New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

Synthesis and Characterization of Water Soluble Gold Nanoclusters of Varied Core Size

A series of water soluble gold nanoclusters of variable core size were prepared and characterized. The clusters were synthesized using a ligand substitution reaction between CH₃(OCH₂CH₂)₃SH and hexanethiol encapsulated gold nanoclusters, and characterized using NMR, FTIR, and UV/Vis spectroscopy, as well as TGA, DSC, and TEM. Conductivity values were obtained for the clusters and increased as expected along with the gold core size. The substitution reaction used in the synthesis was followed by ¹H NMR and found to produce only partial ligand substitution after 24 h, consistent with solubility observations and necessitating use of a two-step preparation scheme.     

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Summary Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.²⁹Si and¹³C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.     

Application of cationic polymerization to grafting and coating of silica particles

The cationic polymerization of electron rich monomers such as vinyl ethers, vinyl furane, and cyclopentadiene on silica surfaces can be initiated by aryl methyl halides. The reactions yield always soluble polymers (by heterogeneous catalysis) and novel polymer/silica hybrid materials. The link between polymer and solid is caused by covalent Si-O-C bonds, by network formation of the polymers during the chain growth, or by a combination of both of them. The analysis of the polymer structures on the surface by ¹H MAS NMR spectroscopy in suspension and by solid state ¹³C CP MAS NMR spectroscopy is described. Proof of Si-O-C bonds via DRIFT spectroscopy and ¹³C CP MAS NMR spectroscopy is given. The most effective method of irreversibly linking the polymer to the silica surface is the network formation. Polyvinyl ethers are bound strongly to the surface, as can be shown by FTIR measurements, but the linkage is not stable due to the Si-O-C bonds' susceptibility to hydrolysis. Poly-cyclopentadienes (PCPD) are linked to the surface by Si-O-C bonds, which show an extraordinary high resistance to acids and bases. Si-O-C bond formation of poly-2-vinyl furane could not yet be detected by ¹³C CP MAS NMR spectroscopy and DRIFT spectroscopy. In this case the high degree of coating derives from the bifunctionality of 2-vinyl furane: it may undergo Friedel-Crafts-alkylation at the 5-position of the furane ring as well as chain polymerization via the vinyl group at the 2-position.     

Fingerprints of Phalaenopsis Tissues in Growth and Spike Induction Periods—A Solid‐state 13C NMR Approach

Solid‐state Nuclear Magnetic Resonance (ss‐NMR) ¹³C single‐pulse excitation spectroscopy in combination with the magic‐angle spinning (MAS) technique was applied to a series of Phalaenopsis tissues, including the leaf, sheath, stem, and root, at different growth and spiking periods. Compared with{¹H}/¹³C cross‐polarization MAS spectra, the ¹³C single‐pulse excitation MAS spectra displayed very distinct spectral patterns, recognizable as fingerprints of the tissues studied. ¹Here, we demonstrate that solid‐state ¹³C single‐pulse excitation NMR spectroscopy provides a direct and robust analytical tool for studying the various tissues of Phalaenopsis in different growth and spiking induction periods.     

Substitution clustering in a non-stoichiometric celsian synthesized by the thermal transformation of barium exchanged zeolite X

The thermal transformation of Ba exchanged zeolite X to celsian has been studied by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the ²⁹Si NMR spectra after thermal treatment at 850 °C. Confirmation is provided by the ²⁹Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed ²⁹Si chemical shifts. The ²⁷Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian.     

Solid State NMR Characterisation of Encapsulated Molecules in Mesoporous Silica

Ibuprofen molecules have been encapsulated in mesoporous MCM-41 type-silica functionalised or not by amino groups. They have been characterised by ¹³C and ¹H solid state NMR spectroscopy. The ¹³C MAS single pulse or cross polarization NMR spectra, as well as the ¹H MAS NMR spectra demonstrate an extremely high mobility of the ibuprofen molecules when the matrix is not functionalised. On the contrary, when the silica matrix is functionalized by amino groups, the ¹³C NMR response shows less mobility suggesting the existence of interactions between the amino groups and the carboxylic groups. Benzoic acid as well as benzamide have also been encapsulated and their NMR responses compared to that of ibuprofen.     

