Substitution Of Silicon Research Articles

Substituent effects on rates of rhodium-catalyzed allene cyclopropanation

Rates of cyclopropanation for mono- and disubstituted allenes have been measured relative to standard substrates in reaction with aryldiazoacetate esters catalyzed by Rh 2( S-DOSP) 4. Phenylallene derivatives exhibited a linear correlation of rate with σ + coefficients, indicating a resonance-based effect, though the magnitude of the effect for allenes is less than that reported for other cyclopropanations. Relative reaction rates for aliphatic allenes were found to be similar to those for aryl-substituted allenes, but silicon substitution was found to give a 5- to 14-fold rate increase. The rate enhancement effect for 1-silyl allenes can partially make up for loss of rate and regioselectivity, with 1-trimethylsilyl-1,2-butadiene exhibiting high levels of enantioselectivity and diastereoselectivity in reaction with the chiral catalyst.

ChemInform Abstract: Electronic Structure Calculations on Octanuclear Silsesquioxanes and Aluminosilsesquioxanes.

Semiempirical electronic structure calculations have been performed on a series of octanuclear molecular silicates and aluminosilicates to determine the effects of substitution of tetravalent silicon by trivalent aluminum. By inclusion of molecular species that have already been isolated and characterized, these calculated results have been compared directly with experimental values to determine the reliability of the results obtained. Although these calculations consistently predict Si-O bonds to be slightly longer than observed experimentally, they are shown to be quite successful at reproducing geometric trends within a related series of molecules

ChemInform Abstract: Coupled Substitution of Niobium and Silicon in KTiOPO4 and KTiOAsO4. Synthesis and Nonlinear Optical Properties of KTi1-xNbxOX1-xSixO4 (X: P, As).

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Synthesis and Characterization of Silicon-Rich SAPO-44 Molecular Sieves.

Silicon-rich SAPO-44 molecular sieve was synthesized using cyclohexylamine as templating agent and the material was characterized by various physico-chemical techniques. Isomorphous substitution of silicon for Al-P pairs was observed as the major substitution mechanism.29Si magic angle spinning nuclear magnetic resonance studies showed the existence of aluminosilicate domains in the molecular sieve framework. The acidity of such silicon-rich materials is however comparable to that of other SAPO molecular sieves. The material exhibits high selectivity towards formation of alkenes of lower carbon number while catalyzing the conversion of methanol.

Effects of SiO2 substitution on wettability of laser deposited Ca-P biocoating on Ti-6Al-4V

Silicon (Si) substitution in the crystal structure of calcium phosphate (CaP) ceramics has proved to generate materials with improved bioactivity than their stoichiometric counterpart. In light of this, in the current work, 100 wt% hydroxyapatite (HA) precursor and 25 wt% SiO(2)-HA precursors were used to prepare bioactive coatings on Ti-6Al-4V substrates by a laser cladding technique. The effects of SiO(2) on phase constituents, crystallite size, surface roughness, and surface energy of the CaP coatings were studied. Furthermore, on the basis of these results, the effects and roles of SiO(2) substitution in HA were systematically discussed. X-ray diffraction analysis of the coated samples indicated the presence of various phases such as CaTiO(3), Ca(2)SiO(4), Ca(3)(PO(4))(2), TiO(2) (Anatase), and TiO(2) (Rutile). The addition of SiO(2) in the HA precursor resulted in the refinement of grain size. Confocal laser microscopy characterization of the surface morphology demonstrated an improved surface roughness for samples with 25 wt% SiO(2)-HA precursor compared to the samples with 100 wt% HA precursor processed at 125 cm/min laser speed. The addition of SiO(2) in the HA precursor resulted in the highest surface energy, increased hydrophilicity, and improved biomineralization as compared to the control (untreated Ti-6Al-4V) and the sample with 100 wt% HA as precursor. The microstructural evolution observed using a scanning electron microscopy indicated that the addition of SiO(2) in the HA precursor resulted in the presence of reduced cracking across the cross-section of the bioceramic coating.

