Berichte der Bunsengesellschaft für physikalische Chemie最新文献

The partial excess Gibbs energies of carbon and aluminum in liquid Al-C were used with the equilibrium vapor pressures of pure Al and pure C to obtain the phase diagrams (1) at 1 bar of total pressure, and (2) at pressures sufficient to suppress the gas phase. The first diagram contains [gas+graphite → liquid] tectoid-type reaction at 2900±20K with the tectoid composition of 0.681 mol fraction of aluminum. The [gas/graphite] boundary is subject to large errors due to the large errors in the vapor pressure of pure graphite, but its general form is acceptable. The second diagram consists entirely of the condensed phases with peritectic reaction [liquid+graphite → Al₄C₃] at 2429 K. The [liquid/graphite] phase boundary was obtained from the graphite solubility in liquid solutions at 2429-2800 K, yielding the partial excess Gibbs energy of carbon in solution.

The resulting phase boundary is as reliable as the melting point of graphite, taken to be 4130 K.

Potential energy surfaces for pericyclic reactions can easily be predicted by means of correlation diagrams. Whereas the initially excited state (called 1 B in this paper) may vary from molecule to molecule, its population is collected by a lower, dark state (2A) of always the same nature, which corresponds to a two-electron excitation. From there, the system leaves to the ground states of educt and product(s) via a conical intersection (CI). Using transient absorption and transient ionization spectroscopy, we measured the lifetimes t_1B and t_2A of these states after initiating electrocyclic ring opening of 1,3-cyclohexadiene and some of its derivatives (α-terpinene, α-phellandrene and 7-dehydrocholesterol). t_2A can be considered the reaction time constant. Whereas t_2A = 5.2 ps for dehydrocholesterol. it is 80–100 fs for the other three molecules. We suggest that the former system needs some time to find the exit (the CI) from the 2A surface, whereas in the latter cases the CI is located right in the line of the steepest descent. We apply the same scheme to, and compile the time constants for, some other fast processes: the [1.3] sigmatropic reactions of norbornene and norbornadiene (t_2A = 220 and 210 fs) and the fast internal conversions of cycloheptatriene (80 fs) and several UV stabilizers. The latter two processes are suggested to take the same pathway as the pericyclic hydrogen shifts in these molecules. We also point out the similarity of the pericyclic potential surfaces to those of cis-trans isomerizations and give evidence that cis-stilbene (t_2A < 300 fs), too, is accelerated directly in the direction of the CI. For cis-trans isomerization of longer polyenes there are however several 2A minima and conical intersections. We also give examples where additional branchings and competing reactions play a role.

Liquid state ¹H and ¹⁹F NMR experiments in the temperature range between 110 and 150 K have been performed on mixtures of tetrabutylammonium fluoride with HF dissolved in a 1:2 mixture of CDF₃ and CDF₂Cl. Under these conditions hydrogen bonded complexes between F⁻ and a varying number of HF molecules were observed in the slow proton and hydrogen bond exchange regime. At low HF concentrations the well known hydrogen bifluoride ion [FHF]⁻ is observed, exhibiting a strong symmetric H-bond. At higher HF concentrations the species [F(HF)₂]⁻, [F(HF)₃]⁻ are formed and a species to which we assign the structure [F(HF)₄]⁻. The spectra indicate a central fluoride anion which forms multiple hydrogen bonds to HF. With increasing number of HF units the hydrogen bond protons shift towards the terminal fluorine's. The optimized gas-phase geometries of [F(HF)_n]⁻, n = 1 to 4, calculated using ab initio methods confirm the D_∞h, C_2v, D_3h and T_d symmetries of these ions. For the first time, both one-bond couplings between a hydrogen bond proton and the two heavy atoms of a hydrogen bridge, here ¹J_HF and ¹J_HF where |¹J_HF|≥|¹J_HF'|, as well as a two-bond coupling between the heavy atoms, here ²J_FF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. ¹J_HF and ²J_SF>0, and ¹J_HF|. <0. Ab initio calculations of NMR chemical shifts and the scalar coupling constants using the Density Functional formalism and the Multi-configuration Complete Active Space method show a reasonable agreement with the experimental parameters and confirm the covalent character of the hydrogen bonds studied.

