Pore formation during the pyrolysis of C-Nb2O5 and C-Ta2O5 xerogels

Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb₂O₅ or Ta₂O₅ sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I Type IV) and of the hysteresis loops (H4H2H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Determination of highly polar compounds in water samples around former ammunition plants

Summary A method was developed for the determination of nitrobenzoic acids and nitrophenols as well as of diaminoaromatics and was applied to the analysis of water samples from the former ammunition plant at Elsnig (Saxony, Germany). The procedure is based on a preseparation into a neutral, an acidic and a basic fraction by multi-step extraction at different pH values followed by HPLC analysis with UV and electrochemical detectors, coupled in series. Applying optimized enrichment conditions, all investigated compounds were extracted from spiked distilled water with recoveries >80% and variation coefficients <7%. Similar results were obtained with spiked ground water samples. After enrichment, all compounds can be analysed by HPLC with UV detection at concentrations below 100 ngL⁻¹. The electrochemical detector (ELCD) allowed a selective and sensitive detection of the nitrophenols and especially of the diaminoaromatics and, therefore, provides, some advantages in the analysis of real samples. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Synthesis and reactivity of metal-containing monomers

Acid and neutral Co^II, Cu^II, Ni^II, Zn^II, Fe^II, and Fe^III maleates, fumarates, and itaconates were obtained and characterized. The methods for their synthesis were optimized, and the valence state and coordination of metals were studied. Co^II and Fe^II hydrogen maleates, Co^II maleate, and Co^II fumarate were examined by X-ray diffraction analysis. The ligands based on unsaturated dicarboxylic acids can be mono-, bi-, and tetradentate, which results in the formation of acid salts, chain and three-dimensional coordination polymers, whose double bond is not involved in the coordination. The strong antiferromagnetic exchange (μ_elf=1.41 and 0.34 μ_B at 290 and 80 K, respectively) was detected in Cu^II itaconate. Based on the data of Mössbauer spectroscopy, the partial reduction of Fe^III to Fe^II during the synthesis of Fe^III maleate was shown to occur: δFe=0.43 and 1.27 mm s^?1, ΔE _Q=0.57 and 3.13 mm s^?1 and Γ=0.37 and 0.28 mm s^?1 atT=298 K for Fe^III and Fe^II, respectively.     

The thermal decomposition of NaHCO3

New EGA findings revealed that the small endothermal event preceding that of the main decomposition of commercial NaHCO₃ involves the simultaneous evolution of water and CO₂. At very high sensitivity, EGA experiments evidenced that the above (limited) evolution of gases also took place from the recrystallized material for which thermal methods gave no indication of endotherms.Careful reexamination of previous DSC results indicated that for one kind of recrystallized material a very small endotherm had been neglected. Renewed experiments revealed that this endotherm can be enhanced if the samples are prepared by crushing and sieving in a wet atmosphere. Parallel FT-IR experiments on commercial and recrystallized materials demonstrated the presence of carbonate in samples that had previously been taken just beyond the first small endotherm; this confirmed the EGA results. SEM experiments showed that surface texture changes take place when samples are heated to temperatures just above that of the preliminary endotherm. On the basis of these new findings, the interpretation previously given to the small endotherm is revised and detailed knowledge is gained on the mechanism of decomposition of NaHCO₃.The authors express their gratitude to Dr. Stephen B. Warrington (Thermal Analysis Consultancy Service, Leeds Metropolitan University) for having communicated the EGA results that led to the present report. The authors also feel indebted to Dr. Mario Paolieri of the Centro Interdipartimentale per la Microscopia Elettronica e la Microanalisi (M.E.M.A.) for his help in performing and interpreting the SEM experiments, and to Mr. Paolo Parri for his careful preparation of the illustrative material. Financial support from the University of Florence (ex 60% MURST) is greatly appreciated.     

