1H MAS Research Articles

Ferroelastic property of tetramethylammonium tetrachlorozincate tetrachlorocuprate, [N(CH3)4]2Zn1−xCuxCl4 (x = 0, 0.1, 0.3, 0.5, and 1)

The various crystallographic structures of [N(CH3)4]2Zn1−xCuxCl4 (x = 0, 0.1, 0.3, 0.5, and 1) may be understood by considering the different chemical shifts observed in 1H MAS and 13C CP/MAS NMR spectra.

Synthesis and characterisation of ZSM-5/SBA-15 composite material

ZSM–5/SBA–15 composite materials were successfully synthesised by a hydrothermal treatment method including three steps: 1) microporous phase seed formation (seed 1); 2) precursor addition (seed 2); 3) formation of the ZSM–5/SBA–15 composite material. The obtained samples were characterised by XRD, SEM, TEM, TPD–NH3, 27Al MAS NMR, 1H MAS NMR, and N2adsorption/desorption. It could be shown that the formation of both micro– and mesopores strongly depends on the duration and temperature of the above–mentioned crystallisation steps. The results obtained from XRD, SEM, TEM, and N2 adsorption/desorption indicated that the ZSM–5/SBA–15 composite materials have both microporous and mesoporous properties. The composite nature of the samples was demonstrated by SEM and TEM studies. The high acid sites number and strength of the ZSM–5/SBA–15 composite materials were shown using 27Al MAS NMR, 1H MAS NMR, and TPD–NH3 methods.

Sn surface-enriched Pt–Sn bimetallic nanoparticles as a selective and stable catalyst for propane dehydrogenation

A new one pot, surfactant-free, synthetic route based on the surface organometallic chemistry (SOMC) concept has been developed for the synthesis of Sn surface-enriched Pt–Sn nanoparticles. Bu3SnH selectively reacts with [Pt]–H formed insitu at the surface of Pt nanoparticles, Pt NPs, obtained by reduction of K2PtCl4 by LiB(C2H5)3H. Chemical analysis, 1H MAS and 13C CP/MAS solid-state NMR as well as two-dimensional double-quantum (DQ) and triple-quantum (TQ) experiments show that organo-tin moieties Sn(n-C4H9) are chemically linked to the surface of Pt NPs to produce, in fine, after removal of most of the n-butyl fragment, bimetallic Pt–Sn nanoparticles. The Sn(n-CH2CH2CH2CH3) groups remaining at the surface are believed to stabilize the as-synthesized Pt–Sn NPs, enabling the bimetallic NPs to be well dispersed in THF. Additionally, the Pt–Sn nanoparticles can be supported on MgAl2O4 during the synthesis of the nanoparticles. Some of the Pt–Sn/MgAl2O4 catalyst thus prepared exhibits high activity in PROX of CO and an extremely high selectivity and stability in propane dehydrogenation to propylene. The enhanced activity in propane dehydrogenation is associated with the high concentration of inactive Sn at the surface of Pt nanoparticles which “isolates” the active Pt atoms. This conclusion is confirmed by XRD, NMR, TEM, and XPS analysis.

Investigation of Cs(H2PO4)1 − x (HSO4) x (x = 0.15–0.3) superprotonic phase stability

A detailed investigation of the highly conductive Cs(H2PO4)1−x (HSO4) x (x = 0.15–0.3) proton electrolyte, its structural properties, and ageing behavior was carried out using X-ray diffraction, DSC, and impedance and NMR spectroscopy. The high conductivity of electrolytes (∼2 × 10−2 S/cm) remains stable during long-term ageing at 180–200°C due to stabilization of the high temperature phase to lower temperatures. The room temperature 1H MAS NMR spectrum of (CsH2PO4)1−x (CsHSO4) x demonstrates the predominantly highly mobile protons present in these materials with the residual low-mobile protons, which agrees with the XRD data. According to XRD and 1H NMR data, the cubic phase of Cs(H2PO4)1 − x (HSO4) x (x = 0.15–0.3) that stabilizes at room temperature gradually transforms to a low-temperature monoclinic one. The kinetics of the phase transformation for mixed salt depends markedly on the relative air humidity. A possible stabilization mechanism of the Cs(H2PO4)1 − x (HSO4) x superionic phase with high proton mobility at low temperatures is discussed.

