °C. Our study suggests that a synergistic effect created by nanosizing, large lattice distortions, and robust interfaces between MgD2 and the Mn matrix can effectively and concurrently improve the kinetics and thermodynamics of MgD2 in Mg0.25Mn0.75. Our work demonstrates the possibility of utilizing the immiscibility of metals with Mg to synthesize a robust nanostructure that can alter the kinetics and stability of MgH2.

High hydrogen density and low material costs make Mg as one of the most promising candidates for solid-state hydrogen storage. However, the practical applications of Mg are restricted by high reaction temperature and slow kinetics of hydrogen absorption/desorption. Here we present the improvements of both thermodynamics and kinetics of the hydride/deuteride of Mg (MgH2/MgD2) by utilizing the immiscible Mg–Cr system. Nanometer-sized MgD2 domains with the average crystallite size of ~10 nm embedded in a Cr matrix are formed in deuterated Mg0.25Cr0.75. X-ray diffraction and nuclear magnetic resonance spectroscopy studies show that the MgD2 domains are heavily distorted, which leads to the thermodynamic destabilization lowering the reaction temperature. Mg0.25Cr0.75 can reversibly absorb and desorb hydrogen/deuterium at a low temperature of 473 K. The enthalpy ΔH for deuterium desorption of Mg0.25Cr0.75−D is 72.1 kJ mol−1−D2, which is lower than ~74 kJ mol−1−D2 for bulk MgD2. The apparent activation energy for hydrogen desorption of Mg0.25Cr0.75−H is decreased to 75 kJ mol−1 from ~160 kJ mol−1 for bulk Mg, in which the dehydrogenation of nanometer-sized MgH2 is controlled by one-dimensional diffusion of hydrogen. Our work demonstrates that MgH2 nanostructured by an immiscible matrix is a useful strategy to alter the thermodynamic and kinetic properties.

Porous boron nitride (BN) was synthesized using a pyrolysis method in conjunction with varying NH3 flow rates, followed by washing as a post-treatment. The performance of this material as a solid base catalyst was assessed. It was found that the post-treatment rather than the synthesis conditions significantly improved the activity of the BN during the nitroaldol reaction. The BN catalyst obtained after washing gave a ten times higher product yield. Both an increase in the surface areas of the material and the emergence of micropores after washing were observed. 11B solid-state nuclear magnetic resonance spectroscopy demonstrated that boron oxide species were present in the BN after synthesis but were removed by the washing process. B K-edge X-ray absorption fine structure analyses indicated the formation of oxygen-substituted trigonal boron sites on the BN surface. Fourier transform infrared spectroscopy showed that amino groups on the samples functioned as moderately strong basic sites.

Through various in situ analyses, we have revealed the structural changes that occur during the reversible melting-solidification process of layered alkylsiloxanes (CnLSiloxanes) with carbon numbers (n) of 18 and 16. In situ high-resolution solid-state (13)C nuclear magnetic resonance (NMR) analysis at controlled temperatures indicates drastic conformational changes of the long alkyl chains during the melting-solidification process. A (13)C NMR signal at 33 ppm, which shows the highest intensity at room temperature (RT), is assigned to an inner methylene group with an all-trans conformation. As the temperature increases, the 33-ppm signal intensity decreases while the signal intensity at 30.5 ppm simultaneously increases. The 30.5 ppm signal is assigned to an inner methylene group with a trans-gauche conformation. Subsequently, upon cooling, the signal at 33 ppm recovers, even after CnLSiloxanes have melted. In situ X-ray diffraction measurements at controlled temperatures reveal that the ordered arrangement of the long alkyl chains becomes disordered with elevating temperatures and reordered upon cooling to RT. In situ high-resolution solid-state (29)Si NMR analysis shows that the melting-solidification process progresses without any structural change in siloxane sheets of the CnLSiloxanes. Thus, the in situ analyses show that disordering of the long alkyl chains causes the CnLSiloxanes to melt. Because the majority of long alkyl chains are packed again in the ordered arrangement with the all-trans conformation upon cooling, the CnLSiloxanes are reversibly solidified and the CnLSiloxane structure is recovered.

Inorganic solid acid salts with hydrogen bond networks frequently show very long spin-lattice relaxation times even for

Mechanisms of the

Acid properties of ZSM-5 type zeolite are studied by solid-state NMR using trimethylphosphine oxide (TMPO) as a probe molecule. TMPO is introduced from the gas phase at 373 K. The amounts of the loaded TMPO are quantified by the signal intensities of 1 H MAS NMR spectra, which are compared with the numbers of Bronsted acid sites estimated by the Si/A ratio of the framework derived from 29 Si MAS NMR spectra and the content of fourfold-coordinated Al from 27 Al MAS NMR spectra. The use of 373 K for loading the probe molecules increases the loading level of TMPO, and thus almost all the acid sites are probed. The 31 P chemical shifts in the range between 90 and 60 ppm are attributed to TMPO adsorbed on Bronsted acid sites, whereas the signals between 60 and 40 ppm are ascribed to TMPO adsorbed on other sites than Bronsted acid sites. The 13 C chemical shift of methyl groups in the adsorbed TMPO depends mainly on the confined space as well as on the acid strength.

The adsorption state of trimethylphosphine oxide (TMPO) on siliceous MFI-type zeolite, silicalite, has been studied by solid-state NMR. TMPO was loaded by vapor as well as the solvent methods, and ...

Ion dynamics in alkali borohydrides, LiBH4, NaBH4, and KBH4, was studied by solid-state NMR. NMR spectra and spin–lattice relaxation times, T1, of 1H and 11B were measured and analyzed. The T1 results of 1H and 11B indicated that two types of motions take place in the low-temperature (LT) phase of LiBH4. The second moments of 1H and 11B NMR spectra suggested that the sum of the two types of motions resembles isotropic reorientation of the BH4 unit. The T1 minimum values of 1H and 11B supported that the C3 rotation takes place at first. The second type of motion might be either C3 rotation or C2 rotation. The second moments of NMR spectra and the T1 minimum values of 1H and 11B supported that the BH4 unit reorients isotropically in NaBH4 and KBH4.

Hybrid materials of CsHSO4 and silica nanoparticles were prepared by mechanical milling, and hydrogen bond states and proton dynamics were studied by means of 1H solid-state NMR. 1H MAS NMR spectra demonstrated that three types of domains are present in the milled materials. Domain A has hydrogen bond states similar to those in the bulk compound. With respect to hydrogen bonds, domains A-II and A-III are similar to phases II and III of CsHSO4, respectively. Protons in domain A-II undergo translational diffusion, and the diffusion is faster than in phase II of bulk CsHSO4. Domain B is originated by mixing of CsHSO4 with silica nanoparticles, presumably locating at the boundary region. Protons in this domain also undergo translational diffusion. The motional rate is faster than in phase II of bulk CsHSO4 but is slower than in domain A-II. In domain C protons are contained as OH groups on the surface of silica nanoparticles. Protons are immobile in this domain.