Hydridosilazanes hydrolysis-condensation reactions studied by <Superscript>1</Superscript>H and <Superscript>29</Superscript>Si liquid NMR spectroscopy

Hydridosilazane compounds containing Si–N and Si–H bonds can be used as precursors of SiO_x materials. The hydrolysis-condensation reactions of tetramethyldisilazane, as a polyhydridosilazane model compound, were investigated by ¹H and ²⁹Si liquid NMR spectroscopy. These reactions were carried out at room temperature for up to 120 min in presence of water. The identified products are short linear siloxane species (hydride terminated polydimethylsiloxanes M^HD_xM^H) and cyclosiloxanes. Silicon hydride persistence in the reactional mixture suggested that silazane group is more sensitive to hydrolysis reaction than silicon hydride group. Moreover, additional experiments evidenced that the low steric hindrance of the silicon hydride influences the silazane hydrolysis kinetic. Hence the presence of ammonia released during silazane hydrolysis reaction was demonstrated to be a catalyst of the silicon hydride hydrolysis reaction.     

Vibrational and 29Si NMR spectroscopies of soluble silicate oligomers

Vibrational spectroscopy has been used to correlate the features of silicate solution spectra with ²⁹Si NMR spectra, as a function of SiO₂:Na₂O ratio and SiO₂ concentration. Assignments of infrared and Raman component bands have been made for different anion types and are consistent with results reported for crystalline silicates and silicate glasses. The extent of depolymerization of larger anions has been found to differ, depending on both the degree of dilution and alkalinity. The relative amounts of polymeric (Q⁴) silica and larger three dimensional (Q³_4R) anions are greater for the more siliceous solutions (3.3 and 2.95 ratio) at high concentrations. These large anions continue to be major contributors to the anion distribution even at 15 to 50 fold dilution. In the case of more alkaline solutions, especially below 2.5 ratio, the relative contribution of the large anions to the distribution drops rapidly, even with 2 or 3 fold dilution. At low concentrations, in very alkaline solutions, the anion distribution is mainly monomer, dimer and cyclic trimer. Vibrational spectroscopies show great promise for observing changes in anion distribution on a much shorter time scale than is possible with ²⁹Si NMR and can be useful for following reactions involving silicate solutions.     

State of silicon in silicate and silica-containing solutions and their binding properties

Relationship between the distribution of silicon atoms over connectivities in concentrated silicate and silica-containing alkaline solutions differing in chemical composition and preparation methods, on the one hand, and the binding properties of these solutions, on the other, was studied by ²⁹Si NMR spectroscopy.     

Aluminum-27 NMR Investigations of Aqueous and Methanolic Aluminosilicate Solutions

Aluminum-27 NMR spectroscopy was used to characterize aqueous and methanolic alkaline solutions of tetramethylammonium (TMA) aluminosilicates. Aluminosilicate solutions have been prepared with different concentrations of silicon [0.577–1.24% (w/w)], aluminum [0.0022–0.239% (w/w)], methanol [0.0–0.70% (w/w)] and H₂O [0.23–90% (w/w)]. All solutions contain the same ratio of Si/TMA = 1 and Si/Al molar ratios between 0.5 and 25.²⁷Al NMR spectra of TMA aluminosilicate solutions are characterized by a variety of aluminosilicate species such as q¹(Al1OSi), q²(Al2OSi), q³(Al3OSi) and q⁴(Al4OSi). Aluminum-27 NMR spectra of TMA aluminosilicate solutions indicate that considerable changes occurred by changing the Si/Al ratio. The distribution of aluminosilicate species was affected by the presence of the methanol and the method of mixing the silicate and aluminosilicate solutions. A methanolic aluminosilicate solution needs about twice the time required for an aqueous aluminosilicate solution to reach a steady state, i.e., the latter takes 36 h to reach steady state. Results with the same concentration of silicon and aluminum show that the formation and distribution of aluminosilicate species are strongly dependent on the solvent comprising the silicate and aluminate solutions.     

Aluminum-27 NMR studies of alcoholic (2-hydroxyethyl)trimethylammonium aluminate solutions

²⁷Al NMR spectroscopy is a power tool for investigation of the aluminate species existing in both aqueous and non-aqueous solutions. Aluminum-27 nuclear magnetic resonance (NMR) spectroscopy also can be used to determine thermodynamic properties of complexes in the solution. In this report, ²⁷Al NMR spectroscopy was used to characterize species present in alkaline alcoholic aluminate solutions. (2-Hydroxyethyl)trimethylammonium (2-EHTMA) hydroxide was used as base. In solution of CH₃OH and H₂O in a mole ratio of 64:1 it was possible to detect five signals by aluminum-27 NMR, indicating formation of [Al(OH)_4−n(CH₃OH)_n]⁽ⁿ⁻¹⁾⁺ (n = 0,1, 2, 3 and 4) species. Aluminum-27 NMR spectroscopy has also used for investigation of the species present in the ethanolic 2-HETMA aluminate solutions. The equilibrium constants for the formation of aluminate complexes were also determined for both methanolic and ethanolic aluminate solutions. Aluminum-27 NMR spectra of propanolic and butanolic 2-HETMA aluminate solutions were also studied.     

