Silicon Carbide Nanostructures from Reactions between Vapors of Organochlorosilanes and Liquid of Sodium‐Factors Affecting Morphology and Composition

Cubic shells and spherical nanoparticles of β‐SiC were produced at 1273 K by processing the ceramic precursors formed from the reactions between vapors of organochlorosilanes, Me₂SiCl₂, MeSiCl₃, MeSiHCl₂, and PhSiCl₃, and liquid Na at 523‐723 K. From Me₂SiCl₂, a flexible linear polycarbosilane precursor was synthesized and covered the NaCl byproduct surface to form a cubic shape. Hollow cubic β‐SiC shells were produced after the NaCl templates were removed. From MeSiCl₃, a rigid cross‐linked polycarbosilane was produced and phase segregated from the NaCl byproduct. The precursor was transformed into nanoparticles without special morphology. MeSiHCl₂ produced a cross‐linked polysilane precursor at low temperatures, which can be converted into a mixture of β‐SiC and Si nanoparticles. At high temperatures, the polysilane converted to polycarbosilane and produced hollow cubic β‐SiC shells. The carbon‐rich PhSiCl₃ generated cube‐like particles as the final product, which contained β‐SiC and carbon.     

ZrB<Subscript>2</Subscript>/HfB<Subscript>2</Subscript>–SiC Ultra-High-Temperature Ceramic Materials Modified by Carbon Components: The Review

The review has been made of recent publications on modification of ZrB₂/HfB₂–SiC ultra-hightemperature ceramic composite materials (UHTC) by carbon components: amorphous carbon, graphite, graphene, fibers, and nanotubes. Available data have been presented on some aspects of oxidation of such materials at temperatures ≥1500°C and both at the atmospheric pressure and at the reduced oxygen partial pressure; structural features of the formed multilayer oxidized regions have been noted. It has been considered how the type and content of the carbon component and the conditions (first of all, temperature) of UHTC production affect the density, flexural strength, hardness, fracture toughness, and thermal and oxidation resistance of the modified ceramic composites.     

High performance liquid chromatography on the chiral stationary phase (R,R)-DACH-DNB using carbon dioxide-based eluents

Fast and efficient separations of chiral stereolabile compounds were obtained at very low temperature on a π-acid chiral stationary phase (R,R-DACH-DNB) using carbon dioxide-based mobile phases containing alcoholic polar modifiers. Furthermore, efficient separations of the newly discovered spherical carbon cluster buckminsterfullerene (C₆₀) and the related higher fullerenes (C₇₀, etc.) have been performed on the same stationary phase using eluents based on either n-hexane or carbon dioxide.     

A Noncovalent Binding Strategy to Capture Noble Gases,Hydrogen and Nitrogen

A molecular design strategy to develop receptor systems for the entrapment of noble gases, H₂ and N₂ is described using M06L‐D3/6‐311++G(d,p)//M06L/6‐311++G(d,p) DFT method. These receptors made with two‐, three‐, four‐ and five‐fluorinated benzene cores, linked with methelene units viz. R_I, R_II, R_III and R_IV as well as the corresponding non‐fluorinated hydrocarbons viz. R_IH, R_IIH, R_IIIH and R_IVH show a steady and significant increase in binding energy (E_int) with increase in the number of aromatic rings in the receptor. A stabilizing “cage effect” is observed in the cyclophane type receptors R_IV and R_IVH which is 26–48% of total E_int for all except the larger sized Kr, Xe and N₂ complexes. E_int of R_IV…He, R_IV…Ne, R_IV…Ar, R_IV…Kr, R_IV…H₂ and R_IV…N₂ is 4.89, 7.03, 6.49, 6.19, 8.57 and 8.17 kcal/mol, respectively which is 5‐ to9‐fold higher than that of hexafluorobenzene. Similarly, compared to benzene, multiple fold increase in E_int is observed for R_IVH receptors with noble gases, H₂ and N₂. Fluorination of the aromatic core has no significant impact on E_int (∼ ±0.5 kcal/mol) for most of the systems with a notable exception of the cage receptor R_IV for N₂ where fluorination improves E_int by 1.61 kcal/mol. The E_int of the cage receptors may be projected as one of the highest interaction energy ranges reported for noble gases, H₂ and N₂ for a neutral carbon framework. Synthesis of such systems is promising in the study of molecules in confined environment. © 2018 Wiley Periodicals, Inc.     

