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.     

<Superscript>29</Superscript>Si NMR Spectroscopy Investigation of Alcoholic Phenyltrimethyl Ammonium Silicate Solutions

²⁹Si NMR spectroscopy is a powerful tool for studies of the silicate species existing in both aqueous and non-aqueous solutions. In this report ²⁹Si NMR spectroscopy was used to characterize species present in alkaline alcoholic silicate solutions. Phenyltrimethylammonium (PTMA) hydroxide was used as a base. The effects of polymerization/depolymerization of silicate anions in alcoholic alkaline solutions were investigated with different alcohols by ²⁹Si NMR spectroscopy. The esterification of monomeric silicate, Si(OH)₄, in the presence of different alcohols was also studied. Esterification depends on the alkyl chain as well as number of hydroxyl groups in the alcohol.     

Correlations between data of <Superscript>29</Superscript>Si NMR and thermogravimetry for silica/monetite nanocomposites

Silica/monetite nanocomposites were synthesized through controlled hydrolysis of tetraethoxysilane at concentrations of 5, 10, 15, and 20% mol/mol of calcium phosphate forming the solids named CaPSil1, CaPSil2, CaPSil3, and CaPSil4, respectively. XRD patterns showed formation of nanocomposites with a decrease in crystallinity. The NMR ²⁹Si spectra suggested an increase in the content of incorporated silica with reduction of Q³ (–SiOH) signal, which contributes for mass loss, in agreement with thermogravimetry. The incorporation of silica increased the chemical stability of the precursor phosphate in an acidic medium.     

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.     

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.     

Structure of palladium(II) complexes with nitrilotrimethylenephosphonic acid in aqueous solutions according to <Superscript>31</Superscript>P and <Superscript>1</Superscript>H NMR data

The complex formation in the K₂PdCl₄-nitrilotrimethylenephosphonic acid (NTMP) system with a metal to ligand molar ratio of 1: 1 and 1: 2 was studied by ³¹P and ¹H NMR spectroscopy. The formation of equimolar complexes with NTMP coordinated in the bidentate ([N,O]) and tridentate ([N,O,O]) fashions depending on the reactant and chloride ion concentrations and solution pH was observed.     

The state of Al(III) in alcohol solutions of aluminum alkoxide as probed by <Superscript>27</Superscript>Al and <Superscript>13</Superscript>C NMR and small-angle X-ray scattering

Solutions of aluminum alkoxides obtained by interaction of aluminum metal with methyl, ethyl, and isopropyl alcohols were studied by ²⁷Al and ¹³C NMR and small-angle X-ray scattering. Alkoxides with a tetrahedral environment of aluminum prevail in methanol solutions, and those with an octahedral environment of aluminum predominate in ethanol solutions. In isopropyl alcohol at 293 K, polynuclear alkoxides with tetrahedral, octahedral, and pentacoordinated aluminum environments coexist. The structure of polynuclear complexes was refined by comparison of their calculated dimensions with small-angle X-ray scattering data.     

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.     

27Al and 29Si solid-state NMR characterization of calcium-aluminosilicate-hydrate

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.     

Kinetics of the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) by <Superscript>29</Superscript>Si NMR spectroscopy and mathematical modeling

Tetraethoxysilane (TEOS) is widely used to synthesize siliceous material by the sol–gel process. However, there is still some disagreement about the nature of the limiting step in the hydrolysis and condensation reactions. The goal of this research was to measure the variation in the concentration of intermediates formed in the acid-catalyzed hydrolysis by ²⁹Si NMR spectroscopy, to model the reactions, and to obtain the rate constants and the activation energy for the hydrolysis and early condensation steps. We studied the kinetics of TEOS between pH 3.8 and 4.4, and four temperature values in the range of 277.2–313.2?K, with a TEOS:ethanol:water molar ratio of 1:30:20. Both hydrolysis and the condensation rate speeded up with the temperature and the concentration of oxonium ions. The kinetic constants for hydrolysis reactions increased in each step k_h1?<?k_h2?<?k_h3?<?k_h4, but the condensation rate was lower for dimer formation than for the formation of the fully hydrolyzed Si(OH)₄. The system was described according to 13 parameters: six of them for the kinetic constants estimated at 298.2?K, six to the activation energies, and one to the equilibrium constant for the fourth hydrolysis. The mathematical model shows a steady increase in the activation energy from 34.5?kJ?mol^?1 for the first hydrolysis to 39.2?kJ?mol^?1 in the last step. The activation energy for the condensation reaction from Si(OH)₄ was ca. 10?kJ?mol^?1 higher than the largest activation energy in the hydrolytic reactions. The decrease in the net positive charge on the Si atom contributes to the protonation of the ethoxy group and makes it a better leaving group.