Hydrolysis behavior of a precursor for bridged polysilsesquioxane 1,4-bis(triethoxysilyl)benzene: a <Superscript>29</Superscript>Si NMR study

The hydrolysis behavior of 1,4-bis(triethoxysilyl)benzene (BTB), a precursor of bridged polysilsesquioxane, was investigated with high-resolution ²⁹Si nuclear magnetic resonance (²⁹Si NMR) spectroscopy at ambient temperature in a system with BTB:ethanol:water:HCl = 1:10:x:0.8 × 10⁻⁴ (x = 3, 6 or 9). Signals due to hydrolyzed triethoxysilyl groups as well as unhydrolyzed triethoxysilyl groups [−Si(OEt)₃, −Si(OEt)₂(OH), −Si(OEt)(OH)₂ and −Si(OH)₃ (OEt = OCH₂CH₃)] formed four sub-regions based on the number of hydroxyl groups bound to a silicon atom. In addition, one silicon environment influenced the other silicon environment by an intra-molecular interaction between two silicon atoms, and each sub-region for monomeric species thus contained four signals. Based on the development of signal intensity, it is revealed that one of the two triethoxysilyl groups in BTB is hydrolyzed preferentially. Thus, when a triethoxysilyl group is hydrolyzed, the −Si(OH) _x (OEt)_3−x (x = 1, 2) groups formed undergo further hydrolysis, which is opposite to the tendency expected from the hydrolysis behavior of organotrialkoxysilanes under acidic conditions.     

Mechanochemical Changes in the Structure of Layered Silicates according to <Superscript>27</Superscript>Al and <Superscript>29</Superscript>Si NMR Data

Mechanochemical structural changes in layered silicates were studied by ²⁷Al and ²⁹Si NMR spectroscopy.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 2, 2005, p. 344.Original Russian Text Copyright © 2005 by Pashchenko.     

Aluminum Solvate Complexes Forming in Acidic Methanol-Acetone Mixtures Studied by <Superscript>27</Superscript>Al NMR Spectroscopy

²⁷Al NMR spectroscopy is a powerful tool for the study of coordination and solvation in both aqueous and nonaqueous solutions. In this study, the complexes coexisting upon dissolution of AlCl₃ in acidic acetone + methanol solutions are shown to consist essentially of mixed hexacoordinated species of the general formula [Al(CH₃OH)_6−n (CH₃COCH₃) _n ]³⁺ (n=1,2 and 3), all exhibiting distinctly different ²⁷Al shielding effects. The relative populations of the various mixed species are found to be highly dependent upon the acetone:methanol mole ratio that in the more acetone-rich mixtures with aluminum become appreciably coordinated by acetone. The results demonstrate that the key factor for the formation of acetone-containing species in acidic methanolic solutions is having the CH₃COCH₃:CH₃OH mole ratio at 3:1.     

Silicon-29 NMR studies of aqueous silicate solutions: Part II. Isotopic enrichment

Silicon-29 NMR spectra have been recorded for aqueous solutions of sodium silicate in the alkaline pH range using silica enriched in the ²⁹Si isotope. Work at low concentration shows that essentially only monomeric orthosilicate ions are present at ca. 0.01 M in silica. At higher concentrations, peaks which lack fine structure indicate species whose silicon nuclei are all equivalent. The experiments have assisted in the difficult process of assigning the spectra of silicate solutions.     

Investigation of the structural directing effect of (2-hydroxyethyl)trimethylammonium on distribution of the silicate anions using 29Si NMR spectroscopy

The effect of (2-hydroxyethyl)trimethylammonium (2-HETMA) cation on the equilibrium of silicate oligomers in aqueous alkaline silicate solutions was investigated using (29)Si NMR spectra. The results indicate role of structural directing of 2-HETMA in which it particularly directs the silicate species to the Q(4)(1)Q(4)(3)Q(4)(4) silicate anion. Results reveal that composition of the alcohols in solution affect the distribution of anionic species. The effect of methanol concentration is also discussed.     