Characteristic IR C═C Stretch Enhancement in Monolayers by Nonconjugated, Noncumulated Unsaturated Bonds

Control over and understanding of single-molecule covalent coatings becomes increasingly important in tailoring surfaces during the fabrication of nanoscale electrical or optical elements, such as organic field-effect transistors and light-emitting devices as well as microelectromechanical systems as the relevant feature sizes decrease. In this work, we develop a model based on IR spectra from public databases and DFT calculations that can be used to semiquantitatively assess the level of double bonds in monolayer coatings. We use the model to show the enhancement of the C=C vibrational mode due to silicon substitution and also from additional unsaturated bonds. Simple models for other functional groups in organic monolayers could be produced similarly.

Thorough analysis of silicon substitution in biphasic calcium phosphate bioceramics: A multi-technique study

Four samples of composition Ca(10)(PO(4))(6-x)(SiO(4))(x)(OH)(2-x), with x=0.0, 0.1, 0.2 and 0.5, were prepared and characterized using powder X-ray and neutron powder diffraction, and (1)H, (31)P and (29)Si nuclear magnetic resonance (NMR) spectroscopy. The composition of the Si-substituted HAp phases was determined by joint Rietveld refinements from powder X-ray and powder neutron diffraction data. Taking into account electroneutrality, a chemical formula for the Si-substituted HAp phases with indication of the incorporated silicate amount is proposed. Solid-state (29)Si NMR confirms the presence of only Q(0) species, in good agreement with the presence of substituted HAp and beta-TCP phases only. Thanks to NMR spectroscopy, two types of protons in the Si-substituted HAp phase were identified, the new site corresponding to species engaged in hydrogen bonding with silicate anions. This allowed further refinement of the formulae for these phases with very good quantitative agreement for populations derived from the refinement and integration of NMR data.

Synthesis and characterization of composite molecular sieves M 1-MFI/M 2-MCM-41(M 1, M 2 = Ni, Co) with high heteroatom content and their catalytic properties for hydrocracking of residual oil

The novel rich heteroatom-containing (15 wt.%) composite molecular sieves M 1-MFI/M 2-MCM-41 have been prepared from the ionic complex [ ( C 4 H 9 ) 4 N ] 2 + [M 1(EDTA)] 2− and [ C 16 H 33 ( CH 3 ) 3 N ] 2 + [M 2(EDTA)] 2− as the organic templates, which could effectively introduce a large amount of heteroatoms into the framework of molecular sieves. The products were characterized by XRD, XRF, SEM, HRTEM, N 2 adsorption–desorption, H 2-TPR, FT-IR, TG and DSC techniques. The catalytic performances of the composite materials were investigated by means of residual oil hydrocracking. Characterization results showed that the composite materials simultaneously possessed a typical orthogonal MFI phase and a hexagonal MCM-41 phase and the two phases were composite rather than the physical mixture. H 2-TPR data indicated all of the heteroatoms were incorporated by isomorphous substitution of silicon in framework. N 2 adsorption–desorption analysis exhibited that these samples possessed regular and stable structure with high specific surface area and large pore diameters of 550–620 m 2 g −1, 3.5 nm and 5.5 nm, respectively. M 1-MFI/M 2-MCM-41 presented excellent activities in hydrocracking of residual oil, which were superior to the pure materials of MFI/MCM-41.

First-Principles Calculations and the Thermodynamics of Cementite

Thermodynamic data for the substitution of silicon and manganese in cementite have been estimated using first-principles methods in order to aid the design of steels where it is necessary to control the precipitation of this phase. The need for the calculations arises from the fact that for silicon the data cannot be measured experimentally; manganese is included in the analysis to allow a comparison with its known behaviour. The calculations for Fe3C, (Fe11Si4c)C4, (Fe11Si8d)C4, (Fe11Mn4c)C4 and (Fe11Mn8d)C4 are based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The output includes the ground state lattice constants, atomic positions and bulk moduli. It is found that (Fe11Si4c)C4 and (Fe11Si8d)C4 have about 52 and 37 kJ greater formation energy when compared with a mole of unit cells of pure cementite, whereas the corresponding energy for (Fe11Mn4c)C4 and (Fe11Mn8d)C4 is less by about 5 kJ mol1. These results for manganese match closely with published trends and data; a similar comparison is not possible for silicon but we correctly predict that the solubility in cementite should be minimal.

