Untersuchungen über benzylübergangsmetallverbindungen : III. Darstellung und eigenschaften des tetrabenzylthoriums

Tetrabenzylthorium can be obtained in crystalline form by interaction of benzyllithium with thorium tetrachloride. All properties of the compound indicate σ-bonds between the metal atom and the benzyl groups. The IR spectrum of tetrabenzylthorium is discussed together with that of tetrabenzylzirconium.     

29Si-NMR-spektroskopische untersuchungen an substituierten disilanen

The ²⁹Si chemical shifts δ(Si) and coupling constants J(SiSi) and J(SiC) for a series of substituted tetra- and penta-methyldisilanes are reported and discussed as well as the corresponding data of di- and poly-silanes from the literature. The substituent effects of /gd(Si) can be explained satisfactorily by a simple quantum-chemical model without consideration of d-orbital effects and are mainly affected by the distribution of the /gs-electron density at the Si atom under study. The influence of steric interactions on δ(Si) is investigated in the case of alkoxy-substituted disilanes. A short discussion of the substituent effects on the coupling constants J(SiSi) and J(SiC) is given.     

Adsorption study of acetylcholine and cholinergic and anticholinergic drugs onto an electrode surface

As a type of interaction of acetylcholine and cholinergic and anticholinergic drugs with a charged surface, their adsorption onto a gold electrode has been examined by measuring the specular reflectivity and differential capacity of the electrode-solution interface.Based on the adsorption potential profiles, the drugs have been divided into three groups: (I) carbamylcholine and methacholine; (II) tetraethylammonium ion and hexamethonium; (III) nicotine, atropine, scopolamine and pilocarpine. Group I includes cholinergic drugs. Group II and III include antichilinergic drugs whose adsorptivity is stronger than that of acetylcholine. The results of the adsorption of acetylcholine and drugs onto a charged electrode surface should provide information about the competitive inhibitory effects on the attachement of acetylcholine to receptor sites.     

Austauschreaktionen an organostannylphosphinen: Identifizierung einer stabilen zwischenstufe

Monomeric 1 : 1 adducts of di-t-butyl(chlorodimethylstannyl)phosphine and dimethyltin dichloride are formed in solution by mixing equimolar amounts of the two components or by mixing dimethyltin dichloride and di-t-butyl(trimethylsilyl)phosphine in a 2 : 1 molar ratio. NMR, IR, Raman and ^{119 m}Sn Mössbauer data are in agreement with a symmetrical structure with two equivalent (CH₃)₂SnCl groups bridged by di-t-butylphosphine and chloro groups. The bonding and stability of the adduct are discussed.     

On the influence of ionic strength on the equilibrium constant of an ion-molecule reaction

Concentration formation constants for the 18-crown-6-sodium ion complex in anhydrous methanol solutions were measured as a function of the ionic strength of the solution. The K_c values remained reasonably constant for I_c ? 0.05 mol dm^?3 At higher ionic strengths the K_c values begin to decrease. The infinite-dilution formation constant was 2.2 × 10⁴.     

Explicit finite-difference simulation of a dropping mercury electrode utilizing an exponentially expanded space grid

An exponentially expanded space grid has been applied in the explicit finite-difference computer simulation of a dropping mercury electrode (DME). The simulator has been proved to be considerably faster than previously described models using uniform grids (S.W. Feldberg, J. Electroanal. Chem., 109 (1980) 69) and easily adaptable to different electrode reaction stoichiometries and forms of excitation potential.     

Tunnel electrode: Part II. Electron-transfer theory at an n-type semiconductor tunnel electrode

The formula for the cathodic tunnel current at an n-type semiconductor tunnel electrode was derived from the double adiabatic perturbation theory considering the vibration in the first coordination sphere. In the low-temperature limiting case, the apparent difference between the normal and the abnormal regions was observed in both the transfer coefficient α and the activation energy E* as a function of potential. It was concluded that the semiconductor tunnel electrode has advantageous characteristics for investigation of the electron-transfer mechanism at high overvoltage.     

Sodium N-chlorobenzenesulfonamide as an analytical reagent: Determination of amino acids and their metal complexes

Simple, elegant, and reproducible back-titration methods for the determination of arginine, histidine, threonine, glutamine, alanine, lysine, glycine, and serine in free state and in Zn, Cd, Ni, Cu, Ba, Sr, Mn, and Ag metal complexes with chloramine-B have been developed. The oxidation reaction involves a four-electron change per mole of amino acid, but a six-electron change is noticed with glycine and serine in various solvent and buffer media. The proposed analytical procedures for the assay of amino acids are useful for computing the number of amino acid ligands present in the respective metal complexes. The aromatic sulfonyl haloamine used, CAB, was prepared and then characterized by ¹H and ¹³C FT-NMR spectral data.     

Multicomponent gas separation by an asymmetric permeator containing two different membranes

A numerical analysis of an asymmetric permeator containing two different kinds of membranes and capable of separating a ternary feed gas mixture into three product streams has been carried out. For negligible axial pressure drops in the gas streams, equations have been developed and calculation methods illustrated for four flow patterns: cross, parallel, countercurrent and perfectly mixed. The main parameters used in the analysis are pressure ratios, the ratio of the two membrane areas and the permeabilities. Results have been presented for a 50% H₂10% CO₂40% N₂ feed with a permeator having one cellulose acetate (CA) type membrane and one silicone membrane. For both permeate streams, countercurrent achieves highest permeate qualities and requires least membrane area amongst all four flow patterns. The presence of a silicone membrane produces a richer H₂ permeate from the CA membrane than that from a CA-membrane-alone configuration. Simultaneously, an enriched CO₂ permeate is obtained from the silicone membrane. Under the ideal condition of zero pressure ratios, the asymmetric permeator has also been evaluated against the series configuration of a permeator containing a CA membrane only followed by a permeator containing only a silicone (S) membrane. The asymmetric configuration usually performs better than this series configuration for the chosen range of parameters.     