Structure of binuclear platinum(III) acetamidate complexes in solutions as probed by <Superscript>195</Superscript>Pt, <Superscript>13</Superscript>C,and <Superscript>1</Superscript>H NMR spectroscopy

¹⁹⁵Pt, ¹H, and ¹³C NMR spectroscopy was used to study the structure of binuclear platinum(III) acetamidate complexes with 1,10-phenanthroline and 2,2′-bipyridine ligands [Pt₂(phen)₂(acam)₄](NO₃)₂ (1) and [Pt₂(bipy)₂(acam)₄](NO₃)₂ (2) in aqueous solutions. The ¹⁹⁵Pt NMR spectra of solutions of complexes 1 and 2 in D₂O exhibit two signals with satellites due to the ¹⁹⁵Pt–¹⁹⁵Pt spin-spin coupling (¹ J(Pt–Pt) ≈ 6345 Hz), whereas their ¹H and ¹³C NMR spectra contain four sets of signals for the protons and the carbon atoms of the heterocyclic and acetamidate ligands. The signals were assigned using the COSY, NOESY, and HSQC/ HMBC experiments and comparing the coordination shifts of the signals for the protons of heterocycles. These data allowed us to draw a conclusion that binuclear complexes 1 and 2 in solution have a head-to-head structure with nonequivalent platinum(III) atoms (coordination cores PtN₅ and PtN₃O₂), the axial-equatorial coordination of the bidentate heterocyclic molecules, and two bridging and two terminal acetamidate ligands.     

“Extracting” the Key Fragment in ETS‐10 Crystallization and Its Application in AM‐6 Assembly

The mechanism of crystallization of microporous titanosilicate ETS‐10 was investigated by Raman spectroscopy combined with ²⁹Si magic‐angle spinning (MAS) NMR spectroscopy, DFT calculations, and SEM imaging. The formation of three‐membered ring species is shown to be the key step in the hydrothermal synthesis of ETS‐10. They are formed by means of a complex process that involves the interaction of silicate species in the reaction mixture, which promotes the dissolution of TiO₂ particles. These insights into the mechanism of ETS‐10 growth led to the successful development of a new synthesis route to the vanadosilicate AM‐6 that involves the use of intermediates that contain three‐membered ring species as an initiator.     

Zum Anionenaufbau von Tetra-n-butylammoniumsilicaten und ihren wäßrigen Lösungen

On the Anion Constitutions of Tetrabutylammonium Silicates and their Aqueous Solutions The anion distribution of tetra-n-butylammonium-(TBA)-silicate solutions with molar TBA/SiO₂ ratios between 0.6 and 4 and silica concentrations between 0.1 M and 2.2 M has been investigated by trimethylsilylation and ²⁹Si NMR techniques. In contrast to concentrated tetramethylammonium- and tetraethylammonium silicate solutions in TBA silicate solutions a preference of double ring silicate anions does not occur. In TBA silicate solutions a broad distribution of silicate anions consisting of monomeric, oligomeric chain and ring, as well as polymeric silicate anions has been observed. Crystalline TBA silicates with TBA/SiO₂ ratios of 0.78 to 1 contain mainly double five-ring silicate anions Si₁₀O₂₅^10? whereas for TBA/SiO₂ ratios higher than 1.4 the double three-ring anion Si₆O₁₅^6? predominates. A recently prepared TBA silicate with low TBA content (TBA/SiO₂ = 0.23) has been found to consist of double four-ring silicate anions with 6 SiOH groups per double four-ring.     

Unusual transformations of difluorosilanone F<Subscript>2</Subscript>Si=O

The structure and properties of fluorosilicon polymer (fluorosil) formed by the reaction of phenyltrifluorosilane with aliphatic alcohols have been studied by the methods of IR, ¹⁹F, ²⁹Si NMR spectroscopy, high temperature mass-spectrometry, derivatography and atom emission analysis. Due to its high reactivity, this polymer readily reacts with glass of the reaction vessel extracting the ions of all metals entering into its composition. Fluorosil formed in a quartz, teflon or polypropylene reactors is characterized by low stability and is slowly decomposed to SiF₄ and SiO₂. Apparently, fluorosil is the product of incorporation of SiF⁴ into the SiO₂ matrix.     

Über die Silicatanionenkonstitution in Tetraethylammoniumsilicaten und ihren wäßrigen Lösungen

On the Constitution of Silicate Anions in Tetraethylammonium Silicates and their Aqueous Solutions Investigations by paperchromatography, ²⁹Si-NMR spectroscopy and trimethylsilylation method show that concentrated solutions of tetraethylammonium (TEA) silicates with molar TEA:Si ratios from 2.8 to 1 contain mainly double three-ring silicate anions. Besides of these small amount of mono-, di-, tri-, tetra-, cyclotri-, cyclotetra-, double four-ring- and other polycyclic silicate anions are present. From these solutions a crystalline double three-ring silicate of the formula [N(C₂H₅)₄]₆[Si₆O₁₅] · 57 H₂O could be obtained by crystallization at low temperature. Concentrated solutions with TEA:Si ratios of 0.8 to 0.6 contain mainly double three-, double four-, double five- an probably double six-ring silicate anions. From such solutions always the solid TEA-double four-ring silicate is obtained by crystallization. The reasons for the prefered formation of double ring silicate anions in TEA-silicate solutions and their crystallization are discussed.     