On the electronic structure and stability of icosahedral r-X2Z10H12 and Z12H122− clusters; r = {ortho, meta, para}, X = {C, Si}, Z = {B, Al}

We report on the electronic structure of the 12-vertex icosahedral clusters r-X(2)Z(10)H(12) and Z(12)H(12)(2-), where X = {C, Si} and Z = {B, Al}. The least stable cluster--with the lowest HOMO-LUMO gap (E(g))--corresponds to the ortho-X(2)Z(10)H(12) isomers for all values of X = {C, Si} and Z = {B, Al}. The well-known energetic order E(para) < E(meta) < E(ortho) for r-carboranes is also valid for all compounds except r-C(2)Al(10)H(12). Substitution of two atoms of carbon or silicon into the icosahedral cage B(12)H(12)(2-) enhances considerably the stability of the system as analyzed from E(g) gaps, as opposite to Al(12)H(12)(2-), where similar gaps are found upon double carbon or silicon substitution regardless of the positions in the cage. In order to highlight similarities and differences in the title clusters, topological analysis of the electron density was performed, together with analysis of the deviation from polyhedron icosahedral form with (i) volumes, skewness and kurtosis calculations; and (ii) continuous shape measures.

Aerosol deposition of silicon-substituted hydroxyapatite coatings for biomedical applications

Silicon-substituted hydroxyapatite (Si-HA) coatings on commercially pure titanium (Ti) were prepared by aerosol deposition using Si-HA powders. Si-HA powders with the chemical formula Ca 10(PO 4) 6 − x (SiO 4) x (OH) 2 − x , having silicon contents up to x = 0.5 (1.4 wt.%), were synthesized by solid-state reaction of Ca 2P 2O 7, CaCO 3, and SiO 2. The Si-HA powders were characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometry, and Fourier transform infrared spectroscopy. The corresponding coatings were also analyzed by XRD, scanning electron microscopy, and electron probe microanalyzer. The results revealed that a single-phase Si-HA was obtained without any secondary phases such as α- or β-tricalcium phosphate for both the powders and the coatings. The Si-HA coating was about 5 µm thick, had a dense microstructure with no cracks or pores, and showed a high adhesion strength ranging from 28.4 to 32.1 MPa. In addition, the proliferation and alkaline phosphatase activity of MC3T3-E1 preosteoblast cells grown on the Si-HA coatings were significantly higher than those on the bare Ti and pure HA coating. These results revealed the stimulatory effects induced by silicon substitution on the cellular response to the HA coating.

Silicon-substituted calcium phosphates – A critical view

Nowadays, the scientific community widely accepts the statement that silicon-substituted calcium phosphates have better biological properties compared to pure calcium phosphates. For example, a review published in this journal in 2007 started with the sentence “Silicon (Si) substitution in the crystal structures of calcium phosphate (CaP) ceramics such as hydroxyapatite (HA) and tricalcium phosphate (TCP) generates materials with superior biological performance to stoichiometric counterparts” [1]. A critical look at published articles demonstrates that this sentence is controversial and somehow misleading, because there is no experimental evidence that Si ions are released from Si-substituted calcium phosphates at therapeutic concentrations, and because there is no study linking the improved biological performance of Si-substituted calcium phosphates to Si release. The aim of this article is to explain this statement in more details.

Brønsted Sites on Acid-Treated Montmorillonite: A Theoretical Study with Probe Molecules

The effect of isomorphic substitution on Brønsted acid sites, generated during acid treatment of montmorillonite, was studied using the density functional theory (DFT) and the cluster model method. Cluster models describing acid montmorillonite were constructed and used as a matrix for the protonation of several test molecules under different types of isomorphic substitution. Our results concerning geometries, energies, and infrared vibrational frequencies indicate that isomorphic substitution of aluminum with magnesium in the octahedral sheet leads to stronger acid sites and more effective protonation of test molecules than those produced by substitution of silicon with aluminum in the tetrahedral sheet.