Ammonolysis of 1,2‐Bis[dichloro(methyl)silyl]ethane. On the Structure of 1,3,6,8‐Tetramethyl‐2,7,11,12‐tetraaza‐1,3,6,8‐tetrasila‐tricyclo[6.2.1.13,6]‐dodecane

Ammonolysis of 1,2‐bis[dichloro(methyl)silyl]ethane afforded a crystalline tricyclic silazane along with polymeric material. The crystalline material could be isolated in pure state. It was analyzed by ¹H, ¹³C, ¹⁵N and ²⁹Si NMR spectroscopy in solution, by ¹³C, ¹⁵N and ²⁹Si MAS NMR spectroscopy in the solid state, as well as by single‐crystal and powder X‐ray diffraction. The title compound exists as a single isomer in solution, whereas in the solid state the presence of several modifications is indicated, in particular by the solid‐state MAS NMR spectra.     

Charged Si9 Clusters in Neat Solids and the Detection of [H2Si9]2− in Solution: A Combined NMR,Raman, Mass Spectrometric,and Quantum Chemical Investigation

Polyanionic silicon clusters are provided by the Zintl phases K₄Si₄, comprising [Si₄]⁴⁻ units, and K₁₂Si₁₇, consisting of [Si₄]⁴⁻ and [Si₉]⁴⁻ clusters. A combination of solid‐state MAS‐NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum‐chemical investigations was used to investigate four‐ and nine‐atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹Si isotope‐enriched samples. ²⁹Si‐MAS NMR and Raman shifts of the phase‐pure solids K₄Si₄ and K₁₂Si₁₇ were interpreted by quantum‐chemical calculations. Extraction of [Si₉]⁴⁻ clusters from K₁₂Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹Si‐MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹Si NMR in solution. ²⁹Si and ¹H NMR spectra reveal the presence of the [H₂Si₉]²⁻ cluster anion in solution.     

Charged Si9 Clusters in Neat Solids and the Detection of [H2Si9]2− in Solution: A Combined NMR,Raman, Mass Spectrometric,and Quantum Chemical Investigation

Polyanionic silicon clusters are provided by the Zintl phases K₄Si₄, comprising [Si₄]⁴⁻ units, and K₁₂Si₁₇, consisting of [Si₄]⁴⁻ and [Si₉]⁴⁻ clusters. A combination of solid‐state MAS‐NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum‐chemical investigations was used to investigate four‐ and nine‐atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹Si isotope‐enriched samples. ²⁹Si‐MAS NMR and Raman shifts of the phase‐pure solids K₄Si₄ and K₁₂Si₁₇ were interpreted by quantum‐chemical calculations. Extraction of [Si₉]⁴⁻ clusters from K₁₂Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹Si‐MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹Si NMR in solution. ²⁹Si and ¹H NMR spectra reveal the presence of the [H₂Si₉]²⁻ cluster anion in solution.     

Synthesis of Mesoporous Silica Immobilized with 3-[(Mercapto or amino)propyl]trialkoxysilane by a Simple One-pot Reaction

Sodium silicate from rice husk ash (RHA) was transformed to functionalized silica with 3‐(mercaptopropyl)‐trimethoxysilane (MPTMS) or 3‐(aminopropyl)triethoxysilane (APTES) via a simple sol‐gel technique in a one‐pot synthesis to give RHAPrSH and RHAPrNH₂. The ²⁹Si MAS NMR of RHAPrSH and RHAPrNH₂ showed the presence of T¹, T², T³, Q², Q³ and Q⁴ silicon centers. The ¹³C MAS NMR showed that RHAPrSH had chemical shifts at δ 16.59, 32.73, consistent with two of the carbon atoms of the MPTMS moiety, while the ¹³C MAS NMR of RHAPrNH₂ had chemical shifts at δ 14.58, 26.13, 47.87, consistent with the three carbon atoms of the APTES moiety. The presence of carbon, silicon, sulfur and nitrogen in RHAPrSH and RHAPrNH₂ was determined by a combination of elemental analysis and EDX study.     

The structure of phosphate and borosilicate glasses and their structural evolution at high temperatures as studied with solid state NMR spectroscopy: Phase separation,crystallisation and dynamic species exchange

