Synthesis of BaCuO2 by Different Ways

In order to optimize the synthesis of BaCuO₂, different modes were developed and estimated. Wet chemical powders produced by coprecipitation of oxalates or hydroxycarbonates were characterized and their reactivity is compared with the conventional ceramic process of firing a mixture of BaCO₃ and CuO by DTA, XRD and IR-investigations. This becomes possible, because the decomposition of these chemical precursors to BaCO₃ and CuO is finished up to 600 °C. Hence, the same reaction for the formation of BaCuO₂ is monitored.     

On the preparation of tungsten oxides by chemical transport with TeCl4

The Tungsten oxides WO₃, W₂₀O₅₉, W₁₈O₄₉ and WO₂ have been transported by TeCl₄ in monocrystalline Form. W₃O is not a equilibrium phase of the system, and therefore it is impossible to obtain this phase by CTR. The value of the lower phase boundary of tungsten trioxide is 2.96.     

Untersuchungen zum Zustandsdiagramm EuCl2–EuCl3

Investigations on the Phase Diagram EuCl₂–EuCl₃ The phase diagram EuCl₂–EuCl₃ was determined by difference‐thermoanalysis and the barogram of the system was determined by total pressure measurements of different compositions in function of temperature. By the results the phase diagram was precised. Eu₅Cl₁₁ decomposes at 565 °C in to EuCl_2,s and Eu₄Cl_9,s and shows a transition at 510 °C. Eu₄Cl₉ decomposes at 603 °C in to EuCl_2,s and Eu₁₄Cl_33,s. Eu₁₄Cl₃₃ decomposes at 612 °C peritectically in EuCl_2,s and a EuCl₃‐rich melt. Eu₁₄Cl₃₃ shows two high temperature transition points at 570 °C and 580 °C. The endothermal effect of compositions near Eu₁₄Cl₃₃ was interpreted as an endothermal formation of this compound. The eutectikum was detected between Eu₁₄Cl₃₃ and EuCl₃ to 60 ± 1 Mol‐% EuCl₃ at 530 °C.     

Zum chemischen Transport von Bismutoxidhalogeniden BiOX (X = Cl,Br, I) mit Halogen,Halogenwasserstoff sowie Wasser und Bestimmung der Molwärmen von BiOX

Chemical Transport of Bismuth Oxide Halides BiOX (X = Cl, Br, I) with X₂, HX and H₂O, and Determination of the Molar Enthalpies of BiOX By comparison of calculated and experimental chemical transport behaviour of BiOX (X = Cl, Br, I) with X₂, HX, and H₂O it was shown, that we understand the transport of BiOCl, BiOBr and BiOI with X₂ and HX in terms of the well known gaseous spezies in the systems. The existence of gaseous complexes Bi(OH)₂X is be concluded from high transport rates of BiOX with water, and their enthalpies and entropies were derived. The molar enthalpies and standard entropies of BiOX were determined by low temperature C_p measurements. (Data see Inhaltsübersicht)     

Exciton decay mechanism in DNA single strands: back-electron transfer and ultrafast base motions

The photochemistry of DNA systems is characterized by the ultraviolet (UV) absorption of π-stacked nucleobases, resulting in exciton states delocalized over several bases. As their relaxation sensitively depends on local stacking conformations, disentangling the ensuing electronic and structural dynamics has remained an experimental challenge, despite their fundamental role in protecting the genome from potentially harmful UV radiation. Here we use transient absorption and transient absorption anisotropy spectroscopy with broadband femtosecond deep-UV pulses (250–360 nm) to resolve the exciton dynamics of UV-excited adenosine single strands under physiological conditions. Due to the exceptional deep-UV bandwidth and polarization sensitivity of our experimental approach, we simultaneously resolve the population dynamics, charge-transfer (CT) character and conformational changes encoded in the UV transition dipoles of the π-stacked nucleotides. Whilst UV excitation forms fully charge-separated CT excitons in less than 0.3 ps, we find that most decay back to the ground state via a back-electron transfer. Based on the anisotropy measurements, we propose that this mechanism is accompanied by a structural relaxation of the photoexcited base-stack, involving an inter-base rotation of the nucleotides. Our results finally complete the exciton relaxation mechanism for adenosine single strands and offer a direct view into the coupling of electronic and structural dynamics in aggregated photochemical systems.

Despite its key role in DNA photochemistry, the decay mechanism of excitons in stacked bases has remained difficult to resolve. Ultrafast polarization spectroscopy now reveals a back-electron transfer and ultrafast base motions in adenosine strands.     

Abscheidung von Cu2O aus der Gasphase im strömenden Medium

CVD-experiments in the system Cu O H Cl N result in the growth of Cu₂O single crystals. The characteristic faces of the various forms are the {100} ones. A thermodynamic analysis permits to predict such conditions under which Cu₂O is deposited without deposition of any other phases.     

On the chemical transport of some niobium oxides with TeCl4 (II). The transport behaviour of the equilibrium NbO2.417 to NbO2.5

A report is given on the transport behaviour of the equilibrium phases NbO_2.417 to NbO_2.483 and of niobium pentoxide during chemical transport with TeCl₄. – It became apparent that phase-pure NbO_2.417 and Nb₂O₅ are deposited from the investigated equilibrium phases. Chemical transport of the phases NbO_2.453 to NbO_2.483 produces a mixture of several phases. This indicates a strong inhibition of the equilibration in the starting solid. Chemical transport of the niobium pentoxide caused the deposition of monocrystals of the upper and lower phase boundary. The experimental behaviour agrees very well with the calculated results. Monocrystals of the niobium pentoxide forms H, M, N, B and P are deposited in a well identifiable manner by variation of the temperatures (T₂, T₁).     

Zum chemischen Transport der Übergangsmetalldioxide mit Tellurhalogeniden

The thermodynamic relationship between the transition metal dioxides and their tetrahalogenides is demonstrated, and a comprehensive survey on the thermodynamic values of the compounds is given. – All dioxides of the transition metals existing to 1000°C in the solid state are transportable by TeCl₄. Only those dioxides are transported by TeBr₄ which have corresponding oxide-bromides existing in the gaseous phase. – The forms of crystallographic growth are very similar. The favoured growth direction is [001], the crystal faces are {110}- and {100}-prisms, {101}- and {111}-pyramids, respectively.     

Thermisches Verhalten und Kristallstruktur von YAl3Cl12

Thermal Behaviour and Crystal Structure of YAl₃Cl₁₂ We determined the thermodynamic data of YAl₃Cl₁₂ ΔH = ?739.9 ± 3 kcal/mol and S = 136.1 ± 4 cal/K · mol by total pressure measurements and ΔH = ?739.1 ± 1.6 kcal/mol by solution calorimetry. Using DTA-investigations we established the phase diagram in the system AlCl₃–YCl₃. The crystal structure was refined on the basis of single crystal data (P3₁ 12; Z = 3; a = 1 046.8(2); c = 1 562.3(3) pm).