Berichte der Bunsengesellschaft für physikalische Chemie最新文献

Rate constants for the gas-phase reaction of NO₃ radicals with 1-butene, trans-butene, 2-methyl-2-butene and 2,3-dimethyl-2-butene have been obtained in a flow system at 298±2 K and a pressure of 3 mbar He using NO₃ LIF measurements. NO₃ radicals were produced via thermal decomposition of N₂O₅ at ca. 400 K, i.e. N₂O₅+M → NO₃+NO₂+M.

Applying pseudo-first-order conditions, the rate constants for 1-butene and trans-butene were found to be (1.04±0.11)·10⁻¹⁴ and (3.74±0.45)·10⁻¹³, respectively. For 2-methyl-2-butene and 2,3-dimethyl-2-butene the rate constants were obtained under second-order conditions to be (1.03±0.09)·10⁻¹¹ and (5.49±0.42)·10⁻¹¹, respectively (unit: cm³ molecule⁻¹ s¹).

The spectroscopic properties of 4-methyl-6-phenyl-2,2-difluoro-1,3,2-dioxaborine 1 and of several of its derivatives 2–5 substituted in para position on the phenyl ring or bearing an alkylene chain between their carbo-cyclic and heterocyclic rings have been studied in solution of different polarities and in the solid state. In solution, an important intramolecular charge transfer (ICT) process is characterized in the excited state. This charge transfer occurs in a planar configuration of the molecules, as demonstrated by comparison of the data obtained for 1 and 3 with those of the rigid molecules 6 and 7, respectively. The singlet excited state of molecules 1–5 is typical of species having a free rotating substituent and for which the small fluorescence quantum yield and lifetime increase when a planar configuration can be stabilized by electron-donor substitution. The low fluorescence quantum yield found for molecules 1 and 2, with respect to molecules 3–5, has been attributed to an important deactivation channel resulting from the rapid free rotation of the phenyl moiety of the molecule and not to an efficient intersystem crossing to the triplet excited state, as this process has a quantum yield lower than 0.01. Theoretical calculations lead to a low oscillator strength for molecules 1 and 2 which are more stable with a planar configuration, while a significant higher oscillator strength is calculated for compounds 3 and 4 substituted with electron-donating groups. The singlet excited state of compounds 4 and 5 might be stabilized by the formation of a twisted species. In the solid state, the molecules are shown by X-ray analysis to be planar and packed in such a way that two molecules lie head-to-tail in two parallel planes separated from each other by ca. 360 pm. As a result, this proximity enables the development of a ground state charge transfer. This charge transfer species has been confirmed by increasing the concentration of compound 2 in dichloromethane solution, giving rise to a new fluorescence band in the long wavelength region.

Steel electrodes were investigated under electrochemically controlled conditions at pH 13. The linear reflectivity at 1064 nm and the second harmonic signal (SHG) at 532 nm generated in reflection were measured for different polarizations and recorded simultaneously with cyclovoltammograms. The potential was varied between −1.4 V and 0.5 V vs. SCE and the optical signals were recorded after repetitive cycling. The reflectivity which probes the whole thickness of the electrochemically active layer and the non-linear optical signal which, in contrast, only probes the electrode surface and its immediately vicinity, exhibit very different behavior with respect to their potential dependence. In a cathodic scan the increase of the respective optical signals is associated with different peaks of the cyclovoltammogram. Furthermore, the SHG signal is strongly dependent on the polarization. The pronounced changes around −1.25/-1.1 V in the cathodic/anodic scan direction, which are observed for pp-polarization, are absent in sp- and ms-polarization. The sp-signal shows features which are paralleled by a decrease in the linear reflectivity. Contrary to the other polarizations, the ms-polarized signal does not show any particular features correlated with peaks of the cyclovoltammogram. The potential dependence of the surface charge expressed by a simple parabolic model describes the SHG signal over extended parts of the potential range. The differences between the potential dependence of the SHG signal and the change of the linear reflectivity fully agree with the two layer model of electrochemically formed iron oxides.

Multistep protolysis or metal-complex systems can be divided into their individual titration equilibria by the concept of projective mapping using diagrams which contain only absorbance data. In this way, the individual equilibria constants and their ratios can be very easily determined. The concept can be transfered to the spectroscopic-kinetic analysis of linear reaction systems. Reaction systems which are described by two linearly independent concentration variables (s = 2) can be reduced to systems which depend only on one linearly independent concentration variable (s = 1) by replacing the perspective projection by a parallel projection. The new method is demonstrated by application to the spontaneous hydrolysis of Boc-gly-ONP (N-tert.-Butoxycarbonyl glycine p-nitrophenylester) and oNPA (o-nitrophenyl acetate) in borax buffer (pH = 8.7).

With recent developments in solid state detectors combined with X-rays at glancing angles of incidence, Extended X-ray Absorption Fine Structure (EXAFS) experiments on arsenic ion-implanted into amorphous silicon surfaces have been obtained at dilutions down to 0.01 at%. Structural relaxation processes in both hydrogen-containing and hydrogen-free material have been examined, including the effects of single doping and of counterdoping. In addition the different careers of solid phase epitaxy and amorphisation have been followed from the standpoint of the arsenic environment. EXAFS has been used to establish the degree of crystallinity in the vicinity of arsenic impurities at each stage so that this can be compared with the overall extent of crystalline order in the ion-implanted silicon.

The field-enhanced conductivity of grain boundary space charge depletion layers in acceptor-doped SrTiO₃ ceramics was investigated by impedance analysis in the time domain. The dependence of the GB conductivity on the external applied electrical field are discussed and interpreted in terms of a Schottky diffusion model.

In this article we discuss the results of molecular dynamics simulations which were performed to get more information on the physical properties of sodium octanoate micelles. The calculations were based on the AMBER forcefield and we succeeded in simulating the dynamic features of the micelle for a period of 300 ps. The aggregate was surrounded by 870 water molecules and the simulation was performed in the isothermic-isobaric constant NPT ensemble at a temperature of 300 K and a pressure of 10⁵ Pa. From the results, thus obtained, it was possible to obtain some typical properties of the micelle. It turns out that the average radius of the micelle coincides pretty well with experimental results of small angle neutron scattering experiments. It was also possible to determine the actual conformations of monomers outside and inside the micelle. These data are in general agreement with nuclear magnetic resonance studies and with raman scattering experiments. On close inspection it was found that the micelle attains a slightly ellipsoidal shape.

While linear and curved phase transition lines in P(T) or H(T) diagrams can be described by adding linear and quadratic terms in a joint expansion of the Landau coefficients a, b in terms of P–P_t and T–T_t where (P_t, T_t) is a critical point, a new class of phase diagram is obtained by allowing for bilinear, ϵ(P–P_t)(T–T_t), and higher product terms. In this contribution we relate these new expansion terms to some hitherto unexplained experimental P–T and H–T phase diagrams.

Spectroscopic studies can provide important information on physico-chemical processes in solids, like, e.g., order-disorder processes, the point defect structure and local defect arrangements, and heterogeneous as well as homogeneous chemical reactions of solids. Results will be reported relating to these topics from experiments that have been performed under in situ conditions, i.e. at high temperatures and under defined conditions of oxygen partial pressures. The examples have been chosen from high-temperature studies using optical spectroscopy and Mössbauer spectroscopy.