Incorporation of tin in different types of pores in SBA-15: Synthesis, characterization and catalytic activity

Mesoporous Sn-SBA-15 has been synthesized by three different methods such as conventional hydrothermal route, using cocatalyst NH 4F and in the presence of organosilane precursor. All the materials are thoroughly characterized by powder X-ray diffraction (XRD), SEM, TEM, N 2 sorption and surface area measurements, diffuse-reflectance UV–visible and FTIR spectroscopy, TG–DTA and elemental analysis through ICP. Nitrogen adsorption data, XRD patterns, and TEM observations suggests that the textural properties are retained during the isomorphous substitution of silicon by tin. ICP chemical analysis indicates that tin can be substituted in the range of Si/Sn = 69–162. UV–visible spectra of samples synthesized by the cocatalytic approach exhibit unique absorption band at 213 nm characteristics of tin atom substituted in the smaller pores (2–3 nm) located inside the walls of mesopores. Further, an additional band at 224 nm can be assigned to Sn atoms located in the distorted tetrahedral position along the primary mesopores. In contrary, only one absorption band centered at 224 nm is observed for all the samples synthesized by conventional hydrothermal as well as in the presence of organosilane precursor. 19F NMR spectra confirmed (no signal) the absence of occluded F − ions in the samples made with NH 4F. Observed high catalytic activity in Baeyer–Villiger oxidation and Meerwin–Pondorf–Verly reduction under the liquid-phase conditions suggest the incorporation of a portion of tin in the smaller pores for the Sn-SBA-15 materials synthesized through cocatalyst method.

How are Heteroelements (Ga and Ge) Incorporated in Silicate Oligomers?

The properties of microporous solids can be strongly influenced during the first stages of solid-state formation from solution. Furthermore, their properties are directly related to the presence, number, and character of intra- or extraframework heteroelements. In this contribution we studied the influence of the substitution of silicon against heteroelements, such as gallium and germanium. The aqueous prenucleating solutions were obtained by hydrolysis of the silicon and heteroelement sources in the presence of tetraalkylammonium cations. The time-resolved stepwise formation of larger oligomers was studied by using electrospray ionization mass spectrometry (ESI-MS). Furthermore, the structures of the obtained signals were verified by using MS/MS experiments, which led to the conclusion that strong structure-directing effects emerge from the introduction of heteroelements into the solutions.

Ion implantation, luminescence, and cluster growth in silica layers

Abstract To activate silica optically our investigations are extended to ion implantation, mainly to overstoichiometric injection or isoelectronic substitution of the both constituents silicon or oxygen, i.e. by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). Such implantation produce new luminescence bands in silica layers, partially with optical electronic–vibronic transitions and respective multimodal spectra. In this context, special interest should be directed to low-dimension nanocluster formation in silica layers. Cathodoluminescence, high resolution transmission (HR-TEM) and scanning transmission electron microscopy (STEM), and energy dispersive X-ray analysis (EDX) have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers and their respective luminescence behavior.

Synthesis, characterization and catalytic performance of Al-MCM-41 mesoporous materials

A series of MCM-41 molecular sieves has been synthesized by using different Si/Al molar ratios and appropriate conditions for the synthesis. Silica fumed and sodium aluminate, both of inorganic origin, have been used in the synthesis, avoiding the use of organic sources. Isomorphous substitution of silicon in tetrahedral framework positions of structure by Al in the Al-MCM-41 material was observed by 27Al NMR and 29Si NMR spectra. The nitrogen adsorption isotherms of the samples showed higher specific surface area and more mesopore volume when there was an increase in the aluminium content. Simultaneously, a smaller thickness of the wall and diameter pore by XRD was observed. At higher degrees of incorporation of aluminium, the acidity of the samples increased, as is clearly shown by the results of temperature-programmed desorption of ammonia and pyridine adsorption by IR. As more aluminium was incorporated into the samples, the higher were the catalytic performances. The increase of the conversion, in hydroisomerization of n-heptane, shows that these materials acted as acid catalysts. This increase in the total conversion is related to the higher number of acid sites in the samples.

Can we make silica luminescent?