In this contribution we present an in-depth study of the network structure of different phosphate based and borosilicate glasses and its evolution at high temperatures. Employing a range of advanced solid state NMR methodologies, complemented by the results of XPS, the structural motifs on short and intermediate length scales are identified. For the phosphate based glasses, at temperatures above the glass transition temperature T_g, structural relaxation processes and the devitrification of the glasses were monitored in situ employing MAS NMR spectroscopy and X-ray diffraction. Dynamic species exchange involving rapid P–O–P and P–O–Al bond breaking and reforming was observed employing in situ ²⁷Al and ³¹P MAS NMR spectroscopy and could be linked to viscous flow. For the borosilicate glasses, an atomic scale investigation of the phase separation processes was possible in a combined effort of ex situ NMR studies on glass samples with different thermal histories and in situ NMR studies using high temperature MAS NMR spectroscopy including ¹¹B MAS, ²⁹Si MAS and in situ ²⁹Si{¹¹B} REAPDOR NMR spectroscopy.     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Preparation of a liquid boron‐modified polycarbosilane and its ceramic conversion to dense SiC ceramics

A boron‐modified ethynylhydridopolycarbosilane (B‐EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9‐borabicyclo‐[3.3.1]nonane (9‐BBN). The as‐synthesized B‐EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B‐EHPCS was investigated by solid‐state NMR. The ¹³C magic angle spinning (MAS) NMR results indicated that the C?C and C?C groups of B‐EHPCS take part in the hydrosilation cross‐linking at a relatively low temperature (170°C). According to the ²⁹Si MAS NMR analysis, the CSiH₃ end groups are most reactive hydride functionality involved in the hydrosilation cross‐linking. With increasing curing temperature, the C₂SiH₂ and CSiH₃ units are completely consumed, while C₃SiH units remain even after curing at 600°C. The TGA results show the 1200°C ceramic yield of B‐EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.     

Modulation of the Activity of Gold Clusters Immobilized on Functionalized Mesoporous Materials in the Oxidation of Cyclohexene via the Functional Group. The Case of Aminopropyl Moiety

Gold nanoclusters and isolated gold atoms have been produced in a two-liquid phase procedure that involves a solution of gold in aqua regia and rosemary essential oil as organic layer. These gold entities have been immobilized on the ordered mesoporous silica material SBA-15 functionalized with different amounts of aminopropyl groups. The resulting materials have been characterized by XRD, N₂ adsorption, chemical analysis, TGA, ²⁹Si MAS NMR, ¹³C CP/MAS NMR, UV-vis spectroscopy, XPS, and STEM. The Au-containing materials retain the ordering and porosity of the pristine support. Gold content varies in the range of 0.07–0.7 wt% as a function of the specific immobilization conditions, while STEM evidences the presence of isolated gold atoms. XPS shows a shift of the Au 4f BE toward values lower than those of metallic gold. The catalytic activity in the oxidation of cyclohexene with molecular oxygen at atmospheric pressure parallels the Au content of the aminopropyl-SBA-15 supports. This activity is higher than that of analogous Au entities immobilized on SBA-15 functionalized with thiol or sulfonate groups, the activity decreasing in the order Au-NH₂ > Au-SO₃⁻ > Au-SH. This behavior has been attributed to differences in the interaction strength between the functional group and the Au entities, which is optimum for the aminopropyl groups.     

Vinylimidazole copolymers: coordination chemistry,solubility, and cross-linking as function of Cu<Superscript>2+</Superscript> and Zn<Superscript>2+</Superscript> complexation

P(1-VIm-co-MMA) copolymers with 4 or 44 wt.% 1-VIm (abbreviated PVM-4 and PVM-44) where polymerized from 1-VIm (1-vinylimidazole) and methylmethacrylate with azobisisobutyronitrile as initiator and reacted with either Cu²⁺ or Zn²⁺. The resulting coordinated polymer complexes were studied using ICP-AES, CP/MAS ¹³C NMR, conductivity measurements, vibrational spectroscopy (mid-FTIR and far-FTIR), DSC, and EPR. It was established by ICP-AES, CP/MAS ¹³C NMR, conductivity, mid-FTIR and EPR measurements that the transition metal ions in the complexes were exclusively coordinated by the imidazole ligand. The coordination geometry is square planar with regard to Cu(II) complexes. The strong interaction between the polymeric imidazole ligand and the transition metal ion cross-links the system, resulting in augmentation of T _g (the glass transition temperature), especially for copolymers with high relative amount of 1-VIm. The effect of changing metal ion is more complicated and depends on both the strength of the coordinate interaction as well as the coordination number. The solubility of the coordinate polymer complex in conventional solvents is low due to the coordinate cross-links. However, the coordinate polymer complexes are soluble in strongly coordinating solvents such as acetonitrile and dimethylsulfoxide.     