Excess molar volumes of (ethyl formate or ethyl acetate + an isomer of hexanol) at 298.15 K

Excess molar volumes V_m^E were determined over the entire composition range at 298.15 K for ethyl formate or ethyl acetate + hexan-1-ol, +2-methylpentan-1-ol, +3-methylpentan-2-ol, +2-methylpentan-3-ol, +3-methylpentan-3-ol, +2-methylpentan-2-ol, +4-methyl-pentan-2-ol, and +hexan-2-ol. Excess volumes were determined from density measurements made with a vibrating-tube densimeter. The V_m^E values were all positive, decreasing with the n value of the ester: C_n?1H_2n?1CO₂C₂H₅.     

Optical autoionization resonance theory as an approach to electron transfer via adsorbed atoms at metal—solution interfaces

Electron transfer between an ion in solution and an impurity atom adsorbed at a metal electrode is sometimes faster than direct electron transfer between an ion and a metal. This process displays close analogies to Fano autoionization resonances in atoms excited to formally bound states broadened by coupling to continuum states. On the basis of this analogy we have derived current—voltage relationships for electron transfer via adsorbed impurity atoms. The most striking effect is that the current passes through a maximum at overvoltages approximately coinciding with the nuclear reorganization enery, then drops, and rises again at still higher voltages. This pattern is quite different from that of direct electron transfer and is caused by the “interference” of metal and impurity wave functions in certain energy ranges. The effect is generally expected only for activationless process, i.e. for large overvoltages, where the current variation directly reflects the behaviour of the pre-exponential factor in the current expressions.The impurity level also modifies the Gibbs energy of nuclear activation due to the solvent configuration dependence of the electron density at the adatoms. This effect is commonly small and in contrast to the electronic structural effects, leads to smooth current-voltage relations.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.     

α-Eliminierungsreaktionen an di-t-butyl(trichlorgermyl)phosphin: Ein weg zu phosphinogermylenen und triorganophosphin-dichlorogermylen-komplexen

The complex PtClMe(2,2′-bipy)(MeO₂CCCCO₂Me) decomposes to PtClMe(2,2′bipy) and MeO₂ CCCCO₂ Me by a complex mechanism involving preliminary ionisation of the chloride ligand, but the dissociation of CF₃CCCF₃ from PtlMe(2,2′-bipy)(CF₃CCCF₃) takes place without ionisation of iodide.     

An electrochemical study of the adsorption of two isomeric chromium(III) complexes on mercury electrodes: Thioether as an anchoring group

Cyclic voltammetry and chronocoulometry were utilized to examine the electrochemistry and adsorption on mercury of two isomeric complexes of Cr(III) with a multidentate ligand bearing a thioether group, thiobis(ethylenenitrilo) tetraacetic acid, S[CH₂CH₂N(CH₂COOH)₂]₂. One isomer contains a sulfur-chromium bond and is exceedingly strongly adsorbed. The second isomer lacks this bond and is adsorbed much less strongly. Back-bonding from chromium to sulfur is argued to play an important role in the adsorption of the first isomer. Upon reduction, both isomers yield the same Cr(II) product. A dimeric form of the second isomer in which the two Cr(III) centers are bridged by an acetate group is proposed to form at certain pH values. The different coordination environments of the two Cr(III) centers in the dimer cause them to be reduced at different potentials.     

Antiferromagnetic complexes involving metalmetal bonds : I. Synthesis and molecular structure of an antiferromagnetic dimer with a CrCr bond

The binuclear complex (C₅H₅)₂Cr₂(S)(SCMe₃)₂ was prepared by refluxing a solution of chromocene and t-butylmercaptane in heptane. The structure of the product was determined by single crystal X-ray diffraction. The chronium atoms are linked by a sulphide bridge (SCr 2.24 Å;, <CrSCr 74.1° and two SCMe₃ bridges (CrS 2.38 Å;, <CrSCr 68.3–69.3°). The two cyclopentadienyl ligands (CC 1.41 Å;, CrC 2.23 Å;) are in apical positions, their ring planes being parallel to each other. The complex is an antiferromagnet (?2J cm^?1) despite the small CrSCr angles and short chromiumchromium distance (2.689 Å;) indicative of strong CrCr bonding.     

Couple axial gradients of potential and concentration in a cylindrical pore electrode: an impedance model

The impedance of a cylindrical pore electrode in the case where the potential gradient due to the electrolyte resistivity is coupled to the axial concentration gradient of reacting species has been calculated semi-analytically from the approximate solution reported previously for the steady-state concentration and current profiles in the pore. Complex plane impedance plots, computed by an iteration technique for the transmission line, indicate: (i) a quasi-semi-circular diffusion loop at low frequencies due to diffusion control; and (ii) a high frequency loop in which the frequency dispersion is strongly dependent on the electrode parameters (electrolyte resistivity, diffusion coefficient of the reacting species, pore depth, Tafel coefficient of the electrochemical reaction and overall current flowing through the pore).     

Noble-gas broadening of an optical transition of I2 measured by coherent transients. III. Two-pulse photon echo and free induction decay

We report two-pulse photon echo decay and free induction decay measurements of iodine as a function of noble-gas pressure. The non-exponential behavior of the two-pulse photon echo decay which in contrast to the free induction decay shows a t³ dependence, is extensively discussed. A comparison with the results of the three-pulse stimulated echo measurements is made. The results are interpretated in terms of a quantum mechanical transport equation, and analytical expressions are derived for the three-pulse stimulated echo, two-pulse echo and free induction decay.