It is still a challenge to develop optoelectronic or even photonic devices based on silicon technology. Silicon and its oxide, as well-known, do not possess direct electronic band transitions and, therefore, are not luminescent. The remaining weak light emission is based on intrinsic and extrinsic defect luminescence. Thus our investigations are extended to ion implantation, mainly to over-stoichiometric injection or isoelectronic substitution of the both constituents silicon or oxygen, i.e. by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). Such implantation produces new luminescence bands in silica layers, partially with optical electronic–vibronic transitions and respective multimodal spectra. In this context, special interest should be directed to low-dimension nanocluster formation in silica layers.

Methylt-Butyl Ether and Methyl Trimethylsilyl Ether Ions Dissociate near Their Ionization Thresholds: A TPES, TPEPICO, RRKM, and G3 Investigation

The threshold photoelectron spectra and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of methyl t-butyl ether, (CH(3))(3)COCH(3) (MTBE), and methyl trimethylsilyl ether, (CH(3))(3)SiOCH(3) (MTMSE), have been measured using synchrotron radiation. The effect of silicon substitution on the unimolecular dissociation processes and the threshold photoelectron spectrum has been investigated. Both molecular ions dissociate at low internal energies. For ionized MTBE, the parent ion is no longer observed at an internal energy of only 0.2 eV. For this reason, it was not possible to fit the TPEPICO data to extract reliable thermochemical information. G3 level calculations place the molecular ion 5 kJ mol(-1) above the lowest-energy dissociation products, (CH(3))(2)COCH(3)(+) + (*)CH(3), suggesting the participation of an isomer, potentially the distonic ion (*)CH(2)(CH(3))(2)CO(+)(H)CH(3), in the dissociation. However, the calculations are not considered accurate enough to reliably determine the role this isomer plays, if any. RRKM modeling of the threshold region of the TPEPICO breakdown curves for ionized MTMSE leads to an E(0) for methyl loss of 63 +/- 2 kJ mol(-1), in good agreement with the G3 value of 66 kJ mol(-1). The resulting Delta(f)H(0) for (CH(3))(2)SiOCH(3)(+) of 384 +/- 10 kJ mol(-1) (Delta(f)H(298) = 361 +/- 10 kJ mol(-1)) is 28 kJ mol(-1) lower than the G3 value of 412 kJ mol(-1) due to the G3 Delta(f)H(0) for neutral MTMSE being 16 kJ mol(-1) higher than the previously reported value and the fact that the experimental IE(a) is 6 kJ mol(-1) lower than the G3 estimate. Appearance energy values for higher-energy fragmentation channels up to 36 (for MTBE) and 32 eV (for MTMSE) are reported and compared to literature values. An investigation of fragment ion peak broadening at high internal energy indicated that the two doubly charged molecular ions are not stable on the microsecond time scale. Each was found to dissociate into two singly charged ions along one or more neutral species.

Silicon-substituted hydroxyapatite composite coating by using vacuum-plasma spraying and its interaction with human serum albumin

The incorporation of silicon can improve the bioactivity of hydroxyapatite (HA). Silicon-substituted HA (Ca(10)(PO(4))(6-x )(SiO(4))( x )(OH)(2-x ), Si-HA) composite coatings on a bioactive titanium substrate were prepared by using a vacuum-plasma spraying method. The surface structure was characterized by using XRD, SEM, XRF, EDS and FTIR. The bond strength of the coating was investigated and XRD patterns showed that Ti/Si-HA coatings were similar to patterns seen for HA. The only different XRD pattern was a slight trend toward a smaller angle direction with an increase in the molar ratio of silicon. FTIR spectra showed that the most notable effect of silicon substitution was that -OH group decreased as the silicon content increased. XRD and EDS elemental analysis indicated that the content of silicon in the coating was consistent with the silicon-substituted hydroxyapatite used in spraying. A bioactive TiO(2) coating was formed on an etched surface of Ti, and the etching might improve the bond strength of the coatings. The interaction of the Ti/Si-HA coating with human serum albumin (HSA) was much greater than that of the Ti/HA coating. This might suggest that the incorporation of silicon in HA can lead to significant improvements in the bioactive performance of HA.