Synthesis of Poly(silyne-co-hydridocarbyne) for Silicon Carbide Production

Pre-ceramic polymers have previously been shown to be polymeric precursors to silicon carbide, diamond and diamond-like carbon. Here, we report the synthesis of a pre-ceramic polymer, poly(silyne-co-hydridocarbyne), which was electrochemically synthesized from one monomer containing both silicon and carbon in its structure. The polymer is soluble in common solvents such as CHCl₃, CH₂Cl₂ and THF. Since the polymer contains both silyne and carbyne on its backbone, it can be easily converted to silicon carbide upon heating under an ambient inert atmosphere, or to SiO₂ under ambient air atmosphere. Poly(silyne-co-hydridocarbyne) was characterized with UV/Vis spectroscopy, FTIR, ¹H-NMR, GPC and Raman spectroscopy. Conversion of the polymer to SiC ceramic was accomplished by heating at 1000 and 750°C under an argon atmosphere and characterized with optical microscopy, SEM, X-Ray and Raman spectroscopies.     

Some topical applications of plasma atomic spectrochemical methods for the analysis of ceramic powders

The scope of a number of plasma spectrochemical methods for the determination of the main components and impurities in ceramic powders is described. These methods meet the requirements for the analytical characterization of new structural and functional ceramics for modern industrial applications and electronic devices. For ceramic powders, spectrochemical analysis with direct methods as well as analysis subsequent to sample dissolution are discussed. Fusion is a powerful method for the dissolution of ZrO₂ ceramic powders, provided the fluxes are pure enough. For determinations in Al₂O₃, SiC and ZrO₂, it will be shown that ICP-MS is very useful. This is especially true for trace analysis after matrix removal. The latter can easily be performed on-line in the case of the analysis of Al₂O₃ powders. For direct analysis of ceramic powders, the direct insertion of samples into the plasma, spark and arc ablation, laser ablation, electrothermal vaporization and slurry nebulization are discussed. Particular attention is given to the direct analysis of ceramics in powder form (Al₂O₃, SiC, Si₃N₄, B₄, WC) using ICP-OES with slurry nebulization as well as with direct sample insertion (DSI) and with electrothermal vaporization (ETV). For the two latter methods, the use of chemical modifiers for volatile compound formation will be shown to be of great importance, and its features will be explained using thermochemical considerations. Received: 18 February 1998 / Revised: 13 May 1998 / Accepted: 9 June 1998     

Crystal and molecular structure of cis- and trans-2,4-dihydroxy-2,4-dimethylcyclohexanetrans-1-acetic acid γ-lactone

During the transformation process of limonene to tetrahydrofuran derivatives, the title compounds (±)-( 4 ) have been obtained as crystalline products and subjected to X-ray analysis. The crystals of trans-( 4 ) are orthorhombic, space group P2₁2₁2₁, with the lattice constants a = 7.0445(5) Å, b = 10.0908(4) Å, c = 14.0309(6) Å; the absolute configuration at atoms C1, C2, and C4 is R_c1, S_c2, and R_c4, respectively. The isomeric form cis-( 4 ) crystallizes in the monoclinic system, space group P2₁, with the following unit-cell parameters: a = 10.8275(4) Å, b = 8.6994(5) Å, c = 16.4722(6) Å, β = 106.515(3)°. The asymmetric part of the unit cell of cis-( 4 ) contains three independent molecules. Each of these three molecules has the identical absolute configuration at all centers of chirality: S_c1, S_c2, and R_c4. © John Wiley & Sons, Inc.     

