Berichte der Bunsengesellschaft für physikalische Chemie最新文献

The simplicity of density functional methods and the increasing availability of computer-processing power have enabled ab initio modelling techniques to be applied to very large systems. Our own code, AIMPRO, enables the relaxation of clusters as large as 800 atoms while still maintaining full self-consistency and a realistic basis. Moreover, the use of pseudopotentials means that applications to Ge clusters are no more time consuming than those built of C. An important thrust of our work has been the computation of accurate vibrational modes of defects and this has caught the attention of several prominent experimental groups with the consequence that joint studies of rather complicated defects have been successfully made. We outline the procedures used and discuss applications that have been made to a wide variety of non-metallic systems such as vacancy defects in diamond, interstitial centres in silicon and carbon-related defects in III–V materials.

A time domain reflectometer for dielectric relaxation spectroscopy of electrolyte solutions in the temperature range of −;45 ≤ ϑ /°C ≤ 90 is presented. With a set of cutoff cells differing in their cell constants, pure solvents covering the permittivity range of 7 ≤ ϵ ≤ 228 and their concentrated electrolyte solutions (κ > 1 Ω⁻¹ m⁻¹) can be investigated at frequencies of 0.05 ≤ v/GHz ≤ 9. In combination with the transmission line system of our laboratory (8.5 ≤ v/GHz ≤ 89) dielectric dispersion, ϵ′(v), and total loss, η″(v) = ϵ″(v) + κ/(2π vϵ₀) can be determined with 2% accuracy relative to ϵ.

An oscillating cup viscometer interfaced to an automatic data acquisition and processing system (computer, interface, software) was built for viscosity measurements in molten salts below 1400 K.

In order to avoid the “meniscus effect”, the main source of errors of the method, the measurements were carried out in the so-called “full cup” condition, i.e. with the height of the liquid column (h) of 11.2 cm, the overall height of the cup being 11.9 cm. An adequate software supplied η values at each temperature, as function of R, I, δ, h, ρ and T values in air and with liquid containing cup. The viscosity of molten KNO₃ and NaCl, both of “standard salt purity”, was reinvestigated over a temperature range of 100 K above their melting points. The standard deviation of the data obtained was ±0.38%. These data were systematically lower (3% for η_NaCl and 2% for η) than those obtained earlier by the same method, but agree within ±0.9% with the recommended Janz's “calibration quality viscosity data”.

One can conclude that the method gives accurate results only if operated in “full cup” condition.

Vapour pressures of di-n-propylether with ethanol and with 1-butanol were measured at eight temperatures between 278.15 and 323.15 K by a static method. Reduction of the vapour pressures to obtain activity coefficients and excess molar Gibbs free energies was carried out by Barker's method. Mixtures containing ethanol show azeotropy with a minimum boiling temperature. G^E, H^E, and V^E, at 298.15 K, of mixtures of di-n-propylether + methanol, + ethanol, + 1-propanol, + 2-propanol, and + 1-butanol are compared with predictions of the ERAS model observing that the model reproduces acceptably the experimental behaviour.

Using a numerical simulation method, the electrostatic properties of grain boundaries in titanate ceramics (SrTiO₃ and BaTiO₃) are calculated by the combination of a defect chemistry model and the general potential equation. Based on these results and a simple three-dimensional “brick-wall-model” of the ceramic microstructure, an equivalent network is developed for the ceramic body which consists of a great number of R C-branches. Using this network it is possible to calculate the frequency dependence of the complex impedance of the ceramic. The results of the impedance simulation can be compared to experimental results. This comparison yields information about the charge transport across and along the grain boundaries and about the influence of the temperature, the bulk dopant concentrations, and the donor- or acceptor-like grain boundary interface states on the electrical properties of the grain boundaries. By using the simulation technique one gets additional information to interpret and understand the experimental results and to model the physical and electrical behaviour of the grain boundaries which mainly determine the electrical characteristics of the ceramic.

The reaction of labile square-planar diethylenetriaminepalladium(II) complexes with hydrogen carbonate ion was studied by stopped flow spectrophotometry as a function of [HCO⁻₃] and [Cl⁻] and constant [Pd], ionic strength and temperature to investigate the substitution behaviour with a nucleophile capable of affecting the solution pH. The kinetic results significantly deviate from those for normal square-planar substitution behaviour. The reaction comprises two consecutive equilibrations, which have complex kinetics compared to the simple two-term rate law normally encountered, and which represent hydrogen carbonate inhibited and induced substitution reactions, respectively. The suggested mechanism and data treatment account for the observed deviating kinetics and provide alternative ways of obtaining formerly determined aquation and anation rate constants with comparable accuracy, even for complexes too labile to be monitored directly.

Fast scan voltammetry gives kinetic evidence for the reversible dimerization of the radical cations of thianthrene (T) and 2,3,7,8-tetra-methoxythianthrene (TMOT) in acetonitrile. At 256 K an equilibrium constant K_Dim ≈ 900 for TMOT (K_Dim ≈ 1.1·10⁴ for T) and rate constants k_f ≈ 1·10⁷ and k_b ≈ 1.2·10⁴ were determined. Using the semi-empirical PM3 method, the formation of σ-bonded dimers is indicated. For the dimerization reaction in acetonitrile, enthalpy values of −79 kJ/mol for T and −56 kJ/mol for TMOT respectively were calculated. Despite the folded structure of the neutral T and TMOT systems and their planarization during the radical cation formation, the heterogeneous electron transfer kinetics was found to be very fast in both cases (1.6 cm/s; 298 K). The activation enthalpy was determined to be 14 kJ/mol in the case of TMOT. Calculations of the inner-sphere reorganization energy using the Marcus theory produced ΔH^#(∞) = 4.6 kJ/mol for TMOT and 3.9 kJ/mol for T respectively at infinite temperatures. This explains the fast electron transfer kinetics and their low activation enthalpies.