Cyclisation reactions with hexafluoroacetone and phosphonous acid derivatives containing the trifluoromethyl and pentafluoropropenyl group; molecular structures of two 1,3,2λσ-dioxaphospholanes

1,3,2λ⁵σ⁵-Dioxaphospholanes have been obtained from trifluoromethylphosphonous aciddiethyl ester, the corresponding bis(diethyldiamide), (Z)-1,2,3,3,3-pentafluoropropenyl-phosphonous acid diethyl ester and hexafluoroacetone. (Z)-1,2,3,3,3-Pentafluoropropenylphosphonousacid bis(diethylamide) furnished, unexpectedly, 2-bis(diethylamino)-2,3-difluoro-4,5,5-tris(trifluoromethyl)-1,2λ⁵σ⁵-oxaphospholene-(3). The structures of the twoCF₃ group-containing dioxaphospholanes were confirmed by single-crystal X-ray investigationsand shown to exhibit distorted trigonal bipyramids with CF₃ occupying the axialposition.     

A Sn NMR and Mössbauer study of some di- and tri-alkyltin(IV) alkoxides

¹¹⁹Sn chemical shifts and ^119mSn Mössbauer parameters of a series of tributyltin alkoxides and cyclic and acyclic dibutyltin alkoxides have been recorded, in order to study the self-association of these molecules in the solid and in the liquid states.

The results show that the degree of association is markedly dependent upon the size and number of alkoxide groups attached to the tin atom. Dilution and temperature effects on the ¹¹⁹Sn chemical shifts are also described.     

On the relative strengths of amide…amide and amide…water hydrogen bonds

We have performed Hayes—Stone intermolecular perturbation theory (IMPT) calculations on amide…water and amide…amide complexes in order to estimate the change ΔW in intermolecular interaction energy associated with the hydrogen bond exchange process amide(NH)…water+water…(OC)amideamide(NH)…(OC)amide+water…water. ΔW is found to be small and varies by almost 5 kJ/mol and in sign for the amides formamide, acetamide, N-methyl formamide and N-methyl acetamide. The main variations in the amide hydrogen bond energies occur in the electrostatic and exchange-repulsion contributions. This reflects the variation in the charge distributions of the hydrogen bonding groups between the different amides. Thus, we cannot quantify an isolated hydrogen bond strength with any great accuracy, and care must be used in extrapolating model potentials based on small model systems to peptides and proteins.     

Formation and structure of [Co{Ph2PN(Bu)PPh2-P,P′}2(CO)][Co)(CO)4] - a cage molecule-pair or an ion-pair complex?

The strong π-acid ligand Ph₂PN(ⁱBu)PPh₂ reacts with Co₂(CO)_S (1:1) to give Co₂[μ-Ph₂PN(ⁱBu)PPh₂] (μ-CO)₂(CO)₄ (1); however, when the ratio is 2:1 a novel species [Co{Ph₂PN(ⁱBu)PPh₂-P,P′}₂(CO)][Co(CO)₄] (2) has been obtained. Crystal data for 2: M_r = 1140.83; triclinic, space group P , a = 12.330(2), b = 13.340(2), c = 18.122(3) Å, = 86.63(1), β = 80.75(1), γ = 84.24(1)°, V = 2924 ų, Z = 2; R = 0.060 for 3711 reflections having I 3σ(I). The results of X-ray diffraction, ESR, variable-temperature magnetic susceptibility, conductivity, and XPS analysis support that the species 2 is a d⁹-d⁹ cage molecule-pair. The mechanism for the formation of the species 2 has been investigated initially by ³¹P NMR.     