Insights into the functional group transformation of a chinese brown coal during slow pyrolysis by combining various experiments

The effects of heat treatment on the structural transformation of a Chinese brown coal were studied using Fourier transform infrared spectroscopy (FT-IR), high resolution solid-state nuclear magnetic resonance (SSNMR), and Raman spectroscopic techniques. The wide applicabilities of 13C dipolar-decoupling magic-angle-spinning (DDMAS), 1H MAS, 1H–1H homonulcear double-quantum (DQ) MAS and 1H–13C cross-polarization heteronuclear correlation (CP-HETCOR) SSNMR approaches that directly probed the proton and carbon spatial interactions within functional groups of Shengli (SL) brown coal char were tentatively demonstrated. Two distinct transformation intervals, as a function of pyrolysis temperature, were clarified in the SL brown coal char. The initial decomposition (at <300°C) of coal, with slight change in either functional groups or microstructures, may be caused by the loss of adsorbed gaseous materials and partial scission of weak bonds (such as methylene and ether bridges). Moreover, the progressive dehydrogenation and splitting off of side-chains at 400–700°C resulted in the drastically cracking of aliphatic groups. However, a significant amount of oxygen-containing functional groups were also found, such as heterocyclic, phenolic hydroxyl groups and crystalline hydrates. The thermal treatment was a crystallite-perfecting process, during which the fraction of the amorphous carbon in the coal chars, determined by Raman spectroscopy, was gradually decreased with increase in the heat-treatment temperature; however, the carbon crystallite size did not show any apparent change.

Structures in solid state and solution of dimethoxy curcuminoids: regioselective bromination and chlorination

BackgroundSeveral papers described the structure of curcumin and some other derivatives in solid and in solution. In the crystal structure of curcumin, the enol H atom is located symmetrically between both oxygen atoms of the enolone fragment with an O···O distance of 2.455 Å, which is characteristic for symmetrical H-bonds. In the solution, the geometry of the enolone fragment is attributed to the inherent disorder of the local environment, which solvates one of the basic sites better than the other, stabilizing one tautomer over the other. In this paper, how the position of methoxy groups in dimethoxy curcuminoids influence the conformation of molecules and how the halogen atoms change it when they are bonded at α-position in keto-enol part of molecules is described.ResultsSix isomers of dimethoxy curcuminoids were prepared. Conformations in solid state, which were determined by X-ray single crystallography and 1H MAS and 13C CPMAS NMR measurements, depend on the position of methoxy groups in curcuminoid molecules. In solution, a fast equilibrium between both keto-enol forms exists. A theoretical calculation finding shows that the position of methoxy groups changes the energy of HOMO and LUMO. An efficient protocol for the highly regioselective bromination and chlorination leading to α-halogenated product has been developed. All α-halogenated compounds are present mainly in cis keto-enol form.ConclusionsThe structures in solid state of dimethoxy curcuminoids depend on the position of methoxy groups. The NMR data of crystalline solid samples of 3,4-diOCH3 derivative, XRD measurements and X-ray structures lead us to the conclusion that polymorphism exists in solids. The same conclusion can be done for 3,5-diOCH3 derivative. In solution, dimethoxy curcuminoids are present in the forms that can be described as the coexistence of two equivalent tautomers being in fast equilibrium. The position of methoxy groups has a small influence on the enolic hydrogen bond. Theoretical calculations show that the energy gap between HOMO and LUMO depend on the position of methoxy groups and are lower in solution. Chlorination and bromination on α-position of 1,3-diketone moiety do not change the preferential form being cis keto-enol as in parent compounds.