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First-principles simulations of the <Superscript>27</Superscript>Al and <Superscript>17</Superscript>O solid-state NMR spectra of the CaAl<Subscript>2</Subscript>Si<Subscript>3</Subscript>O<Subscript>10</Subscript> glass

The local and medium-range structure of the 20CaO·20Al₂O₃·60SiO₂ glass generated by classical molecular dynamics simulations has been compared to NMR experiments by computing the ²⁷Al and ¹⁷O NMR parameters and NMR spectra from first-principles simulations. The calculation of the NMR parameters (chemical shielding and quadrupolar parameters), which are then used to simulate solid-state MAS and 3QMAS NMR spectra, is achieved by the gauge including projector augmented-wave and the projector augmented-wave methods on the DFT-PBE relaxed structure. The NMR spectra calculated with the present approach are found to be in excellent agreement with the experimental data, providing an unambiguous view of the local and medium-range structure of aluminosilicate glasses.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XV. Relativistic calculations of <Superscript>29</Superscript>Si NMR chemical shifts of silanes

Calculations of ²⁹Si NMR chemical shifts of 68 silanes possessing various substituents, in particular, with heavy halogens attached to silicon atom, were carried out applying an efficient calculation scheme of locally dense basis set in the framework of the electron density functional theory utilizing the Keal–Tozer functional combined with relativistic Dyall basis sets on a four-component relativistic level. The main factors of calculation accuracy of silicon chemical shifts were analyzed including the relativistic effects, environmental impact, and vibrational corrections. The mean absolute calculation error for the studied compounds series accounting for all mentioned factors was 14.0 ppm for the nonrelativistic calculation and 6.7 ppm for the four-component relativistic calculation at the range of silicon chemical shifts variation of ~250 ppm.     

Sol-gel polycondensation of methyltrimethoxysilane in ethanol studied by <Superscript>29</Superscript>Si NMR spectroscopy using a two-step acid/base procedure

Sol-gel polymerization of methyltrimethoxysilane (MTMS) in ethanol using a two-step acid/base catalyzed procedure (B2) is followed by ²⁹Si NMR spectroscopy. Analysis of the structural evolution of the B2 system shows that esterification of monomeric and end silicon species is rate-limited while that of linear and cyclic species is able to reach pseudoequilibrium in the second basic step. Condensation reactivity is reduced with increasing network connectivity, however, to a much less degree under B2 conditions than MTMS polymerization under acidic conditions. Steric effects as well as many other factors are attributed to this trend. The concentration of cyclic and polycyclic species of the B2 system is nearly 3 times lower compared to the acid-catalyzed system. The empirical degree of condensation at the gel point is determined to be 0.88. The effects of cyclization and phase separation on MTMS gelation are discussed for both B2 and acid-catalyzed systems. Based on these results it is believed that MTMS-based gels form for B2 and not acid-catalyzed conditions due to reduced cyclization, rapid hydrolysis and condensation, effective use of functional groups, and effective contribution of branched and polycyclic species as crosslinking points to connect polymeric chains in the B2 system.     