Inelastic neutron scattering studies of amorphous network solids

The inelastic neutron scattering technique provides information on the vibrational spectra of amorphous network glasses which extends that available from optical and resonance spectroscopy. A range of instrumentation is available to cover energies corresponding to relaxation phenomena (< 1 μeV) up to hydrogen stretching frequencies (500 meV). The resolution attainable is now sufficient to give data, the line width of which is dominated by the natural broadening caused by the structural variations in the networks. Additional information on atomic motions within a vibrational mode is given by the dependence of the scattered intensity on the magnitude of the scattering vector, Q, as revealed by the dynamical structure factor, S(Q, E). Low energy excitations (<10 meV) characteristic of the amorphous state have been related to anomalous low temperature thermal properties and also contain information on the range of topological order within the network. Experimental investigations using a variety of neutron inelastic scattering instrumentation to study vitreous silica, vitreous opal, vitreous boron trioxide, the amorphous germanium-selenium system and amorphous silicon containing hydrogen or deuterium are described. The results are used to discuss the possibilities and limitations of the method for testing structural models of network glasses.     

ZrO2-SiO2 materials prepared by sol-gel

Gels with composition xZrO₂−(100−x)SiO₂, X = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors.

Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment.

Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.     

Determination of diffusion coefficient of titanium ion in TiO2---SiO2 wet gel prepared from metal alkoxides during leaching

It was found that the radial gradient refractive index (r-GRIN) glass rod could be produced by the leaching of wet gel prepared by a sol-gel method from metal alkoxides and heat-treatment of the gel. The concentration gradient of the doped component resulting from its diffusion in the wet gel was studied. In order to estimate the exact diffusion coefficient of the titanium ion in the binary TiO₂---SiO₂ wet gel, its value was measured by using two methods. The observed diffusion coefficient by colorimetric analysis was about 1.1 × 10⁻⁶ cm²/s, while the value obtained from analysis of EPMA was about 1.2 × 10⁻⁶ cm²/s, and both values were nearly equal to each other. The activation energy of titanium ion diffusion in the wet gel was estimated to be about 11 kcal/mol.     

RHEED studies of MOMBE growth using TMGa or TEGa with As2

MOMBE and CBE growth has until recently been based on largely empirical studies of the epitaxial process. We have used reflection high energy electron diffraction (RHEED), previously applied to the study of MBE, to study the growth GaAs using TMGa and As₂. In this work we have extended our previous studies to include a detailed study of the effect of As₂ flux on growth rate and to compare data on singular and vicinal plane surfaces cut off orientation in two orthogonal {110} directions. Clear evidence for site blocking mechanisms is observed together with an indication that the concentration of elemental Ga present on the surface during growth is negligible even under conditions where the arrival rate of TMGa exceeds that As₂. We have compared this behaviour with that observed using TEGa and As₂ under identical conditions. Using TEGa a conversion from a (2×4) to (4×2) reconstructed surface is observed under As deficient conditions indicating the presence of elemental Ga on the surface. This is accompanied by an abrupt change in growth rate similar to that secn in MBE.     

A structural study of Y2O3---Al2O3---SiO2 glass employing partial RDFs obtained by anomalous scattering

A structural study of Y₂O₃---Al₂O₃---SiO₂ glass has been performed using the partial radial distribution function (RDF) obtained by anomalous scattering at the Y absorption edge. The partial RDF consists of contributions from atom pairs, such as Y---O, Y---M(Si, Al) and Y---Y, related to the local arrangements around the Y atoms, while those of all of the constituent atom-pairs are convoluted into the ordinary RDF. The M(Si, Al)---O, Y---O, M---M, Y---M, and Y---Y interatomic distances and the coordination numbers around the Al and Y atoms are obtained from the two RDFs. The local structures around the Al and Y atoms in the glass are discussed.     

Influence of oxygen contamination during Si low presure vapour phase epitaxy on epitaxial layer quality

The influence of oxygen contamination on Si low pressure vapour phase epitaxy (LPVPE) at 800°C in the SiCl₂H₂−H₂ system has been investigated. O₂ was added intentionally with partial pressures between 10⁻⁸ and 2×10⁻⁴ mbar. The quality of the epitaxially grown silicon layers was determined by comparing surface morphology, defect density, Schottky diode characteristics and SIMS measurements. These four parameters are correlated and they reveal a drastic decrease in epitaxial layer quality for O₂ pressures above 15×10⁻⁶ mbar. The critical oxygen pressure which has been until now considered as a limit for epitaxial growth can therefore be exceeded by one order of magnitude.