The Role of 2-(Methacryloyloxy) Ethyl Acetoacetate in the Polymerization of Hybrid Multicomponent (Si, Ti, Zr) Sols

2-(Methacryloyloxy) ethylacetoacetate (MEEAH) was used as stabilyzing agent to control the chemical reactivity of Ti and Zr monomeric alkoxides in a Si—Ti—Zr sol. The organic and inorganic polymerizations were carried out simultaneously. The organic polymerization was performed using benzoyl peroxide as the initiator. The inorganic polymerization was done via the sol-gel process. The polymerization was studied by several spectroscopic techniques, including Fourier Transform Infrared (FTIR), UV-Vis, ²⁹Si NMR, Small Angle X-ray Scattering (SAXS) and X-ray diffraction (XRD). Theoretical calculation and experimental results showed that MEEAH acted as a monodentate ligand, mainly. This led to a high condensation extent of the Si species in the sol. A short range order gel was obtained, according to the SAXS results. The Si—O—Ti and Si—O—Zr bonds were detected by FTIR. The bonds remained stable, because a sample calcined at 1173 K was amorphous according to the XRD results, indicating structural homogeneity.     

Polysiloxane-Diamine Modified Gel as Precursor to Crystalline/Amorphous Si3N4

New hybrid organic-inorganic materials are prepared by reaction of polymethylhydrosiloxane (PMHS) with aniline (An) or 4,4-diamino,diphenyl methane (APM) through a condensation between NH₂-terminated amines and Si—H groups. The obtained modified-polysiloxane is a liquid polymer (PS-An) in the first case and a cross-linked gel (PS-APM) in the second one. The chemical structures of PS-An and PS-APM were investigated by ²⁹Si nuclear magnetic resonance and were proved to be formed of created (Si—O)₂(Me)Si—N(H)— and unreacted (Si—O)₂(Me)Si—H sites. The pyrolysis of the diamine-based gel has been carried out in N₂ atmosphere up to 1450°C. FTIR, ²⁹Si MAS NMR and X-ray diffraction have shown that Si—O and Si—C bonds are totally broken during pyrolysis and that the final product is formed of crystalline and amorphous silicon nitride in the presence of a free carbon phase.     

Elaboration and Characterisation of Hybrid Materials of Polymethylhydrosiloxane Modified by Diamines of Various Lengths

The polymethylhydrosiloxane (PMHS) modified by bifunctional organic compounds (diamines), offer the possibility of producing organic-inorganic hybrid materials. These materials present excellent opto-electronic properties and find numerous applications such as the manufacture of electroluminescent diodes and ion or radiation sensors.This work shows that monolithic and transparent hybrid gels were obtained by reaction at room temperature of PMHS with diamines in tetrahydrofuran, using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as catalyst. The products have been characterized by infrared and ²⁹Si MAS-NMR spectroscopy. The results show that the diamines have reacted with the PMHS leading to the monolithic and transparent gels in which both organic-inorganic —Si—(H)N—(CH₂) _n —N(H)—Si— bridges are formed (n = 3, 4 and 6). The thermal analysis of the xerogels was determined by TGA and DTA. The structure and texture of the obtained materials, were studied by Chemical Analysis and the Brunauer-Emmett-Teller (BET) method.     

Design of Homogeneous Hybrid Materials through a Careful Control of the Synthetic Procedure

Sols for the synthesis of hybrid organic-inorganic materials have been prepared by mixing zirconium n-propoxide and methacryloxypropyltrimethoxysilane (MPS). The synthesis was done in two steps: a 15 minute hydrolysis of a MPS : H₂O : EtOH 1 : 1 : 2 mixture and then addition of 0.5 molar equivalent of zirconium alkoxide. All the experimental parameters—hydrolysis ratio, pH, dilution, pre-hydrolysis time—have been optimized through a detailed ²⁹ Si and ¹⁷O NMR analysis. Immediately after the addition, 94% of the initial water was consumed for the formation of Si–O–Zr bridges. Cleavage of these bonds, associated with formation of Si–O–Si and Zr–O–Zr bridges are then observed during the aging time.     