Atomic structure and dehydration mechanism of amorphous silica: Insights from 29Si and 1H solid-state MAS NMR study of SiO2 nanoparticles

Detailed knowledge of the atomic structure of hydrous species on surface of amorphous silica and the effect of temperature and particle size on their atomic configurations are essential to understand the nature of fluids–amorphous silicates interactions and the dehydration processes in the amorphous oxides. Here, we report the 29Si, 1H MAS, and 1H–29Si heteronuclear correlation (HetCor) NMR spectra of 7nm and 14nm amorphous silica nanoparticles—a model system for natural amorphous silica—where previously unknown details of changes in their atomic structures with varying dehydration temperature and particle size are revealed. Diverse hydroxyl groups with varying atomic configurations and molecular water apparently show distinct dehydration trends. The dehydration (i.e., removal of water) of amorphous silica nanoparticles mostly results in the increase of isolated silanol by removing water molecules from hydrogen-bonded silanols associated water molecules. With further increase in dehydration temperature, the intensity of isolated silanol peak decreases above ∼873K, suggesting that the condensation of isolated silanol may occur mainly above ∼873K. The entire dehydration (and dehydroxylation) process completes at ∼1473K. Both the water (i.e., physisorbed water and hydrogen-bonded water) and hydrogen-bonded silanol species show a dramatic change in the slope of intensity variation at ∼873K, indicating that most of silanols is hydrogen-bonded to water rather than to other silanols. The fraction of hydrogen-bonded proton species is also much smaller in 14nm amorphous silica nanoparticles than in 7nm amorphous silica nanoparticles mainly due to the presences of larger fractions of water and hydrogen-bonded silanol species. 29Si NMR results show that with increasing dehydration temperature, the fraction of Q4 species apparently increases at the expense of Q2 and Q3 species. The fractions of Q2 and Q3 structures in 7nm amorphous silica nanoparticles are larger than those in 14nm amorphous silica nanoparticles. Dehydration of 7nm amorphous silica nanoparticles occurs at a lower temperature than that of 14nm amorphous silica nanoparticles. 29Si MAS NMR results show that a possible simultaneous dehydroxylation can also occur with removal of the hydrogen bonded silanol in the 7nm silica nanoparticles. The energy penalty of dehydroxylation estimated from 29Si MAS NMR spectra varies with Q species and is smaller in 7nm than in 14nm amorphous silica nanoparticles. These results demonstrate that the particle size of nanoparticles plays an important role in controlling the hydrogen contents, and thus overall hydrogen bond strength of hydroxyl groups and atomic structure of silanols can control dehydroxylation of amorphous silica nanoparticles. The structural information and mechanistic details obtained from the current study provide insights into the structure of hydrous species and dehydration mechanisms in crystalline and amorphous silicates in diverse geological settings, highlighting usually unknown effects of particle size on the dehydration processes.

Molecular Salts of Anesthetic Lidocaine with Dicarboxylic Acids: Solid-State Properties and a Combined Structural and Spectroscopic Study

Four lidocaine molecular salts of dicarboxylic acids (oxalic, fumaric, malonic, and succinic) were synthesized and characterized by a combined use of X-ray powder and single-crystal diffraction, differential scanning calorimetry, Fourier transform infrared spectroscopy (FT-IR), and solid-state NMR (1H MAS and CRAMPS, and 13C and 15N CPMAS): all molecular salts show a dramatic increase in their melting point with respect to both lidocaine and lidocaine hydrochloride, and a higher dissolution rate and thermodynamic solubility in physiological solution with respect to the free base.