Chimeric supramolecular synthons in Ph2Te2(I2)Se

Iodination of Ph₂Te₂Se by molecular iodine is directed towards the Te atom and yields {diiodo[(phenyltellanyl)selanyl]‐λ⁴‐tellanyl}benzene, PhTeSeTeI₂Ph or C₁₂H₁₀I₂SeTe₂. The molecule can be considered as a chimera of PhTeSeR, PhTeSeTePh and R′TeI₂Ph fragments. The crystal structure features a complex interplay of the supramolecular synthons Te…π(Ph), Se…Te and I…Te, combining molecules into a three‐dimensional framework. Their combination affords long‐range supramolecular synthons which are fused in a way resembling the mythological chimera and could be defined as chimeric supramolecular synthons. The energies of the intermolecular interactions have also been calculated and analyzed.     

Supramolecular interactions in salts/cocrystals involving pyrimidine derivatives of sulfonate/carboxylic acid

The crystal structures of three compounds involving aminopyrimidine derivatives are reported, namely, 5-fluorocytosinium sulfanilate–5-fluorocytosine–4-azaniumylbenzene-1-sulfonate (1/1/1), C₄H₅FN₃O⁺·C₆H₆NO₃S⁻·C₄H₄FN₃O·C₆H₇NO₃S, I , 5-fluorocytosine–indole-3-propionic acid (1/1), C₄H₄FN₃O·C₁₁H₁₁NO₂, II , and 2,4,6-triaminopyrimidinium 3-nitrobenzoate, C₄H₈N₅⁺·C₇H₄NO₄⁻, III , which have been synthesized and characterized by single-crystal X-ray diffraction. In I , there are two 5-fluorocytosine (5FC) molecules (5FC-A and 5FC-B) in the asymmetric unit, with one of the protons disordered between them. 5FC-A and 5FC-B are linked by triple hydrogen bonds, generating two fused rings [two R₂²(8) ring motifs]. The 5FC-A molecules form a self-complementary base pair [R₂²(8) ring motif] via a pair of N—H…O hydrogen bonds and the 5FC-B molecules form a similar complementary base pair [R₂²(8) ring motif]. The combination of these two types of pairing generates a supramolecular ribbon. The 5FC molecules are further hydrogen bonded to the sulfanilate anions and sulfanilic acid molecules via N—H…O hydrogen bonds, generating R₄⁴(22) and R⁶₆(36) ring motifs. In cocrystal II , two types of base pairs (homosynthons) are observed via a pair of N—H…O/N—H…N hydrogen bonds, generating R₂²(8) ring motifs. The first type of base pair is formed by the interaction of an N—H group and the carbonyl O atom of 5FC molecules through a couple of N—H…O hydrogen bonds. Another type of base pair is formed via the amino group and a pyrimidine ring N atom of the 5FC molecules through a pair of N—H…N hydrogen bonds. The base pairs (via N—H…N hydrogen bonds) are further bridged by the carboxyl OH group of indole-3-propionic acid and the O atom of 5FC through O—H…O hydrogen bonds on either side of the R₂²(8) motif. This leads to a DDAA array. In salt III , one of the N atoms of the pyrimidine ring is protonated and interacts with the carboxylate group of the anion through N—H…O hydrogen bonds, leading to the primary ring motif R₂²(8). Furthermore, the 2,4,6-triaminopyrimidinium (TAP) cations form base pairs [R₂²(8) homosynthon] via N—H…N hydrogen bonds. A carboxylate O atom of the 3-nitrobenzoate anion bridges two of the amino groups on either side of the paired TAP cations to form another ring [R₃²(8)]. This leads to the generation of a quadruple DADA array. The crystal structures are further stabilized by π–π stacking ( I and III ), C—H…π ( I and II ), C—F…π ( I ) and C—O…π ( II ) interactions.     

Crystal and molecular structure of 9-exo-(4-nitrophenyl)thio-10endo-chlorotricyclo[6.3.1.02,7]dodeca-2(7),3,5-triene

Crystal and molecular structure of 9-exo-(4-nitrophenyl)thio-10-endo-chlorotricyclo[6.3.1.0^2,7]dodeca-2(7),3,5-triene (I) was determined by X-ray diffraction analysis: space group P2₁/c, a=9.514(1), b=13.457(1), c=13.163(2) Å, β=104.72(1)°, Z=4, R=0.041 (CAD-4 automatic diffractometer, λCuK_α, 2747 independent reflections with I≥3σ). The framework of molecule I consists of three condensed rings: the benzene ring, the cyclopentene ring having an envelope conformation with a flap at the bridging C atom, and the cyclohexane ring having a distorted chair conformation. The-SAr and Cl substituents have a trans-diaxial orientation at the cyclohexane ring. Molecule I is sterically hindered; it has appreciably reduced interatomic contacts: Cl…C₂, Cl…C₇, S…C₁₂, etc.     