<Superscript>27</Superscript>Al NMR Study of the Metal Cluster Compound Al<Subscript>50</Subscript>C<Subscript>120</Subscript>H<Subscript>180</Subscript>

We present an ²⁷Al NMR study of the metal cluster compound Al₅₀Cp*₁₂ which is composed of (identical) Al₅₀ clusters, each surrounded by a Cp* ligand shell, and arranged in a crystalline 3D array (here Cp* = pentamethylcyclopentadienyl = C₅(CH₃)₅). The compound is found to be non-conducting, the nuclear spin-lattice relaxation in the temperature range 100–300 K being predominantly due to reorientational motions of the Cp* rings. These lead to a pronounced maximum in the relaxation rate at T ∼ 170 K, corresponding to an activation energy of about 850 K. Data for the related compound Al₄Cp*₄, containing very much smaller Al₄ clusters are also presented. A comparison is drawn with the quadrupolar relaxation recently observed for the non-conducting fraction of Ga₈₄ molecules in the metal cluster compound Ga₈₄[N(SiMe₃)₂]₂₀-Li₆Br₂(thf)₂₀·2toluene. It is our pleasure to dedicate this paper to our colleague professor Günter Schmid at the occasion of his 70th birthday.     

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.     

Ammonia Catalyzed Hydrolysis-Condensation Kinetics of Tetraethoxysilane/Dimethyldiethoxysilane Mixtures Studied by <Superscript><Emphasis Type="Bold">29</Emphasis></Superscript>Si NMR and SAXS

In-situ ²⁹Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the ammonia catalyzed hydrolysis and condensation of the mixed systems of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) dissolved in methanol. With ammonia catalysis, the hydrolysis reaction orders for TEOS and DDS in the mixed systems remained first order, which is similar to that observed for their corresponding single silane component precursor systems. The hydrolysis rate constant for TEOS in the mixed systems was larger than that of TEOS in the single silane component precursor systems. Meanwhile, the hydrolysis rate constants of DDS in the mixed precursor systems were smaller than those of DDS in the single silane component precursor systems. The hydrolysis and condensation kinetics showed more compatible hydrolysis-condensation relative rates between TEOS and DDS, which remarkably affected the final microstructure of the resulting silica particles. Small angle X-ray scattering (SAXS) experiments showed a typical double fractal structure in the particulate networks.     

<Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR spectra of some drimanic sesquiterpenoids

One- and two-dimensional homo- and heteronuclear correlation proton, carbon, proton—proton, and proton—carbon NMR spectra of fifteen drimanic sesquiterpenoids: 11,12-dibromodrima-5,8-dien-7-one, drim-8-en-7-one, 11-hydroxydrim-8-en-7-one, 11,12-dihydroxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 8,9-epoxydriman-7-one, 8,9-epoxydriman-7-ol, 11,12-diacetoxydrim-8-en-7-ol, drimane-7,8,11-triol, 7,8-isopropylidenedioxydriman-11-al, 9, 11-dihydroxydrim-7-en-6-one, drimane-7,8,9-triol, drimane-7,8,11-triol, and drim-8-ene-7,11,12-triol were studied. The proton and carbon chemical shifts were assigned.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2589–2594, December, 2004.     

29Si and 27Al MAS NMR spectra of mullites from different kaolinites

Mullites synthesized from four kaolinites with different random defect densities have been studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and X-ray diffraction (XRD). All these mullites show the same XRD pattern. However, 29Si and 27Al MAS NMR spectra reveal that the mullites derived from kaolinites with high defect densities, have a sillimanite-type Al/Si ordering scheme and are low in silica, whereas those mullites derived from kaolinites with low defect densities, consist of both sillimanite- and mullite-type Al/Si ordering schemes and are rich in silica.     

Magic angle spinning29Si and27Al NMR study of mordenite dealumination

Magic angle spinning²⁹Si NMR presents a rapid qualitative method of assessing the degree of dealumination of sodium mordenite modified by acid leaching and heat treatment. A quantitative determination of the²⁹Si coordination sphere is hampered by the overlap of chemical shift ranges for Si?O?Al and Si?O?H species. MAS²⁷Al NMR indicates the presence of octahedrally coordinated Al in interstitial sites in all treated samples. On treating mordenite samples at high temperatures (～700°C) much of the Al becomes invisible to the NMR experiment because of its location in sites of low symmetry and large quadrupole coupling constant.