From molecular crystals to salt co-crystals of barbituric acid via the carbonate ion and an improvement of the solid state properties

Co-crystals formed by neutral barbituric acid (HBA) molecules and alkaline barbiturate salts have been prepared by solvent-free mechanochemical treatment of solid HBA and alkaline carbonates or bicarbonates. During the reaction the carbonate salts release CO2 and H2O, affording the corresponding barbiturate salts which then co-crystallize with the neutral molecule. These “salt co-crystals” have the general formula Mx+(BA−)x·HBA·nH2O with (M = Na and K). The compounds can be obtained in a single step, by directly grinding HBA with M2CO3 (4 : 1 ratio) or MHCO3 (2 : 1 ratio), or, alternatively, in two steps: the preparation of the BA salt followed by a further grinding process or crystallization of the resulting salts with a stoichiometric amount of neutral HBA. All compounds have been characterized by means of single-crystal and powder XRD, solid-state NMR (1H MAS, 13C and 15N CPMAS), vibrational spectroscopies and thermal methods (DSC and TGA). A series of BA− salts (either hydrated or anhydrous) of formula Mx+(BA−)x·nH2O (M = Na, K, Mg and Ca) have also been prepared and reported for comparison.

1H MAS NMRからみた高温におけるゼオライトの水酸基の性質と酸触媒特性

1H MAS NMRからみた高温におけるゼオライトの水酸基の性質と酸触媒特性

1H MAS, 13C CP/MAS, and 2H NMR spectra studies of piperidinium ${\emph{p}}$ -chlorobenzoate

Anomalous H/D isotope effects were detected in the 1H MAS NMR spectra of piperidinium p-chlorobenzoate (C5H10NH $_{2}{^{+}}\cdot $ ClC6H4COO − ) upon deuterium substitution of hydrogen atoms which form two kinds of N-H⋯O H-bonds in the crystal; in contrast to these spectra, only slight chemical shifts were recorded in 13C CP/MAS NMR spectra. 2H NMR spectrum of the deuterated sample show quadrupole coupling constants of 148 and 108 kHz, and reveal that there are a few motions contributing to the electric-field modulation of the 2H nucleus. The 1H MAS NMR spectra of piperidinium p-chlrobenzoate-d 16 (C5D10ND $_{2}{^{+}}\cdot $ ClC6D4COO − ) and -d 14 (C5D10NH $_{2}{^{+}}\cdot $ ClC6D4COO − ) revealed that the change in the envelope is caused by chemical shifts of each signal upon deuteration. Calculations based on the density-functional-theory showed that the N-H distance along the crystallographic a-axis mainly contributes to the anomalous isotope effects on 1H MAS NMR envelopes.

Solid State NMR Study of Thermal Processes in Nanoassemblies Formed by Dipeptides

Three linear dipeptides Phe-Phe (FF), Tyr-Ala (YA) and Asp-Phe(OMe) (DF-OMe, also known as aspartame) were investigated via solid state (SS) NMR spectroscopy, differential scanning calorimetry (DSC), mass spectrometry, and scanning electron microscopy (SEM). Both 1D and 2D SS NMR techniques (1H MAS, 13C CP/MAS, and 1H–13C inverse HETCOR ultrafast MAS) were used to study the thermal stability and chemical processes of the self-assembled structures: peptide nanotubes (PNTs) and peptide nanowires (PNWs). Each of the investigated dipeptides underwent thermal rearrangement to cyclic dipeptides, also known as diketopiperazines (DKP). Employment of variable temperature (VT) 13C NMR measurements revealed that the cyclization of Phe-Phe (FF) PNT began at a temperature of 373 K, which is lower than the temperature reported previously. The process to form FF DKP would be anticipated to occur because of the removal of water from the hydrophilic channel of the PNTs. When FF PNT is thermally treated carefully and the subtle nanostructure is not damaged, the empty channel can be refilled with water during the diffusion process. An analysis of the thermal stability of YA dipeptide revealed that, as in case of FF, a synthesis of YA DKP is a facile process and can be performed in NMR rotor. YA DKP forms PNTs, which are more thermally stable than FF PNTs. Finally, aspartame forms fibrils and peptide nanowires, which is particulary important because it is commonly applied in the food industry.

