Berichte der Bunsengesellschaft für physikalische Chemie最新文献

The rate of the intramolecular charge-transfer state formation has been determined for 4-(N, N-dimethylamino)-benzonitrile (DMABN) in supercritical CF₃H by means of a time-correlated single-photon counting technique. The rate constant increases rapidly with increasing CF₃H density, indicating the importance of the polar environment offered by the CF₃H molecules which are clustering around a DMABN molecule. With increasing clustering number, the activation barrier is lowered and hence the rate increases exponentially. The observation is semiquantitatively interpreted on the basis of a molecular dynamics simulation of clustering in supercritical solutions.

An experimental study is presented of reverse electrodialysis (RED) as a completely different kind of electrical energy storage. The use of RED fuel cells has the potential to overcome common self-discharging problems of the RED accumulator. The membranes used in RED are designed to meet requirements different from those for conventional electrodialysis (ED). This applies particularly to bipolar membranes. An essential criterion for the use of membranes in RED is permselectivity. An improvement in the permselectivity of the membranes was obtained by coating them with an appropriate polymer or cross-linking the membrane polymer.

The results were compared with a mathematical model and some former works in this field.

The guest molecule azobenzene is aligned nearly perfectly by the channels of the molecular sieve AlPO₄-5. After irradiation with light of the wavelength 360 nm the configuration and the orientation of the molecules are changed. This results in changes of the refractive index. It turns out that this reaction is reversible (back reaction by irradiation with 436 nm) and that the change of the birefringence can be very large (δn' = 0,033). It is demonstrated that a crystal of a thickness of 13 μm is sufficient to be switched between a λ-plate and a λ/2-plate. This is an example of an opto-optical microswitch.

In this contribution the synthesis of nanocrystalline metal oxides and the formation of NH₄MnF₃ by microemulsion techniques is investigated in detail. The size and size distribution of the primary reverse micelles are determined by dynamic light scattering. Further it was possible to characterize intermediate species in the reaction process. The size of the final nanocrystal material was evaluated from the line broadening of X-ray reflections. The size distribution is very narrow and for the metaloxides a strong correlation exists between the size of the final product and the reverse micelles.

In this paper we attempt to explain the unusual isotope effect displayed by the ¹H-NMR scalar H-H coupling of the hydrides in complexes [CpIrL(H)₃]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three-coupled well system, but when a deuterium is introduced the system is better described as a double-coupled well. Finally, it is shown that the coupling in the case of a double-well system is greater than in an equivalent triple-well system, in this way giving a plausible explanation to the experimental facts reported.

Because of contradictions in the literature, we reinvestigated the kinetics of the Birch reduction, i.e. the hydrogenation of benzene and its derivatives in metal ammonia solutions (MAS: containing solvated electrons e⁻ and metal cations M⁺) with alcohols to yield the corresponding cyclohexa-1,4-dien compounds (e.g. 2 Li+2CH₃OH+C₆H₆⟹2CH₃OLi+C₆H₈). The kinetics of this reaction are obscured since the hydrogen reaction proceeds parallel to it (2Li+2CH₃OH⟹2CH₃OLi+H₂). The two reactions differ in their activation energies (6.5 and 22.5 kJ/Mol resp.); and in the series of the alkali metals Li, Na and K the rate of the Birch reduction decreases, whereas that of the hydrogen reaction increases. However, in the metal concentration range around 0.01 M. both reactions have within the experimental error the same reaction order with respect to the metal (≈︁0.8). Both are accelerated by addition of alkali cations common to the dissolved alkali metal, and both are decelerated by addition of alkali cation complexing cryptands. Thus we conclude that the cations are involved in the kinetics of both reactions, probably by forming intermediate ion pairs or shifting pre-equilibria in which solvated electrons are involved. The experimental data of both reactions can be described very well with the rate laws v(B) = k_Bf²[e⁻][Li⁺)[CH₃OH][C₆H₆]and v(H)=k_Hf²[e⁻][Li⁺](CH₃OH]resp. (f activity coefficients after Debye-Hückel) inserting the concentrations of e⁻ and Li⁺ as calculated from the known thermodynamics of LiAS. The experimental rate constants k_B and k_H are the products of the rate constants of the rate determining steps and the equilibrium constants of the pre-equilibria.

The Gibbs energy of formation Δ_fG^o of hexagonal α-SiC was determined by electromotive force (emf) measurements between 1200 and 1300 K using the galvanic cell Si, SiO₂|Th(Y)O₂|SiO₂, α-SiC, C which gives (T in K): Δ_fG^o (α-SiC) =-94770+24.24·T J/mol. The third-law enthalpy of formation was calculated as Δ_fH^o₂₉₈ (α-SiC)=-74.4 kJ/mol at 298 K. In order to examine more closely the relative stability of α-SiC and cubic β-SiC, Gibbs energy of transformation Δ_trG measurements were made by the same method between 1100 and 1300 K using the cell β-SiC, C, SiO₂|Th(Y)O₂|SiO₂, C, α-SiC. An emf of about 20 mV of the meta-stable cell was measured up to 5 h cell operation. This observation implies that α-SiC is the more stable modification in the investigated temperature range yielding Δ_trG (βα-SiC) ≈︁ −8 kJ/mol at 1200 K.

The reorientational behaviour of cyclohexane molecules in the rotator phase has been investigated with a Monte Carlo simulation in a cubic box with periodic boundary conditions. A previous simulation for state points at normal pressure is extended to higher pressure. This enables us also to perform simulations along isochors and isotherms, thus to distinguish between isothermal density changes and isochoric temperature changes. Reorientational correlation functions for various molecular axes are calculated in order to analyse the orientational disorder. It was found that in the immediate neighbourhood to the low-temperature phase transition the orientational freedom is limited. At larger distances to the solid-solid transition line no preferred orientations can be detected. The reorientational behaviour changes significantly when a displacement of the molecular centers is allowed, in particular the otherwise retarded decay of the correlation function for the C₃-axis is accelerated near the low-temperature phase boundary of solid I.

This paper summarizes previous work on wetting and adsorption transitions in the Ga-Pb system. It describes a model of these transitions which is based on the regular solution approximation in conjunction with nearest neighbor interactions. It also reviews several recent novel experimental results obtained in Ga-Pb, including: (a) evidence that complete wetting prevails in Ga-Pb alloys within the domain of liquid-liquid coexistence; (b) a demonstration that the thickness of Pb at the surface of Ga diverges logarithmically as liquid-liquid coexistence is approached, as expected for metallic systems characterized by short-range interactions, and (c) studies of adsorption transitions by two complementary experimental approaches which map out the extension of the prewetting line and provide estimates of the prewetting critical point and the wetting temperature.

Several types of closed-loop liquid-liquid immiscibility behavior in binary mixtures have been found with a simple isotropic model fluid. The calculations are based on an equation of state composed by the Carnahan-Starling hard sphere repulsion and the van der Waals-attraction. The extension to binary mixtures is accomplished with the usual quadratic one-fluid mixing rules. The results are presented in a global phase diagram as introduced by van Konynenburg and Scott. Several closed-loop phase diagrams similar to those discovered by Boshkov ten years ago with a Lennard-Jones equation of state have been found. The results are discussed in the context of the topological and physical origin of closed-loop behavior in systems of isotropic mixtures.