Synthesis and Characterization of Silazane-Based Polymers as Precursors for Ceramic Matrix Composites

The goal of this investigation was to optimize the synthesis of silazane-based polymers for processing fibre-reinforced ceramic matrix composites (CMCs). Liquid oligomeric silazanes were synthesized by ammonolysis of chlorosilanes and characterized spectroscopi- cally (FTIR, NMR) as well as by elemental analysis. The silazanes were obtained in high yield and purity. Different functional groups (system S1: Si—H, Si—CH₃, Si—CH=CH₂) and different degrees of branching in the Si—N backbone [system S2; Si(NH)₃, Si(NH)₂] were realized in order to study the properties of the silazanes that are dependent on the molecular structure. For processing ceramics via pyrolysis of pre-ceramic oligomers, molecular weight, rheological behaviour, thermosetting and ceramic yield were investigated systematically and correlated with the molecular structure of the silazanes. Low molecular weights (500–1000 g mol⁻¹) as well as low viscosity values (0.1–20 Pa s) enable processing of the silazanes in the liquid phase without any solvent. Due to the latent reactivity of the functional groups, curing of the polymers via hydrosilylation is achieved. Structural changes and weight loss during polymer curing as well as the organic/inorganic transition were monitored by FTIR spectroscopy and differential thermogravimetric analysis. With increasing temperature (room temperature to 800 °C) the hydrogen content decreases from 7 to < 0.5 wt% due to the formation of gaseous molecules (NH₃, CH₄, H₂). High ceramic yields up to 80% were reached by branching the oligomers, thus reducing the amount of volatile precursor fragments. Up to 1300 °C, ceramic materials remained amorphous to X-rays. At higher temperatures (1400–1800 °C) either SiC or SiC/Si₃N₄ composites were selectively crystallized, depending on the pyrolysis conditions. The utility of the optimized precursors for CMCs has been demonstrated by infiltration of fibre preforms and subsequent pyrolysis. © 1997 by John Wiley & Sons, Ltd.     

Quantitative Thermal Analysis VII. Basic elements of thermal curves in the context of ingradient theory

The supposition of even temperature distribution in the sample mass (‘ingradient’ approach) led to mathematical expressions describing the basic quantitative elements of thermal curves: the transformation duration, the peak height, the initial and final section peak areas and the total area. The simplest expression is that for the total peak area: S=(R²Hd/2k₁)1nR₁/R, where R, H, d, k₁and R₁are the radius, the specific thermal effect of sample transformation, the gravimetric density and the outer layer encircling the sample, respectively. For the other quantitative elements, the dependences are far more complicated, depending on the duration and variants of the transformation process. This revised version was published online in July 2006 with corrections to the Cover Date.     

The analysis of Si3N4, SiC and BN by combustion with elemental fluorine coupled to mass spectrometry

Combustion with elemental fluorine has been used for the decomposition of ceramic powders of Si₃N₄, SiC and BN. Volatile reaction products like N₂, BF₃, SiF₄ and various fluorides of carbon are detected on-line with a quadrupole mass spectrometer. It is shown that thermal separation of the combustion products by controlled release from a cooling trap is useful to decrease limitations by isobaric interferences in quadrupole mass spectrometry. The developed fluorine combustion coupled to a mass spectrometry procedure has been used for the determination of the stoichiometry of Si₃N₄ in which Si could be directly determined.     

Long-range interaction between 1s and 2s or 2p hydrogen atoms

Long-range interaction energy between two hydrogen atoms has been computed in the second order of the perturbation theory. All states of the system arising when one of the atoms is in the 1s and the other in the 2s or 2p state have been considered. The energy represented by a series expansion in inverse powers of the internuclear distance, R, has been computed up to the terms in R^?8. The results are believed to give reliable interaction energies for R > 15 a.u. Accurate interaction energy for two ground-state hydrogen atoms has also been obtained up to the terms in R^?10. Results for the B′ ¹∑ state are employed to discuss the experimental ground-state dissociation energy of H₂, D₂, and HD. For H₂ all values of the dissociation energy obtained from various experimental absorption limits, by using the computed potential energy curve to separate off the effect of rotation, are shown to be satisfactorily consistent. The resulting total energy of H₂ is, however, higher than the most accurate theoretical value.     