On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in the System: Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide

Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5 – 2.0 and their gels after heat treatment at 130°C. The presence of Si—O—Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2–3 ppm compared to corresponding signals of homo-condensed Si—O—Si bonds. The existence of Si—O—Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si—O—Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti—O—Ti and Si—O—Si bonds resulting in a decreased molecular homogeneity of the materials.     

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (Si—O—)₃ZrH and (Si—O—)₂ZrH₂ with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (Si—O—)₃ZrCH₃ and (Si—O—)₂Zr(H)CH₃ as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (Si—O—)₂ZrH₂ are more reactive toward the methane C—H bonds than zirconium monohydrides (Si—O—)₃ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (Si—O—)₂ZrH₂ are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.     

Experimental and quantum-chemical study of complexation of carbene analogs with dinitrogen. Direct IR-spectroscopic observation of Cl2Si·N2 complexes in low-temperature argon-nitrogen matrices

Interaction of dichlorosilylene with dinitrogen in mixed Ar—N₂ matrices at 9 - 10 K was studied by IR spectroscopy. A donor-acceptor complex Cl₂Si·N₂ was found and characterized by six bands of symmetric (at 511.2, 508.9, and 506.5 cm^–1) and antisymmetric (at 500.1, 496.9, and 495.1 cm^–1) stretching vibrations of Si—Cl bonds in the most abundant isotopomers. Two bands at 498.7 and 493.5 cm^–1 observed in mixed matrices were tentatively assigned to Cl₂Si·(N₂)₂ complex. Several stretching vibration bands of minor isotopomers of SiCl₂ were detected for the first time in argon matrices. Assignment has been done for the isotopic structure of SiCl₂ associates with dinitrogen observed in N₂ matrices. Dimerization of SiCl₂ and its complexation with one and two N₂ molecules were studied by quantum-chemical DFT calculations (PBE and B3LYP functionals). The structures, energies, and vibrational frequencies of the Cl₂Si·N₂ and Cl₂Si·(N₂)₂ complexes and the Si₂Cl₄ dimer were determined. The energies of SiCl₂ complexation with one and two N₂ molecules obtained from PBE and B3LYP calculations are 0.3 and 0.6 kcal mol^–1, respectively. More accurate G2(MP2,SVP) calculations using the B3LYP geometries have predicted a higher stability of the Cl₂Si·N₂ complex (1.2 kcal mol^–1). The calculated and experimental vibrational frequencies of reagents and complexes are in good agreement. A correlation has been established between the PBE calculated energies of complexation of EHal₂ (E = Si, Ge, Sn, Pb) with N₂ and the experimentally observed shifts of E—Hal stretching vibrations in EHal₂ upon complexation. The strength of the complexes with N₂ increases on going from dihalosilylenes to dihaloplumbylenes.     

Monomer-Dimer Equilibrium of Rhodamine B Encapsulated in Si–Al and Si–Ti Binary-Oxide Thin Films

Si—Al and Si—Ti binary-oxide thin films including Rhodamine B (RB) have been prepared. They were dip-coated as a function of time after mixing of each sol-gel reaction system. The absorption and fluorescence spectra of the individual films have been observed. These spectra were analyzed in order to clarify the behavior of RB along with the change in the environment around the RB molecules, caused by the progress of the sol-gel reaction, in the fluid sol and the prepared thin films. Some amount of the RB dimers (H- and J-types) were formed in the Si—Al and Si—Ti binary-oxide films (Si : M = 75 : 25) prepared at the initial stage of the sol-gel reaction and aged under relative humidity of 60%. In the case of Si—Al binary-oxide films, the amount of the J-dimer decreased along with the reaction time at which the films were prepared, indicating that growing polymer networks of metal alkoxides around the RB molecules prevent the formation of the J-dimer. On the other hand, larger amounts of the H- and J-dimers were formed in the Si—Ti binary-oxide films prepared at longer reaction time of the solution. RB interacts more strongly with —TiOH compared with —AlOH. In the case of the Si—Ti binary-oxide films, with the progress of the sol-gel reaction, RB molecules in the prepared films easily cohere around the —TiOH and form the dimers because of increase in the amount of the —TiOH and contraction in the volume of the spaces where RB molecules exist.     