Hydrogen H/D Exchange and Activation of C1–n-C4 Alkanes on Ga-Modified Zeolite BEA Studied with 1H Magic Angle Spinning Nuclear Magnetic Resonance in Situ

Kinetics of H/D hydrogen exchange between C1–n-C4 alkanes and Bronsted acid sites (BAS) of both the pure acid-form zeolite BEA (H-BEA) and Ga-modified zeolite BEA (Ga/H-BEA) was monitored by 1H MAS...

ChemInform Abstract: Variable Temperature 1H MAS NMR: A Powerful Tool for the Investigation of Dynamic Properties of Acidic Protons in Zeolites and Heteropoly Compounds

The dynamic nature of the protons in zeolites such as H-ZSM-5, metallosilicates molecular sieves and heteropoly compounds was examined by temperature dependence of 1 H MAS NMR in the range of 298 and 473K. The temperature-dependent lineshape of 1 H MAS NMR spectra of acidic protons in zeolites showed the first and unambiguous evidence for proton migration in zeolite structure. The dynamic property of protons was a more sensitive measure for the activity of protons than the static parameters such as NMR chemical shifi and I R band position of OH bonds. The temperature-dependent 1 H MAS NMR also revealed the mobility of protons in heteropoly compounds (H 3 PW I2 O 40 Ag 3 PW 12 O 40 and palladium metal loaded on H 3 PW 12 O 40 ). This method gave the information of the dynamical process between protons and hydrogen molecules in the gas phase. The variable temperature 1 H MAS NMR is a very useful technique for understanding dynamic properties of protons in solid acids The dynamic property is an important factor infiuencing the catalytic activity of a solid acid catalyst. The various experimental facts described above show that the temperature variable 1 H MAS NMR could reveal the dynamics of protons in solid acids. Information obtained by this technique offers new insights, which could not be obtained by other means, on the acidic properties of solid acids and their catalysis. The significance of proton mobility in catalysts will be able to be discussed in the future.

Crystalline Aluminum Hydroxide Fluorides AlFx(OH)3−x·H2O: Structural Insights from 1H and 2H Solid State NMR and Vibrational Spectroscopy

For the first time, 1H/2H MAS NMR signals of crystalline hydroxide fluorides AlFx(OH)3−x·H2O, as well as of the dehydrated samples, both with pyrochlore structure, were resolved, identified, and assigned in direct correlation with vibrational bands of respective FT IR spectra. The use of magnetically diluted samples in combination with 1H spin−echo experiments, 2H MAS, and 19F−2H CP and 1H−2H CP MAS NMR experiments gave information on different 2H (1H) sites in relation to present structural motifs known from the crystal structure.

H/D isotope effect of 1H MAS NMR spectra and 79Br NQR frequencies of piperidinium p-bromobenzoate and pyrrolidinium p-bromobenzoate

H/D isotope effects onto 79Br NQR frequencies of piperidinium p-bromobenzoate were studied by deuterium substitution of hydrogen atoms which form two kinds of N–H⋯O type hydrogen bonds, and the isotope shift of ca. 100 kHz were detected for a whole observed temperature range. In addition, 1H MAS NMR spectra measurements of piperidinium and pyrrolidinium p-bromobenzoate were carried out and little isotope changes of NMR line shape were detected. In order to reveal effects of molecular arrangements into the obtained isotope shift of NQR frequencies, single-crystal X-ray measurement of piperidinium p-bromobenzoate-d2 and density-functional-theory calculation were carried out. Our estimation showed the dihedral-angle change between piperidine and benzene ring contributes to isotope shift rather than those of N–H lengths by deuterium substitution.