Preparation and Properties of Polyallenes III. Polymerization of Allenes Induced by Al-i-Bu3-VOCl3 Catalyst

The polymerization activity of several allenes under the influence of Al-i-Bu₃-VOCl₃, and structure and properties of the solid polymers obtained, have been studied. Allene; butadiene-1, 2; hexadiene-1, 2; 3-methylbutadiene-1, 2; 3-methylpentadiene-1, 2; and pentadiene 2, 3 could be converted into soluble, solid polymers. 2-Methylpentadiene-2, 3; tetramethylallene; and tetraphenylallene did not react under the polymerization conditions applied. The following order of polymerization activity seems to be valid: CH₂?C?CH₂ > R₁CH?C?CH₂ > R₁R₂C?C?CH₂ ≥ R₁CH?C?CHR₂, > R₁R₂C?C?CHR₃ ≥ R₁R₂C?C-CR₃R₄. The polymers of the homologues of allene were obtained as amorphous solids with the exception of poly(3-methyl-butadiene-1, 2), which was fairly crystalline.     

Synthesis and crystal structures of hydrogen and carbon stabilized lutetium monochloride,LuClHx and Lu2Cl2C

Single crystals of the, basically, four-layer slab structure …?ClLuLuCl…? of the apparently as such nonexisting lutetium monochloride, LuCl, stabilized by interstitial hydrogen or carbon have been obtained from reactions of LuCl₃ and cesium (LuClH_x, due to ubiquitous hydrogen) and C_sCl, lutetium and carbon (Lu₂Cl₂C), respectively, in sealed tantalum containers at elevated temperatures (above 700°C). Both exhibit so-called 3R structures (space group R3 m): LuClH_x (a = 363.83(3), c = 2710.2(3) pm, Z = 6) crystalizes with the ZrCl type structure with hydrogen in tetrahedral interstices and Lu₂Cl₂C (a = 360.17(3), c = 2716.0(3) pm, Z = 3) with the tetradymite type (Bi₂Te₂S) with carbon in octahedral interstices between the double metal layers. Power X-ray diffraction shows that for Lu₂Cl₂C a 1T structure is also adopted (a = 359.72(3), c = 909.25(9) pm, p3 m1, Z = 1).     

Crystal Structures of Mono-, Di-, and Triaminoguanidinium Sulfate,as Well as Azidoformamidinium Sulfate: Important Precursors for Syntheses of Nitrogen Rich Ionic Compounds

Bis(1-aminoguanidinium) sulfate monohydrate (AG₂SO₄ … H₂O, 1), bis(1,3-diamino-guanidinium sulfate (DAG₂SO₄, 2), bis(1,3,5-triaminoguanidinium) sulfate dihydrate (TAG₂SO₄ … 2 H₂O, 3) and bis(azidoformamidinium) sulfate (AF₂SO₄, 5) were synthesized and characterized by multinuclear NMR, IR, and Raman spectroscopy and elemental analysis. In the synthesis of 3, double protonated triaminoguanidinium sulfate (HTAGSO₄, 4) was obtained as a byproduct. The molecular structures of 1–5 in the crystalline state were determined by low-temperature single crystal X-ray diffraction. 1: orthorhombic, Pnma, a = 6.7222 (8) Å, b = 14.153 (2) Å, c = 11.637 (1) Å, V = 1107.1(2) ų, Z = 4, ρ_calc.= 1.586 g cm^?3 R₁ = 0.0442, wR₂ = 0.1007 (all data). 2: hexagonal, P6₁22, a,b = 6.6907 (1) Å, c = 43.4600 (8) Å, γ= 120°, V = 1684.86 (5) ų, Z = 6, ρ_calc.= 1.634 g cm^?3, R₁ = 0.0321, wR² = 0.0714 (all data). 3: monoclinic, C2/c, a = 9.6174 (8) Å, b = 22.858 (1) Å, c = 6.7746 (5) Å, β= 109.49 (1), V = 1404.0 (4) ų, Z = 4, ρ_calc.= 1.620 g cm_?3, R₁ = 0.0292, wR₂ = 0.0781 (all data). 4: monoclini c, P2₁/c, a = 8.9998 (9), b = 6.3953 (6), c = 13.3148(12) Å, β= 99.679 (8), V = 755.44 (13) ų, Z = 4, ρ_calc.= 1.778 g cm^?3, R₁ = 0.0305, wR₂ = 0.0809 (all data); 5: orthorhombic, Pbca, a = 11.3855 (9), b = 7.1032 (6), c = 12.807 (1) Å, V = 1035.74 (14) ų, Z = 4, ρ_calc.= 1.720 g cm^?3, R₁ = 0.0389, wR₂ = 0.0862 (all data).     