Sol-Gel Synthesis and Pyrolysis Study of Oxyfluoride Silica Gels

Silicon oxyfluoride materials are synthesized by the sol-gel method using triethoxyfluorosilane as precursor, bearing the Si—F bond. SiO_(2–0.5x) F _x gel preparation requires peculiar experimental control of hydrolysis and condensation reactions. Maintenance of the Si—F bond during gelling, heating and aging was studied in the case of processes carried out under an argon atmosphere or in air. Fluorine contents in resulting samples were quantified by FT-IR and X-ray photoelectron spectroscopy (XPS); specific surface area and porosity of powdered samples were determined by N₂ adsorption. The thermal stability of oxyfluoride gels was studied by thermogravimetric-mass spectrometric (TG-MS) coupled analyses during heat treatment, under He flow. Mass spectra recorded during principal weight losses indicate the release of variously fluorinated silicon species resulting from Si—F/Si—O exchange reactions. The evolution of these species was observed at different temperatures, depending on gelling conditions. In particular, degradation of Si—F moieties was prominent for gels aged in air, whereas samples processed under an argon atmosphere preserve the Si—F bond up to 300°C.     

Fragmentation behavior of Si2Cln+ cations (n = 1—6)

Sector-field mass spectrometry was used to probe the fragmentation patterns of the cationic silicon chlorides Si₂Cl_n ⁺ (n = 1—6). For almost all Si₂Cl_n ⁺ ions, Si—Si fragmentation predominates the Si—Cl bond cleavage both in the metastable ion and collisional activation mass spectra. Analysis of the fragmentation patterns indicates that the long-lived radical cation Si₂Cl₆ ^·+ corresponds to a complex [SiCl₂·SiCl₄]^·+ rather than the intact molecular ion of hexachlorodisilane. The behavior of Si₂Cl₅ ⁺ is consistent with the formation of the (trichlorosilyl)dichlorosilyl cation Cl₃SiSICl₂ ⁺. Structural aspects are also discussed for the other Si₂Cl_n ⁺ species. A semi-quantitative analysis of the fragmentation patterns in conjunction with the literature thermochemistry data was used to estimate some thermochemical properties of the Si₂Cl_n ⁺ cations.     

Solid State NMR and TG/MS Study on the Transformation of Methyl Groups During Pyrolysis of Preceramic Precursors to SiOC Glasses

The sol-gel method was used to prepare two different starting gels containing SiCH₃-groups for the preparation of SiOC ceramics. To understand the role of Si—H bonds in the incorporation of carbon into the SiOC network, gels prepared from a 1:2 mixture of triethoxysilane and methyldiethoxysilane (T^HD^H2) and solely methyltriethoxysilane (T^Me) were investigated. Thermogravimetric analysis coupled with mass spectroscopy (TG-MS) in inert atmosphere was performed to attain an insight into the decomposition reactions involved during gel-glass transformation. Samples calcined at different temperatures up to 1000°C were characterized by ²⁹Si and ¹³C magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The presence of SiH groups in the starting gel allows an efficient conversion of Si—CH₃ groups into CSi₄ sites at lower temperatures. As a result, despite a much lower amount of carbon in the starting T^HD^H2 gel (C/Si = 0.33) compared to the T^Me gel (C/Si = 1), the amount of carbon inserted into the SiOC network of both glasses is equivalent, but the T^Me sample contains the 10 fold amount of free carbon.     

Synthesis and In Vitro Behavior of Organically Modified Silicate Containing Ca Ions

Organically modified silicates containing calcium ion have a potential to bond to bone via an apatite layer deposited on their surfaces in the body environment. In this study, we examined the relationship between apatite deposition and the microstructure of the organically modified silicates synthesized from tetraethoxysilane (TEOS) and poly (dimethylsiloxane) (PDMS) with a different amount of calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and hydrochloric acid (HCl). Apatite deposition was evaluated in vitro using a simulated body fluid (Kokubo solution). Copolymerization was confirmed between TEOS and PDMS even if PDMS free from —SiOH termination are used as one of the starting materials. The porosity and Ca content incorporated in the structure depended on the amount of HCl, whereas analysis of ²⁹Si MAS NMR spectra indicates that it caused few effects on the local structure around Si atoms. Apatite-forming ability is enhanced by optimal amounts of HCl and Ca (NO₃)₂·4H₂O. The difference in apatite-forming ability among the hybrid gels was attributed to both Ca(II) contents in the structure and aggregation states of the Si—OH groups. Better bioactivity of the hybrid gels is achieved by the release of Ca(II) ions trapped in structure at gelation and the formation of hydrated silica rich in Si—OH.     

