Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: II. Methylation of >Si=O Groups

The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (SiO)₂Si=O with a CH₄ molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (SiO)₂Si(OH)(CH₃) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (Si–O)₂Si(O)(CH₃). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (Si–O)₂Si(OH)(CH₃) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is 70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.     

Neutron activation analysis of zeolite catalysts

A new method for the determination of aluminum and silicon has been developed for zeolite catalysts. In contrast to previous methods, thermal neutrons are used for the analysis of both elements, and cadmium absorbers are not needed. The silicon determination utilizes a one-hour irradiation to observe the³¹Si produced by the (n, ) reaction of³⁰Si. A 15-second irradiation is used for the²⁷Al(n, )²⁸Al reaction. The²⁸Al activity is corrected for the contribution from the²⁸Si(n,p)²⁸Al reaction by using the analyzed weight of silicon in the sample and the data for a silicon standard irradiated simultaneously with the zeolite and the aluminum standard. The quantitation limits are 0.012 g for silicon and 3.3×10^–5 g for aluminum. Sodium presents a significant interference, but this element can be removed by taking advantage of the ion exchange properties of these materials.     

A system for the determination of silicon in the human lung using neutrons from A 2MV Van de Graaff generator

Neutron activation analysis, using inelastic scattering, provides a quantitative, non-invasive technique of studying silica burdens and is potentially useful as a screening procedure for occupationally exposed workers. In this method, silicon is measured using the fast neutron inelastic scattering reaction²⁸Si(n,n )²⁸Si which emits 1779 keV -rays. The method requires a source of fast neutrons (> 2MeV). A 2MV Van de Graaff generator has been developed to produce a pulsed beam of 5.2 MeV neutrons. The pulsed beam has the advantage of improving measurement sensitivity by separating in Bone the inelastic scattering -rays from those due to thermal-neutron capture reactions. The incident neutron energy was chosen to maximise the silicon -ray count rate, while keeping the signal from the competing reaction³¹P(n,)²⁸Al negligible.     

Kinetics and mechanism of oxidation ofd-xylose by Mn(III) pyrophosphate

The oxidation ofd-xylose by Mn(III) pyrophosphate in sulphuric acid has been found to be first order with respect to [Mn(III)]. Variation of rate with [d-xylose] suggests the rapid formation of reversible cyclic complex between Mn(III) and id-xylose, which further disproportionates in a slow rate determining step. Oxidation rate has been found to increase with [H⁺]. Retardation of rate due to [pyrophosphate] and increase due to [Mn(II)] have been also observed. The value of thermodynamic parameters E, S, and G have been found to be 17.6±0.1 kcal/mole, –10.1±0.1 e.u. and 20.6±0.1 kcal/ mole respectively. A mechanism involving a free radical has been proposed for the reaction under study.     

Spectrophotometric determination of palladium with 4-(2-pyridylazo)resorcinol

Summary A simple photometric method for the determination of palladium has been worked out, employing pyridylazo-resorcinol (PAR) as a complexing ligand for the metal ion. The coloured species is extractable into chloroform in the presence of diethylamine, the absorbance of which is measured at 535 nm. The method is free from the interference of a large number of elements particularly the other platinum metals. It obeys Beer's law in the range of 0–3 g Pd/ml with Sandell's sensitivity of 0.0034 g Pd cm^–2. The ratio of metal to ligand in the complex is found to be 1:1. Analysis of various samples has been carried out with satisfactory and reproducible results (standard deviation ±0.002).     

Spectrophotometric determination of trace and ultratrace levels of boron in silicon and chlorosilane samples

Summary Spectrophotometric methods for the determination of boron in the low g/g and ng/g range in high-purity silicon and dichloro- and trichlorosilanes were investigated in detail. The procedures established involve dissolution of silicon samples and the hydrolyzed products of chlorosilane samples in hydrofluoric acid-containing reagents followed by evaporation of the silicon matrix as H₂SiF₆. The boron retained in the treated sample solution was then determined by a spectrophotometric method using carminic acid as a chromatic reagent. Special effort has been paid to the control of the analytical blank and reproducible determination of boron. The results indicate that addition of mannitol and proper control of the evaporation process are effective in preventing volatilization of boron during the evaporation of silicon matrix and can thus attain high recovery of boron and reproducible analysis. Through meticulous control of the analytical blank and experimental conditions, the limit of detection for boron determination with the established method can be as low as ng/g levels. Application of the methods to the determination of boron in various stages of purification of silicon and trichlorosilane as well as in borophosphosilicate film was conducted.     