Effects of methyl substituents on the proton chemical shifts in the NMR spectra of the twenty methyl and ethyl substituted hydrazines. Electronegativity values for hydrazyl groups

The C? H proton NMR spectra of the twenty conceivable methyl and ethyl substituted hydrazines are presented and analyzed with respect to effects on chemical shifts of the C? H protons caused by replacement of hydrogen by methyl and ethyl groups on the C? N? N? C chain. Thirteen different methyl substituent effects and six different ethyl substituent effects are identified and evaluated. Most of the effects are shielding and in accordance with an electron-releasing inductive effect of alkyl groups. A deshielding effect (the ‘C? C bond effect’) is observed when a methyl group replaces the hydrogen on the carbon bearing the hydrogen in focus and primary hydrogen on the carbon bearing the hydrogen in focus and primary hydrogens become secondary, as observed in other systems. On the basis of their effects on the chemical shifts of methyl protons in CH₃X, eighteen different hydrazyl groups (× = ? NR₁NR₂R₃) fall into three classes: I (R₁ = H; R₂, R₃ = H or alkyl); II (R₁ = alkyl; R₂ and/or R₃ = H); III (R₁, R₂ and R₃ = alkyl), with slightly different electronegativities: 2·94, 2·83 and 2·74, respectively.     

Experimental and Theoretical Studies of 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine: A Potential Antibacterial Agent

The title compound ( 1 ), 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine (C₂₀H₁₆Cl₂N₆), was synthesized and structurally characterized by elemental analysis, ¹H NMR and ¹³C NMR and single crystal X‐ray diffraction. The compound crystallizes as a colourless needle shaped in the triclinic system, space group P‐1 with cell constants: a = 10.7557(11) Å, b = 12.7078(17) Å, c = 15.511(2) Å, α = 68.029(4)0, β = 86.637(5)0, γ = 87.869(4)0; V = 1962.4 (4) ų, Z = 4. There are two structurally similar but crystallographically independent molecules (A and B) in the asymmetric unit of the title compound, which is linked via N‐H…Cl hydrogen bond. An intramolecular C‐H…N hydrogen also occurs in each molecule. In the crystal, each of independent molecules forms a centrosymmetric dimer with an R²₂(8) ring motifs through a pair of N‐H…N hydrogen bonds. These dimers are further connected by intermolecular N‐H…Cl and C‐H…Cl hydrogen bonds, forming an infinite two dimensional supramolecular network lying parallel to the [010] plane. The molecular geometry was also optimized using density functional theory (DFT/B3LYP) method with the 6‐311G (d, p) basis set and compared with the experimental data. Mulliken population analyses on atomic charges, HOMO‐LUMO energy levels, Molecular electrostatic potential and chemical reactivity of the title compound were investigated by theoretical calculations. The thermo dynamical properties of the title compound at different temperature have been calculated and corresponding relations between the properties and temperature have also been obtained. The in vitro antibacterial activity has been screened against Gram‐positive (Bacillus cerus and Staphylococcus epidermidis) and Gram‐Negative (Escherichia coli, Acinetobacter baumannii and Proteus vulgaris). The results revealed that the compound exhibited good to moderate antibacterial activity.