Structural Features of Grafted Phosphates in Mesopores of Si-MCM-41 and Ti,Si-MCM-41 Molecular Sieves

We have used ³¹P NMR (MAS) and IR spectroscopy and also chemical analysis to show that when Si- and Ti,Si-MCM-41 mesoporous molecular sieves are treated with phosphorus oxychloride, surface-anchored silicophosphate and titanophosphate compounds are formed of the type (Si—O) _x —{PO(OH)_3–x } and (Ti—O) _x —{PO(OH)_3–x } (x = 1, 2), and also compounds with the structure of titanium hydrophosphate and pyrophosphate.     

Das Dreistoffsystem: Molybdän—Silizium—Kohlenstoff

Neubearbeitung des Randsystems Mo–C. — Strukturermittlung an MoC. — Thermische Untersuchung an dem Paar Si–C. — Aufstellung des Dreistoffsystems Mo–Si–C durch röntgenographische sowie Schmelzpunktsmessungen bzw. mikroskopische Beobachtungen. — Kristallstruktur und Eigenschaften einer neuen ternären Phase.

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Mit 10 Abbildungen.     

Calculations of silicooxygen ring vibration frequencies

In the work model calculations of the vibrations of ideally isolated silicooxygen rings (using PM3 method) have been carried out. three-, four-, and six-membered rings have been considered. It has been found that that the three-membered silicooxygen rings are flat and practically undeformed showing D_3h symmetry. The rings of higher number of ring members (i.e. n>3) are deformed to some extent. The deformation reveals itself most significantly in the Si–O–Si bond angles distribution. In the case of all the rings the bridging Si–O–Si bonds are ca. 0.02–0.04 Å shorter than the non-bridging Si–O⁻ bonds. Hypothetical IR spectra for all the rings considered have been also calculated. Analysis of these hypothetical spectra leads to the conclusion that the whole spectrum can be divided into four wavenumbers regions, 1200–1100 cm⁻¹ stretching Si–O(Si) vibrations; 1000–800 cm⁻¹ stretching Si–O⁻ vibrations; 800–600 cm⁻¹; the region in which a band characteristic of silicooxygen rings appears, and below 600 cm⁻¹ bending ⁻O–Si–O⁻ and (Si)O–Si–O(Si). It has been also found that as the number of ring members increases the ‘ring band’ shifts to lower wavenumbers: 725 cm⁻¹ for three-membered rings, 650 cm⁻¹ for four-membered rings and 610 cm⁻¹ for six-membered rings. Calculated spectra have been compared with the experimental spectra of cyclosilicates. They showed good agreement in the 1200–600 cm⁻¹ region. In the experimental spectra as well as in the calculated ones, with increasing the number of ring members the ‘ring band’ shifts towards lower wavenumbers.     

Toward a More Accurate Silicon Stereochemistry: An Electron Diffraction Study of the Molecular Structure of Tetramethylsilane

The present electron diffraction study of the molecular structure of tetramethylsilane, augmented with theoretical calculations, answers the need for accurate and detailed information on the most fundamental molecules containing silicon. The Si—C bond length is r _g = 1.877 ± 0.004 Å, in perfect agreement with a previous study (Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct. 1971 8 401). The C—H bond length is r _g= 1.110 ± 0.003 Å and the Si—C—H angle is 111.0 ± 0.2°. The experimental data are consistent with a model of T _d symmetry and staggered methyl conformation. The barrier to methyl rotation is estimated to be 5.7 ± 2.0 kJ mol^–1 on the basis of the experimentally observed average torsion of the methyl groups.     

