Composition of saturated vapor over Ytterbium bromides

The vaporization process of ytterbium di- and tribromide was studied using high-temperature mass spectrometry over the temperature range of 850 to 1300 K. It was ascertained that, at the early vaporization stages, the vapor contained molecules YbBr₃, YbBr₂, YbBr, Br₂, Yb₂Br₂, Yb₂Br₃, Yb₂Br₄, Yb₂Br₅, Yb₂Br₆, and atoms Yb and Br. The partial pressures of all components of saturated vapor were calculated. It was found that vapor composition reflected the course of the reactions of decomposition of tribromide and disproportionation of dibromide in the condensed phase. It was concluded that vaporization of di- and tribromide was incongruent at the initial stages; vaporization of both agents acquired a congruent character with the Yb: Br = 1.0: 1.9_±0.2 ratio with time.     

Interaction of Mo(CO)6 and its derivative fragments with the Cu(001) surface: Influence on the decomposition process

A theoretical study on the adsorption and decomposition of molybdenum carbonyl on the copper (001) surface is reported. The adsorption structures and energies of Mo(CO)_n molecules (n = 1 … 6) are computed systematically using density functional theory with Van der Waals corrections. By analyzing the energies of the various conformations, the main factors that determine the stable adsorption geometry are identified. Insight into the thermodynamics of decomposition is gained by calculating the reaction energy for dissociation of Mo(CO)_n into Mo(CO)_n?1 and CO. In the gas phase, this reaction is highly endothermic for all n. On the Cu surface, however, removal of the first CO group (n = 6) becomes strongly exothermic. The subsequent dissociation steps (n < 6), are endothermic even on the surface, but the reaction energies are much reduced. Dissociation is found energetically more favorable than desorption in all cases. The results clearly show that molybdenum carbonyl decomposition is strongly facilitated by the presence of the Cu surface. © 2014 Wiley Periodicals, Inc.     

Interfacial shear rheology

This review summarises the interfacial shear rheology in the context of problems occurring during the measuring process. The main areas covered are surfactants, proteins, macromolecules, monolayers, particles or mixed systems at the gas/liquid and liquid/liquid interface. New developments in measuring techniques, in data analysis, modelling and theory will be discussed, while micro-rheological techniques using optical or magnetic tweezers are not in the scope of this contribution.     

Synthesis and characterisation of coating polyurethane cationomers containing fluorine built-in hard urethane segments

Polyurethane cationomers were synthesised in the reaction of 4,4’-methylenebis(phenyl isocyanate) with polyoxyethylene glycol (M?= 2,000) or poly(tetrafluoroethyleneoxide-co-difluoromethylene oxide) α,ω-diisocyanate and N-methyl diethanolamine. Amine segments were built-in to the urethane-isocyanate prepolymer in the reaction with 1-bromobutane or formic acid, and then they were converted to alkylammonium cations. The obtained isocyanate prepolymers were then extended in the aqueous medium that yielded stable aqueous dispersions which were applied on the surfaces of test poly(tetrafluoroethylene) plates. After evaporation of water, the dispersions formed thin polymer coatings. ¹H, ¹³C NMR and IR spectral methods were employed to confirm chemical structures of synthesised cationomers. Based on ¹H NMR and IR spectra, the factors κ and α were calculated, which represented the polarity level of the obtained cationomers. The DSC, wide angle X-ray scattering and atom force microscopy methods were employed for the microstructural assessment of the obtained materials. Changes were discussed in the surface free energy and its components, as calculated independently according to the method suggested by van Oss–Good, in relation to chemical and physical structures of cationomers as well as morphology of coating surfaces obtained from those cationomers. Fluorine incorporated into cationomers (about 30%) contributed to lower surface free energy values, down to about 15 mJ/m². That was caused by gradual weakening of long-range interactions within which the highest share is taken by dispersion interactions.     

Tuning Deazaflavins Towards Highly Potent Reducing Photocatalysts Guided by Mechanistic Understanding – Enhancement of the Key Step by the Internal Heavy Atom Effect

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo-induced electron transfer, in which their triplet state and semiquinone – the latter is formed from the former after electron transfer from a sacrificial electron donor – are key intermediates. Guided by mechanistic investigations aiming to increase intersystem crossing by the internal heavy atom effect and optimising the concentration conditions to avoid unproductive excited singlet reactions, we synthesised 5-aryldeazaflavins with Br or Cl substituents on different structural positions via a three-component reaction. Bromination of the deazaisoalloxazine core leads to almost 100 % triplet yield but causes photo-instability and enhances unproductive side reactions. Bromine on the 5-phenyl group in ortho position does not affect the photostability, increases the triplet yield, and allows its efficient usage in the photocatalytic dehalogenation of bromo- and chloroarenes with electron-donating methoxy and alkyl groups even under aerobic conditions. Reductive powers comparable to lithium are achieved.     

Structural dependence of non-linear optical properties of 4-Dimethylamino-3-cyanobiphenyl

 4-Dimethylamino-3-cyanobiphenyl (4-DMA-3-CB) was characterized with respect to linear and nonlinear optical properties in a crystal as well as in solution. The crystal structure was studied dependent on the crystallization conditions. It is shown that the crystal structure exhibiting NLO-activity can completely be solved by a combination of electron diffraction and computer modeling. There are four molecules per unit cell in the space group Pna2₁ with dimensions a=10.28 Å, b=22.64 Å, c=5.27 Å. From this model structure the values and orientation of the dipole μ and static second order polarizability β can be calculated. Their relevance to the values obtained by a combination of polarization dependent measurements of Electric Field Induced Second Harmonic Generation (EFISH) and Hyper-Rayleigh-Scattering (HRS) in solution are discussed. The molecular second order polarizability tensor was found to be dominated by one single component. The orientations of the dipole and the vectorial parts of the second order polarizability delivered by the semiempirical calculations are in good agreement with the results of the EFISH and HRS-measurements and allow a deeper insight into the nonlinear optical properties of the crystal. Received: 15 April 1996 Accepted: 21 June 1996     

AES Depth profiling of semiconducting multilayer structures using an ion beam bevelling technique

An ion beam technique has been developed which allows the preparation of bevels from semiconducting heteroepitaxial structures with smooth surfaces and very flat angles in the order of 0.1°. The bevels are used for AES depth profiling of heterostructures by the line scan technique. Measured and calculated line scans from (Al,Ga)As/GaAs and SiGe/Si test structures are compared to estimate the contributions of the electron escape depth and the ion beam mixing to the depth resolution. Received: 21 August 1996 / Revised: 14 January 1997 / Accepted: 18 January 1997