The photophysical and photochemical properties of trans-R-2′,4′-dinitrostilbene (II) and a series of trans-R-2′,4′-dinitrostilbenes (II-R, R: 2-NO₂, 3-NO₂, 4-Br, 4-F, 4-Me, 4-C₃H₇, 4-OCH₂C₆H₅, 3,4,5-(OMe)₃, 4-OEt, 4-NMe₂, 4-NEt₂) were studied in solution as a function of solvent polarity and temperature. The quantum yield of fluorescence (Φ_f) is very small for all II-R at 25°C. At −196°C Φ_f is moderate for several derivatives in 2-methyltetrahydrofuran (MTHF) but strongly enhanced for those bearing electron donating substituents. For the latter compounds the quantum yield of trans → cis photoisomerization is low, but for the other compounds Φ_{t → c} is substantial (0.2-0.5 in toluene or MTHF at room temperature). The triplet state absorbs typically in a broad spectral range; its lifetime (τ_T) lies in the 20-200 ns range and is longer for R = 4-NEt₂; at −196°C τ_T of all II-R approaches milliseconds. The results are compared with those of trans-4-R-4′-nitrostilbenes (I-R, R: NO₂, H, OMe, NH₂, NMe₂, NEt₂). Phosphorescence of singlet molecular oxygen was observed for several mono- and dinitrostilbenes at room temperature. Generally, the quantum yield of singlet oxygen formation is much smaller than that of intersystem crossing into the triplet state. The triplet mechanism accounts for trans→cis photoisomerization and the contribution of this pathway is lowered by intramolecular electron transfer to the nitro group(s). The similarities and differences between I-R and II-R type compounds and the effects of intramolecular charge transfer are discussed.

Vapour pressures of water over (Ag/Cs)NO₃-H₂O (0, 40, 60, and 100% AgNO₃) are reported at three temperatures: 152,179, and 218 °C. The water activities derived from these data are fitted with equations for multicomponent electrolyte solutions developed by Van Laar and Pitzer (three models: Van Laar model, Pitzer-Van Laar model, Pitzer-Simonson model). With these models the mean activity coefficients of the salts and the solubilities of the pure salt components (AgNO₃ or CsNO₃) in the ternary mixture are calculated. The advantages of the Pitzer-Simonson model are underlined.

The reaction of silane with H atoms

was studied behind reflected shock waves at temperatures between 998 K and 1273 K and pressures around 1.5 bar. The thermal decomposition of a few ppm ethyl iodide (C₂H₅I) was used as a well known H-atom source. The atomic resonance absorption spectroscopy (ARAS) was applied for time resolved and simultaneous measurements of H- and Si-atom concentrations. The presence of an excess of SiH₄ causes a fast consumption of H atoms according to reaction (R 5). The signals obtained were kinetically evaluated by computer simulations based on a simplified reaction mechanism. The rate coefficient for reaction (R 5) was found to be:

Activated carbon may be regarded as consisting of graphite nanocrystals forming the walls of the pores. These crystallites contain three graphene layers having sizes of 2-4 nm perpendicular to the “c-axis”. A model has been developed of the electronic properties of the material. The model is based on this nano-structure and on experimental results concerning the variation of the conductivity, the thermoelectric power and the ESR signal of activated carbon, immersed in electrolyte solutions, on changing the electrode potential. According to the model, the graphene layers (“domains”) are charged with one, and preferentially two, electrons (or defect electrons) or uncharged, the π-electron system being delocalized throughout a domain. The various types of domains are in thermodynamic equilibria with one another. Charge transport is regarded as transport of charged domains, resembling the “extra” conduction of protons in H-bonding solvents like water, while uncharged domains (like neutral water clusters) do not contribute to the conductivity. Correspondence with electronic energy band models is obtained by constructing electronic levels from the energetic data. Close structural and mechanistic similarities to conducting polymers suggest the applicability of the model also to this material.

Calcium oxide clusters produced in the direct laser vaporization of different precursor solid samples CaO, Ca(OH)₂ and CaCO₃ have been investigated by a time-of-flight mass spectrometer. Besides [Ca(CaO)_n]⁺ and [(CaO)_n]⁺ clusters, considerable protonated clusters [H(CaO)_n]⁺ and hydrated clusters [H(CaO)_n(H₂O)_m]⁺ have also been found in the TOF mass spectra. The characteristic of the abundance distribution of the clusters shows that the formation process of calcium oxide clusters should proceed along the growth course of the rock salt structure, which is further supported by the reactivity of [H(CaO)_n]⁺ clusters with H₂O produced in the direct laser vaporization.

The sol-gel transition of a novel glycol ester of ortho silicic acid is investigated by small angle neutron scattering technique. We measured the changes of scattering intensity as a function of time during gelation and show the formation of a fractal structure.

When we mix our ester with an aqueous surfactant solution no precipitate is formed as observed in common template synthesis of mesoporous materials. Because of the water solubility of our silicate precursor it can be dissolved into the surfactant phase without phase separation. During hydrolysis of the precursor ethylene glycol is produced which does not affect the hydrophobic interaction of the surfactant.

It is shown by contrast variation that surfactant does not affect the fractal dimension of the final silica gel. Measurements after gelation demonstrate the unchanged presence of surfactant micelles.