Stoichiometry and mechanism of protonation of alkali metal salts of benzophenone radical anions by weak proton donors and its relevance to the base-catalyzed decomposition of benzopinacol

The Stoichiometry of the protonation of lithium and potassium salts of benzophenone radical anions and of the lithium salt of the fluorenone radical anion by methanol has been measured and found to be [(Ar₂C=O)^–]/[MeOH] =21. This result, which was obtained by the method of magnetic titration, implies that paramagnetism decays by the reaction between a ketyl anion and a ketyl radical (i.e., a protonated ketyl anion). The reactivities of alkali metal salts of fluorenone radical anions in relation to methanol exhibit a pronounced dependence on the nature of the counterion. No kinetic deuterium isotope effect has been found for the protonation of the lithium salt of the benzophenone radical anion in tetrahydrofuran (THF) bytert-pentyl alcohol. The lithium salt of the benzophenone radical anion inN,N,N,N-tetramethylethylenediamine (TMEDA) behaves markedly differently. Namely, its protonation by methanol exhibits 1 1 Stoichiometry and it reacts considerably more slowly withsec-butyl alkohol,K(THF)/K(TMEDA) = 2.5. Benzopinacol undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzhydrol. The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentration of the alkoxide. With a very strong base benzopinacol decomposes into two diphenyl ketyl anions. On the basis of this information as well as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pinacols in basic media.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 83–91, January, 1995.     

In-situ HEED structure analysis of AlNx films grown by the simultaneous use of a radical beam source and ICB technique

A reactive ionized cluster beam technique (RICB) which was composed of a conventional ICB source and a radical beam source has been used to deposit stable and metastable polycrystalline AlN_x (0x1) films. Using in-situ high energy electron diffraction (HEED) at grazing incidence geometry, crystallographic properties such as structure, preferred orientation and interplanar dspacing values were determined and the relation to deposition parameters investigated. It could be shown that the simultaneous use of the ICB technique and a radical beam source to separately control the kinetic energy of the Al ions and the dissociation rate of molecular nitrogen, allows AlN films to be deposited with variable composition and crystal structures. In-situ HEED used in the transmission mode is an effective tool to investigate the crystallography of growing compound films such as AlN_x.     

4th order derivative spectrophotometric determination of quinine in soft drinks

A method is developed for the 4rd order derivative spectrophotometric determination of down to 1–6 g/ml of quinine. This method has been applied to its determination in soft drinks. Comparison are made between this method and a spectrofluorimetric method.     

The photochemical addition of aldehydes to vinylsilanes

Conclusions The photochemical reaction of aldehydes of various structures with vinyl and allylsilanes have been investigated. A number of features of this reaction has been established and a convenient method for the synthesis of -silicon-containing ketones, including ketones containing a readily saponifiable alkoxy group on the silicon atom, have been found.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1464–1466, August, 1966.     

Esters of (1,2, 5-trimethyl-4-hydroxy-4-piperedyl)- and phenyl-(1,2, 5-trimethyl-4-hydroxy-4-piperidyl) acetic acids

The methyl ester of a new hydroxy acid of the piperidine series (-phenyl--(1,2, 5-trimethyl-4-hydroxy 4-piperidyl)acetic acid) has been prepared. Ethyl -(1,2, 5-trimethyl-4-hydroxy-4-piperidyl)acetate has been converted into the substituted pyridine, 1,1-diphenyl-2-(2-5-dimethyl-4-pyridyl)ethylene.     

Investigation of electronic structure of impurity and defect centers on the surface of silica by the MNDO method

The electronic structure of cluster models of fragments of a. silica surface with defect centers [stressed disiloxane bridges, radical (Si, SiO) and ionic (Si⁺, SiO^–) groups] and impurity centers (boron and aluminum atoms) has been investigated by the MO-LCAO-SCF method in the MNDO approximation. It has been shown on the basis of an analysis of the energetics of the defect structures (with complete optimization of the geometry of the clusters) that the dehydroxylation of silica with the formation of radical and ionic centers is energetically significantly more favorable than is the formation of disiloxane bridges. The influence of impurity boron and aluminum atoms on the properties of the surface fragments of SiO₂ has been considered. It has been shown that the migration of impurity boron atoms, which isomorphously replace silicon atoms in the lattice, from the bulk to the surface of silica is energetically unfavorable.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 407–418, July–August, 1988.     

Isotopic abundance measurements on solid nuclear-type samples by glow discharge mass spectrometry

A double-focusing Glow Discharge Mass Spectrometer (GDMS) installed in a glovebox for nuclear sample screening has been employed for isotopic measurements. Isotopic compositions of zirconium, silicon, lithium, boron, uranium and plutonium which are elements of nuclear concern have been determined. Interferences arising from the matrix sample and the discharge gas (Ar) for each of these elements are discussed. The GDMS results are compared with those from Thermal Ionization Mass Spectrometry (TIMS). For boron and lithium at g/g-ng/g levels, the two methods gave results in good agreement. In samples containing uranium the isotopic composition obtained by GDMS was in agreement with those from TIMS independently of the enrichment. Attempts for the determination of plutonium isotopic composition were also made. In this case, due to the interferences of uranium at mass 238 and americium at mass 241, the GDMS raw data are complementary with those values obtained from physical non-destructive techniques.     