29Si-NMR-Untersuchungen zur Anionenstruktur von Kristallinen Tetramethylammonium-alumosilicaten und -alumosilicatlösungen

²⁹Si NMR Investigations on the Anion Structure of Crystalline Tetramethylammoniumaluminosilicates and -aluminosilicate Solutions The ²⁹Si NMR spectra of crystalline tetramethylammonium (TMA) aluminosilicates with different Si/Al ratios exhibit up to 4 sharp signals with characteristic chemical shifts which can be assigned to the central Si atom of OSi(OSi)_3?n(OAl)_n building units of double four-ring (DFR) aluminosilicate anions. The number and distributions of the Al atoms in the DFR framework can be derived from the signal intensities in connection with the results of the trimethylsilylation method [1]. A good agreement of the results of both methods has been found. The DFR can exist as monomeric unit or can be connected to polymeric structures by SiOAl bridges, but no information can be obtained about this question by the ²⁹Si NMR spectra. The investigation of the TMA aluminosilicate solutions by ²⁹Si NMR and TMS method [1] show that stable aluminosilicate anions exist in these solutions. The structure of these aluminosilicate anions is different from the structure of the crystalline TMA aluminosilicates obtained from the solutions.     

Aluminium‐27 NMR investigation of the influence of cation type on aluminosilicate solutions

The effects of tetraalkylammonium (TAA) and alkali metal cations on the equilibrium distribution of aluminosilicate oligomers in aqueous alkaline aluminosilicate solutions were investigated using ²⁷Al NMR spectra and their evolution with time. The results indicate that there are no differences in the initial equilibria involving solutions containing both TAA and alkali metal cations on the one hand and those containing alkali metal cations only. However, re‐equilibration of the aluminosilicate species for TAA/Na aluminosilicates is slow (usually not detectable on the time‐scale of the experiments), whereas when purely alkali metal cations are used, the spectra alter over a period of ～1 h, such that resolution is degraded substantially. In the latter case, it is suggested that the anions aggregate into larger systems, although the solutions are still clear. ²⁹Si NMR evidence for slow equilibration of silicate and aluminosilicate solutions at higher concentrations is also discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

Covalent grafting of organoalkoxysilanes on silica surfaces in water-rich medium as evidenced by <Superscript>29</Superscript>Si NMR

This paper addresses two questions related to functionalization of silica particles: (1) is the grafting of hydrophobic organoalkoxysilanes on a silica surface possible in water-rich medium and (2) how to prove the formation of covalent bonds with the surface? Trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane have been reacted with precipitated silica in water-rich medium (water/ethanol 25/75 v/v) and ²⁹Si MAS NMR was used to answer both questions: ²⁹Si chemical shift values of the organosilicon units in the case of trimethylethoxysilane and dimethyldiethoxysilane clearly distinguished between self-condensation reactions and surface reactions through covalent bonds.     

Solid-state <Superscript>29</Superscript>Si MAS NMR studies of diatoms: structural characterization of biosilica deposits

Four different diatom species (Chaetoceros debilis, Chaetoceros didymum, Cylindrotheca fusiformis, Nitzschia angularis) were studied by solid-state (29)Si MAS NMR spectroscopy. To determine the Q(2):Q(3):Q(4) ratios in the biosilica deposits of the diatoms, quantitative (29)Si MAS NMR experiments were performed. This analysis did not reveal any differences regarding the molecular architecture of the silica (i.e. the degree of condensation of the SiOH units (2 identical with SiOH --> identical with Si-O-Si identical with + H(2)O)) from the different diatom species. However, complete cells showed significantly smaller Q(4):Q(3) ratios (1.8-1.9) than extracted cell walls (2.5-2.8), indicating the existence of intracellular pools of less condensed silica.     

<Superscript>13</Superscript>C NMR and mass spectrometry of soil organic matter

Liquid state, high resolution ¹³C NMR spectroscopy and mass spectrometry were used to study the composition and structure of soil organic matter (SOM) using soil extracts from two long-term experiments at the Rothamsted Experimental Station. Both one- and two-dimensional NMR techniques were applied. ¹³C NMR sub-spectra of the CH _n (n=0...3) groups, obtained by the Distortionless Enhancement by Polarisation Transfer (DEPT) technique, were used for the elucidation of the qualitative and quantitative composition of humic and fulvic acids in the soils. The chemical structure of SOM was further analysed at the molecular level through Fast Atom Bombardment Mass Spectrometry (FABMS) and Gas Chromatography-Mass Spectrometry (GC/MS). Humic and fulvic extract results were not only compared to each other, but also to the solid state ¹³C NMR results for the complete soil sample.