1H MAS NMR characterization of hydrous species in volcanic glasses from the Phlegrean Volcanic District (South Italy)

1H MAS NMR characterization of hydrous species in volcanic glasses from the Phlegrean Volcanic District (South Italy)

Dialkylation of Naphthalene with Isopropanol Over H3PO4/MCM-41 Catalysts for the Environmentally Friendly Synthesis of 2,6-Dialkylnaphthalene

AlMCM-41 materials with SiO2/Al2O3 molar ratios 20, 70, 110, 150, 200, and Si-MCM-41 were synthesized following standard procedures, and loaded with different amounts of H3PO4. The catalysts were well characterized by powder X-ray diffraction (XRD), nitrogen adsorption studies, and solid state 29Si, 27Al, 31P, 23Na and 1H MAS NMR spectroscopy. Acidity measurement by FT-IR spectroscopy monitoring of pyridine adsorption reveals that the incorporation of Al in the framework generates Bronsted and Lewis acidity, which increases with the increase in metal content. By loading H3PO4 on the AlMCM-41, Bronsted sites increases substantially, but the Lewis sites decrease to a large extent. Liquid phase isopropylation of naphthalene with isopropanol in n-hexane under autogeneous pressure was performed in the temperature range 473–623 K. The optimum feed molar ratio was found to be 1:2:10 (naphthalene:isopropanol:n-hexane), where the naphthalene conversion reaches to 85.5%. The β-and β,β-selectivities over 30 wt% H3PO4/AlMCM-41(200) were 97.0 and 84.8, respectively, at 85.5% conversion. The main products of the reaction, mono and diisopropylnaphthalenes, were analyzed and identified by gas chromatography and confirmed by GC–MS. The conversion and selectivity of the products are discussed from the point of view of catalyst characteristics and reaction conditions.

Highly Fluorinated Silica Obtained by Direct F2-Gas Fluorination: Stability and Unprecedented Fluorosilicate Species Revealed by Solid State NMR Investigations

The direct F2-gas fluorination of mesoporous silica is a unique method leading to high fluorinated (up to 13 wt % F) and homogeneous powder with a controlled amount of grafted fluorine. Thermogravimetric analysis coupled with mass spectrometry for water, hydroxyl, and fluorine groups have allowed concluding that low fluorine-grafted silicas are thermically stable up to 550 °C whereas high fluorine-grafted silicas start to decompose from 250 °C with departure of SiF4 species. Water adsorption measurements have demonstrated the hydrophobic character of fluorinated silica and proved that direct fluorination is a way to control the hydrophilic−hydrophobic balance of silica. Pristine and fluorinated silicas have been studied by 19F and 1H MAS, and 1H−29Si and 19F−29Si CP-MAS NMR spectroscopies. NMR measurements have revealed various tetrahedral (O2/2SiF2, O3/2SiF) and pentahedral (O4/2SiF) fluorosilicate species previously observed in moderately fluorinated silica, and two unprecedented pentahedral species occ...

Surface Organometallic Chemistry of Titanium on Silica−Alumina and Catalytic Hydrogenolysis of Waxes at Low Temperature

Ti(CH2tBu)4 (1) reacts selectively with the surface silanols of a silica−alumina partially dehydroxylated at 500 °C to provide the monosiloxy species [(≡SiO)Ti(CH2tBu)3]SA (2a) and the bisiloxy species [(≡SiO)2Ti(CH2tBu)2]SA (2b) in a ca. 40:60 ratio, with concomitant evolution of 1.6 ± 0.2 equiv of neopentane per Ti. These surface complexes were characterized via the combined use of several techniques such as IR spectroscopy, 1H MAS, 13C-CP/MAS, 2D 1H−13C HETCOR, and J-resolved solid-state NMR, as well as mass balance analysis. By treatment under hydrogen at 150 °C the neopentyl ligands in complexes 2a,b undergo hydrogenolysis and a mixture of supported titanium species is obtained. IR, ESR, 1H MAS, and DQ solid-state NMR spectroscopies show the presence of ca. 3% [(≡SiO)(MsO)TiH2]SA (3a; Ms = Si, Al), 5% [(≡SiO)(MsO)Ti(Me)−H]SA (3b), 75−80% [(≡SiO)(MsO)2Ti−H]SA (3c), and 14% [(≡SiO)(MsO)2TiIII]SA (3d), along with (SiHx) and (AlHx) fragments whose formation arise from the opening of adjacent Si−O−M bridg...