New packing type of the conducting layer in the structure of the organic semiconductor (ET)6[Pb3Br9PhClMe2CO], the first ET-based lead-containing salt

A new organic semiconductor, (ET)₆[Pb₃Br₉PhClMe₂CO], which is the first radical cation salt based on bis(ethylenedithio)tetrathiafulvalene (ET) with a lead-containing anion, has been synthesized. The composition and crystal and molecular structure of this compound have been established by X-ray structural analysis. The crystal structure can be described as a layer structure. A new type of packing of ET radical cations has been found in the conducting layer. The anionic layer consists of [Pb₃Br₉ ^3–] _n polymeric anions and chlorobenzene and acetone solvate molecules bonded to polymeric anions through Br...H-C hydrogen bonds.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 910–916, May, 1995.This work was financially supported by the International Science Foundation (Grant RE1 000), the Scientific Council on the Problems of High-Temperature Superconductivity (Project No. 93030), and the Russian Foundation for Basic Research (Project No. 94-03-09950).     

Electronic and geometric structure of “vacancy + Li4” complexes in silicon

The electronic and geometric structure of Si₄H₁₂+Li₄ clusters, which serve as models for the vacancy +Li₄ complex in silicon, was calculated using the discrete variational X method. The stable configuration of the complex was found to be cubotetrahedral. The lithium atoms in this configuration are located on axes connecting the centers of the vacancies with the free apices of the cube of the first coordination sphere of the vacancies. The ground state in such a geometric configuration was found to be degenerate, which sould lead to Jahn-Teller distortion of the lattice near the complex. This result is in accord with the available experimental data.Branch of the Institute of Chemical Physics, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 35–40, September–October, 1989.     

(9-(Pyridazinyl-4)-4-azafluorenes and spiro compounds with 4-azafluorene and indeno[1,2-c]pyridazine (4h-5,6-dihydro-1,2-diazepine) fragments

9-(3,6-Diphenylpyridazinyl-4)-4-azafluorene has been synthesized, and its conversions at the C⁹ position have been studied. From its C⁹-hydroxy derivative, 3-phenylspiro-[4-azafluorene-9,5-indeno[1,2-c]pyridazine] has been obtained. From 9-(1,2-dibenzoylethylidene-4-azafluorene, 3,7-diphenylspiro-[4H-5,6-dihydro-1,2-diazepine-5,9-4-azafluorene] has been synthesized.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1357–1360, October, 1992.     

3-Hydroxy-2-(2"-Thienyl)-4H-Chromon-4-one as a Spectrophotometric Reagent for the Trace Determination of Zirconium in an Aqueous Phase

A rapid, simple, selective, and sensitive method for the trace determination of zirconium has been developed based on the reaction of 3-hydroxy-2-(2"-thienyl)-4H-chromon-4-one in an hydrochloric acid medium to form a yellow–colored complex which is rendered water soluble by the micellar action of Triton X-100 and measured at 415 nm. Most of the metal ions do not interfere with the determination. Beer's law is obeyed in the concentration range 0–2.0 g/mL and the molar absorptivity of the complex is 2.73 × 10⁴L mol^–1cm^–1; Sandell's sensitivity is found to be 0.0034 g cm^–2. The method has been applied for the determination of zirconium in various samples, and satisfactory results have been obtained.     

The very low-pressure pyrolysis of phenyl ethyl ether,phenyl allyl ether,and benzyl methyl ether and the enthalpy of formation of the phenoxy radical

A value of the enthalpy of formation of the phenoxy radical in the gas phase, ΔH°_,298K (?O·, g) = 11.4 ± 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether

1 Trivial names for ethoxy benzene, 2-propenoxy (allyloxy) benzene, and α-methoxytoluene, respectively

at very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(?O—H) = 86.5 ± 2 kcal/mol,

2 1 kcal = 4.18674 kJ (absolute)

in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE(RO—H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals into cyclopentadienyl radicals and CO has been observed at temperatures above 1000°K, and a mechanism for this reaction is proposed.     

Synthesis of the perfluoroacetyldiisopropylmethyl radical

A stable trifluoroacetylperfluorodiisopropylmethyl radical has been prepared by interaction of the (-fluorosulfonyloxytetrafluoroethyl)perfluorodiisopropylmethyl radical with anhydrous CsF. This reaction is the first example of a chemical transformation of a C-centered radical which is not resonance-stabilized with retention of the radical center.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 226–227, January, 1993.