Band 214, Heft 1 - 1

The thermodynamic dissociation constants for isoelectric reaction of the type LH + ↔ L + H + (where L = 2,2´-bipyridine or 1,10-phenanthroline) have been determined pH-metrically at 298°K in aqueous binary mixtures of tetrahydrofuran (0-90 wt% of organic co-solvent). The pK-values have been found to decrease continuously. The data have been utilized to calculate the Gibbs energies of transfer of H + -ion from water to tetrahydrofuran-water mixtures. The results have been explained in terms of ion-solvent interactions, structural variations and basicity of aquo-organic mixtures. Attempts have been made to understand the basicity and structuredness of different aquo-ethers (D + H 2 O, THF + H 2 O and DME + H 2 O) from a comparative study of ΔG 0 t (H + ) profiles and excess relative permittivity Δε E r (X) profiles of the aquo-ethers. The order of basicity has been found to be THF + H 2 O > DME + H 2 O > D + H 2 O. DME has been found to be structureforming and D has been found to be structure breaking over the whole composition range (0-87 wt% of organic solvent). But THF is slightly structure forming initially but structure breaking above 40 wt%.

The thermodynamic dissociation constants for the isoelectronic reaction LH + ↔ L + H + [Eq. (1)], (where L = 1,10-phenanthroline or 2,2´-bipyridine) have been determined pH-metrically in urea + water mixtures (0-8 M urea). The data have been utilized to calculate the Gibbs energies of transfer of H + from water to urea + water mixtures. ΔG 0 t (H + ) values have been found to be increasingly negative. The results indicate the increase in basicity of the urea + water mixtures resulting from the breakdown of water structure by urea. The dissociation constants for the reaction UH + ↔ U + H + [Eq. (2)] have also been determined. The variation of the dissociation constant UH + can be properly correlated with the variation of dielectric constants of the urea + water mixtures. A good correlation exists between the variation of basicity of the solvent mixtures and the dissociation constants of uronic acid.

The limitations of Born equation and its different modifications for the calculation of Gibbs energies of hydration of ions have been briefly reviewed. A modified method for the calculation of hydration energies of ions has been suggested. The hydration energies thus obtained have been compared with the data in the literature.

The complex-formation equilibria of mercury(II) with acetylcysteine and captopril have been studied by pH-metric and liquid-liquid extraction methods applying dithizone as a competitive ligand during the latter measurements. The stability constants of three differently protonated Hg(II)/ligand 1:2 complexes of extreme stability have been determined by the combination of extraction and pH-metric data. The formation of the less stabil Hg(II)-ligand 1:3 complexes with the coordination of three sulfhydrilate groups has been followed and the values of β 130 /β 120 determined from potentiometric measurements. The differences in the Hg(II) complex formation and protonation equilibria between the two ligands and the possible binding sites in the complexes have been discussed.

A conductometric method is described and applied for the determination of the Soret coefficient of normal carboxylic acids in aqueous solutions in order to check a new model described in our previous papers. The Soret cell originally developed by Agar and Turner was applied with only minor modifications. The results show that the deviations between the measured and calculated Soret coefficients are lower than 2.5% except for formic acid (33% in average).

A two dimensional model for the substrate inside a pocket of an active site of an enzyme is presented and investigated as a vibrational system. The parameters of the system are evaluated for α-chymotrypsin. In the case of internal resonance it is analytically and numerically shown that the energy concentrated on a certain degree of freedom might be several times larger than in the non-resonant case. Additionally, the system is driven by harmonic excitations and again energy due to nonlinear phenomena is redistributed inhomogeneously. These results may be of importance for the determination of the rates of catalytic events of substrates bound in pockets of active sites.

CdSe films grown on Ti-coated polished steel by constant-current deposition from an aqueous solution containing selenosulfite and cadmium nitrilotriacetate have been studied by spectroscopic ellipsometry under in-situ conditions. Ellipsometric parameters, tanΨ and cosΔ, were measured both as a function of deposition time at λ = 825 nm and of wavelength (λ = 500-900 nm) for constant film thickness from 0.02 μm to 2.3 μm. The model used for data analysis, Ti-substrate | TiO 2 | CdSe-film | solution, with optically homogenous phases was valid for film thickness larger than 0.3 μm. Refractive indices and extinction coefficients of the 2.3-μm film were derived for λ = 500–900 nm. A direct band-gap of 1.72 eV was determined. The faradaic efficiency of electrodeposition was 0.93 for a current density of 50 μA cm -2 .

The reaction of CF 2 (X ~ ) with O 3 has been investigated in a discharge flow reactor at room temperature and low pressure (p = 2.5 mbar). CF 2 (X ~ ) was produced by a microwave discharge of a mixture of CF 2 Br 2 /He. The reactants and products were detected by mass spectrometers (MS), which were connected to the flow systems via a continuous molecular beam sampling system. The following primary products were observed (CF 2 O 3 (m/z = 98); CF 2 O (m/z = 47, 66); O 2 (m/z = 32)). The reaction mechanism: CF 2 (X ~ ) + O 3 ↔ CF 2 O # 3 → CF 2 O 3 → CF 2 O+O 2 (1) explains the observed primary products. The molecule with the empirical formula CF 2 O 3 has been observed directly by MS; the structure is discussed.

The formation of carbon particles following the pyrolysis of C 3 O 2 was investigated behind reflected shock waves towards high temperatures. It is known that in hydrocarbon pyrolysis the temperature range for soot formation extends from about 1300 K to 2200 K. That holds also for C 3 O 2 . Here it could be observed that particle formation starts again above 2300 K and increases towards a maximum at around 3000 K, falling off steeply above 3450 K. This maximum is nearly as high as that of the low ''temperature particle yield curve'' [1] at around 1600 K.