Consecutive reactions of diethoxydimethylsilane and triethoxymethylsilane with methoxide ions in methanol solution

The consecutive reactions of (CH₃)₂Si(OC₂H₅)₂ and CH₃Si(OC₂H₅)₃ with methoxide ions were investigated in methanol solutions. The reverse transesterification reactions with ethoxide ions could be neglected in both cases since the concentration of ethoxide in methanol solution was assumed to be low due to the fast equilibrium reaction C₂H₅O^? + CH₃OH ? C₂H₅OH + CH₃O^?. The progress of the reactions was followed by monitoring the formation of ethanol with a Fourier-transform infrared spectrometer. All rate constants were determined at 295 K. The reactions between the dialkoxydimethylsilanes and methoxide ions were assumed to consist of two consecutive steps that can be represented by the net reaction; (CH₃)₂Si(OC₂H₅)₂ + 2CH₃O^? → (CH₃)₂Si(OCH₃)₂ + 2C₂H₅O^?. The two consecutive rate constants were established as 1.93 ± 0.12M^?1s^?1 and 1.00 ± 0.12M^?1s^?1, respectively. The consecutive rate constants for the reactions between the trialkoxymethylsilanes and methoxide ions can be written according to the total reaction; CH₃Si(OC₂H₅)₃ + 3CH₃O^? → CH₃Si(OCH₃)₃ + 3C₂H₅O^?. The three rate constants corresponding to each consecutive step were established as 1.12 ± 0.09 M^?1s^?1, 0.82 ± 0.10 M^?1s^?1, and 0.51 ± 0.06 M^?1s^?1, respectively. © 1995 John Wiley & Sons, Inc.     

Hydrothermal synthesis of fluorated single crystals: BaMnF4 and BaF2·HF

Investigations to eliminate defects in the incommensurate phase BaMnF₄, led to a method of preparation at a temperature lower than the melting point of BaMnF₄. Qualitative studies of the systems BaF₂ -H₂O -HF and MnF₂ -H₂O -HF showed that very pure single crystals of BaMnF₄ could be grown at a temperature lower than 300°C. Several new phases could be isolated as single crystals, in particular: BaF_2^′HFSingle crystal X-ray analysis revealed the compound to be monoclinic (Space Group P2₁ and Z = 2) and confirmed the formula found by chemical analysis. However, because of the difficulty to distinguish between fluorine and oxygen atoms, some problems remain, which will be solved by neutron diffraction studies. Infrared analysis indicate the presence of HF^?₂ and suggest the following representation: (Ba²⁺)₂(HF^?₂)₂(F^?)₂.It is possible to extend this method of synthesis to the preparation of other fluorides and related compounds, in particular to those of europium and strontium.     

Zur Struktur von N-Benzoylthiobenzamid

Structure of N-Benzoylthiobenzamide N-Benzoylthiobenzamide forms two polymorphic modifications of which the crystal and molecular structures have been determined by X-ray structure analysis. In both modifications the molecules exist as NH tautomeres, but there are differences with regard to their rotameric configurations. In the monoclinic modification both symmetry independent molecules have a Z,Z′ configuration. The triclinic modification also contains two symmetry independent molecules, but now a Z,Z′ and an E,Z′ configuration exist. All four molecules deviate appreciably from planarity. The results of both structure determinations are compared with each other and with the structure of monothiodibenzoylmethane.     

TRANSIENT SPECIES IN THE PHOTOCHEMISTRY OF EOSIN*

Abstract— Photochemical reactions of eosin in aqueous solution were studied using the flash photolysis technique. In deaerated solution the dye was converted quantitatively to the triplet state during flashing. The triplet dye decayed by first and second order reactions which partly regenerated the dye in the ground state and partly produced semioxidized and semireduced eosin. These radical species were formed in an electron dismutation reaction between two triplet molecules and also in a reaction between one triplet and one unexcited molecule. The radicals recombine rapidly to give the dye in the ground state.
An efficient reversible photooxidation reaction was observed in eosin solutions containing potassium ferricyanide. Semioxidized eosin was formed in high yield by reaction between the triplet dye and the oxidant. The dye was regenerated rapidly in a reverse reaction between the products of the oxidation reaction.
An analogous type of reaction was found to occur in eosin solutions containing p -pheny-lene diamine. This reagent reduced the triplet dye to semireduced eosin; the dye was regenerated in the ground state in a very efficient reverse reaction. The protolytic behaviour of semireduced eosin was studied by varying the pH.
Absorption spectra of the transient products were determined and rate constants for the observed reactions were measured. The results are compared with results from previous studies of fluorescein.     

FLUORESCENCE PROPERTIES OF PORPHYRIN-GLOBIN FROM HUMAN HEMOGLOBIN

Fluorescence excitation and emission spectra, decays, and quantum yields are reported for the porphyrin-globin of hemoglobin (Hb^desFe) in aqueous solution of pH 8, at 4°C. A very weak fluorescence was observed in the UV (maximum at 334 nm), due to tryptophan and tyrosine residues, in addition to the strong porphyrin emission in the visible (maxima at 624 and 692 nm) reported previously. The absorption and fluorescence properties of the porphyrins of Hb^desFe were compared to those for free porphyrin in organic solvents and in aqueous solution. The close similarity of the fluorescence decays and quantum yields in Hb^desFe and in solution indicate the absence of stronger, specific porphyrin-protein interactions; however, slight spectral shifts point to the existence of water molecules in the Hb^desFe porphyrin environment. The fluorescence study also demonstrates the existence of efficient Trp-porphyrin energy transfer of Förster type. The extent of transfer is in satisfactory agreement with the value expected from crystallographic data for hemoglobin. The results are discussed and compared to previous fluorescence studies of hemoglobin and apohemoglobin. An improved method for the preparation of Hb^desFe is reported.     

Perron’s Method and Wiener’s Theorem for a Nonlocal Equation

We study the Dirichlet problem for non-homogeneous equations involving the fractional p-Laplacian. We apply Perron’s method and prove Wiener’s resolutivity theorem.