Intrinsic thermal stability and quenching recovery of thin-film superconductors with thermal boundary resistance

The stability behaviour of a thin-film superconductor under a localized release of thermal disturbance is investigated. Two-dimensional conjugate film/substrate conduction equation with anisotropic thermal conductivity of the film, and Joule heat are employed to investigate effects of substrate and thermal properties on the intrinsic stability and quenching recovery. To consider the thermal boundary resistance between film and substrate, an interfacial-layer model (ILM) with very low diffusivity and an acoustic mismatch model (AMM) are employed. Results show that the thermal boundary resistance influences strongly the intrinsic stability. Thermal boundary resistance increases intrinsic stability if the thermal conductivity of the substrate or the disturbance energy is large. Higher Biot numbers and thermal conductivity ratios of film to substrate in longitudinal direction influence stability favorably. We demonstrate also that operation of a film/substrate system, such as YBCO/MgO, is either intrinsically stable or irrecoverably unstable.The authors wish to express their sincere appreciation to Dr. R. C. Chen for his invaluable advice and suggestions during the course of this paper. This research was supported by the National Science Council of the R. O. C. through grant NSC 83-0401-E-009-006. The computations were performed on the IBM ES/9000 at the National Center For High-Performance Computing.     

HPLC of amino acids and oligopeptides by pre-column fluorescence derivatization with N-hydroxysuccinimidyl-α-(9-phenanthrene)-acetate

Summary A sensitive LC method for the detection of amino acids and oligopeptides with pre-column fluorescence derivatization has been developed. Glycine, glycylglycine, triglycine, glutathione, glutamic acid, and cysteine were separated on a reversed-phase C₁₈ column with methanol-water-triethylamine eluent, derivatization and chromatographic conditions were optimized. The six derivatives were eluted in 20 min with good reproducibility. The relative standard derviations (n=6) at an analytical concentration of 2×10⁻⁶ M are <5%. Detection limits (signal-to-noise ratio=3) for the six derivatives are 23–68 fmol.     

Phenol methylation over CrPO4 and CrPO4?AlPO4 catalysts

The vapor-phase catalytic alkylation of phenol with methanol and dimethyl carbonate on a series of differently prepared CrPO₄ (Cr/P=1) and CrPO₄-AlPO₄ (CrAIP) catalysts, has been studied at different temperatures (473–673 K). The reaction is first order in phenol, giving a mixture of O- and C-alkylated products (C-alkylation taking place preferentially at theortho-position). Moreover, dimethyl carbonate is a better methylating agent than methanol.     

Intramolecular cyclization of ketene containing both acyl andN-alkylimidoyl substituents. Synthesis and crystal and molecular structure of 3-benzoyl-5-phenyl-2H,4H-furo[3,2-c] isoquinolin-2-one

Thermal decarbonylation of 1-benzyl-4,5-dibenzoyl-2,3-dihydropyrrole-2,3-dione yields benzoyl[N-benzyl(phenylglyoxalimidoyl)]ketene. The latter undergoes intramolecular cyclization to 3-benzoyl-5-phenyl-2,3,4,5-tetrahydrofuro[3,2-c]isoquinolin-2-one. Which is oxidized to 3-benzoyl-5-phenyl-2H,4H-furo[3,2-c]isoquinolin-2-one under the reaction conditions. The crystal and molecular structure of the title compound was studied by X-ray structural analysis. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 563–565, March, 1997.     

Conformational analysis of 2-oxo-1,2,3,4,-tetrahydropyridine and its alkyl- and phenyl-substituted derivatives

The molecular geometries and inversion barriers of the rings in 2-oxo-1,2,3,4-tetrahydropyridine and its alkyl-substituted (Me, Et, Prⁱ, or Bu^t) and phenyl-substituted derivatives were calculated by the molecular mechanics method. The introduction of substituents has no substantial effect on the equilibrium conformation of the heterocycle (a distorted sofa). For 4-alkyl- and 3-alkyl-substituted derivatives (except for 4-Me and 4-Et derivatives), an axial orientation of the alkyl group is more favorable. The phenyl substituents have equatorial and axial orientations at postions 4 and 3, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1584–1586, September, 1997.     

Contracted polarization functions for B to Ar

Using optimal exponents for B through Ne given by Dunning and those for Al through Ar by Woon and Dunning, d-type contracted polarization functions (2d/1d), (3d/1d), and (3d/2d) are generated from natural orbitals of atomic single and double excitation configuration interaction (SDCI) calculations, where the numbers before and after the slash are those of the primitive and contracted Gaussian type functions. The resulting contracted functions are tested on N₂ and P₂ molecules by self-consistent field and SDCI calculations, which clarify characteristics of the present polarization functions. Received: 5 June 1997 / Accepted: 20 August 1997