Heteroorganic betaines. 6. Structure and reactivity of silicon-containing organophosphorus betaines with the –S—Si—C—P+ fragment: a DFT study

The structures of silicon-containing organophosphorus betaines ^–S—SiR¹ ₂—CR² ₂—P⁺R³ ₃ and their ylide isomers were calculated using the density functional approach with the gradient-corrected PBE functional and extended TZ2P basis set. Three possible pathways of thermal decomposition of these betaines were analyzed. These are (i) cleavage of the central C—Si bond with the formation of a Wittig ylide and silanethione, (ii) intramolecular nucleophilic S _N-substitution with elimination of phosphine PR³ ₃ and the formation of silathiirane (the Corey—Chaikovscky transformation), and (iii) a Wittig-type decomposition followed by the formation of substituted silaethylene.The structures of products and transition states of these reactions were calculated. The cis-gauche conformation of the ^–S—Si—C—P⁺ fragment of betaines was found to be the most stable. This is in agreement with the results of X-ray diffraction study and can be rationalized by strong Coulomb attraction between the cationic and anionic centers. The betaines are stable toward retro-Wittig thermal decomposition. The Corey—Chaikovscky formation of thiirane is preferable under conditions of thermal decomposition. Retro-Wittig-type decomposition of betaines followed by the formation of silanethione is favored by intra- and intermolecular coordination of donor ligands.     

Reactions of cage-like copper/sodium organosiloxanes with CuCl2

The reactions of cage-like copper/sodium organosiloxanes with CuCl₂ were studied. Cage-like copper organosiloxanes containing the Si—O—Cu—Cl group were synthesized for the first time.     

29Si and 17O NMR investigations on Si−O−Ti bonds in solutions of diphenylsilanediol and titanium-tetra-isopropoxide

The structural units in diphenylsilanediol/titanium-isopropoxide solutions with molar ratio Si:Ti between 1:0.1 and 1:5 were examined by means of ²⁹Si and ¹⁷O NMR. The main component in solutions with molar ratio Si/Ti=1:0.1 is the chain-like octaphenyltetrasiloxanediol. With increasing Ti-isopropoxide content (1:0.25–1:05) Si–O–Ti units of the spirocyclic titanosiloxane Ti[O₅Si₄(C₆H₅)₈]₂ prevail in the solutions accompanied by the chain-like tetrasiloxanediol. The ²⁹Si NMR spectra of 1:1 solutions indicate a lot of different Si containing building units with chemical shifts mainly between-40 and-46 ppm. The signals with a chemical shift between-40 and-46 ppm are probably caused by Si atoms which are connected via oxygen bridges directly (Si–O–Ti) or indirectly (Si–O–Si–O–Ti) with titanium. Contrary to the 1:1 solutions only one or two different species with Si–O–Ti units are present in high Ti-alkoxide containing solutions (1:5). ²⁹Si and ¹⁷O NMR results reveal a quick hydrolysis of the Ti–O–Si bonds to titanium-oxo-hydroxo-polymers and phenylsiloxanediols or their isopropyl esters after the addition of water to the solutions. This separation into species only containing either Ti–O–Ti or Si–O–Si bonds can entail a decreased homogeneity of the reaction products on a molecular level.     

Preparation and Characterization of SiO2-MxOy(M = V, Sn, Sb) Thin Films from Silicic Acid and Metal Chlorides

The preparation of SiO₂-M _x O _y (M = V, Sn, Sb) binary oxide thin films by sol-gel method was investigated. The reaction of silicic acid with metal chloride (M = Sn and Sb) or oxychloride (M = V) formed homogeneous solutions. The dip-coating of slide glass and silicon wafer followed by heat treatment gave oxide films having Si—O—M bond. The changes of FT-IR spectra as a function of heat treatment temperature and molar composition confirmed the Si—O—M bonds. The sheet resistance of films increased with an increase on heat treatment temperature and decrease in the content of metal oxide M _x O _y . X-ray diffraction peaks were observed for the SiO₂-V₂O₅ films with high V₂O₅ contents and heat-treated above 250°C, while the others were amorphous. Oxide films heat treated at 500°C had a thickness between 340–470 nm.