Pre-irradiation induced grafting of styrene into crosslinked and non-crosslinked polytetrafluoroethylene films for polymer electrolyte fuel cell applications. II: Characterization of the styrene grafted films

Crosslinked and non-crosslinked polytetrafluoroethylene films (RX-PTFE and V-PTFE films, respectively) were irradiated by γ-ray and then grafted with styrene in liquid phase. Microscope FT-IR spectroscopy, TGA, solid state ¹³C CP/MAS and high resolution HS/MAS NMR spectroscopy, wide-angle X-ray diffraction (WAXD) study were used to get the structural information of the styrene grafted RX-PTFE and V-PTFE films. From microscope FT-IR spectra of the grafted RX-PTFE films, the “grafting front mechanism” was proved. TGA analysis showed that the grafted films have a small degradation step and two main degradation steps. In the ¹³C CP/MAS NMR spectra of the non-grafted films, there are no signal due to the absence of the hydrogen atom. While in the spectra of the grafted films, there are signals attributed to the polystyrene grafts. In the ¹³C HS/MAS NMR spectra of the grafted films, the relative intensity of the peaks attributed to the polystyrene grafts increased while the relative intensity of the peak attributed to PTFE matrix decreased with the increase in the DOG. From WAXD patterns, the intensity of the crystalline peak decrease with the increase in the DOG. The grafted films were sulfonated by chlorosulfonic acid and the results of highest IEC value exceeded 3.0. Those results will be reported in the near future.     

Mechanism Studies of the Conversion of 13C‐Labeled n‐Butane on Zeolite H‐ZSM‐5 by Using 13C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis

By using ¹³C MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively ¹³C‐labeled n‐butane on zeolite H‐ZSM‐5 at 430–470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective ¹³C‐label in the n‐butane molecule, and 2) oligomerization–cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl‐substituted cyclopentenyl cations and condensed aromatic compounds. In situ ¹³C MAS NMR and complementary ex situ GC–MS data provided evidence for a monomolecular mechanism of the ¹³C‐label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to ¹³C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E_a=75 kJ mol^?1 for the scrambling and 71 kJ mol^?1 for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n‐butane as being the rate‐determining stage of the n‐butane conversion on zeolite H‐ZSM‐5.     

Reactivity of Extra‐framework Species of USY Zeolites in Alkaline Medium

Zeolites of type USY (ultra‐stable Y) were obtained by steaming of NH₄NaY modification. Samples were modified by subsequent alkaline treatment in KOH solution. USY and USY‐KOH were characterised by chemical element analysis, XRD, IR, ²⁹Al and ²⁹Si MAS NMR spectroscopic measurements. Correct silicon to aluminium ratios (Si/Al) were determined by XRD and IR (double ring vibration w_DR) data whereas values calculated according to data of ²⁹Si MAS NMR and IR spectroscopy (asymmetrical TOT valence vibration w_TOT) appeared to be too high., In the latter case, the signals of the zeolite framework were strongly superimposed by that of extra‐framework silica gel (EFSi) formed during steaming. It was found that alkaline leaching induces desilication of silicon‐rich area of the zeolite framework and partial dissolution of EFSi. Silicate ions of both react with likewise dissolved extra‐framework aluminium (EFAl) to form X‐ray amorphous aluminosilicate. Consequently, the superposition of the ²⁹Si MAS NMR signals of the zeolite framework by silica gel was reduced for Q⁴(0Al) but increased for Q⁴ (2Al) and Q⁴(3Al) structure units. A reinsertion of EFAl into the zeolite framework has not been observed.     

Accurate Backbone 13C and 15N Chemical Shift Tensors in Galectin-3 Determined by MAS NMR and QM/MM: Details of Structure and Environment Matter

Chemical shift tensors obtained from solid-state NMR spectroscopy are very sensitive reporters of structure and dynamics in proteins. While accurate ¹³C and ¹⁵N chemical shift tensors are accessible by magic angle spinning (MAS) NMR, their quantum mechanical calculations remain challenging, particularly for ¹⁵N atoms. Here we compare experimentally determined backbone ¹³C^α and ¹⁵N^H chemical shift tensors by MAS NMR with hybrid quantum mechanics/molecular mechanics/molecular dynamics (MD-QM/MM) calculations for the carbohydrate-binding domain of galectin-3. Excellent agreement between experimental and computed ¹⁵N^H chemical shift anisotropy values was obtained using the Amber ff15ipq force field when solvent dynamics was taken into account in the calculation. Our results establish important benchmark conditions for improving the accuracy of chemical shift calculations in proteins and may aid in the validation of protein structure models derived by MAS NMR.