A potentiometric study of the water-soluble silver(I) complexes of n,n'-bis(2-hydroxyethyl)dithiooxamide : Part III. Determination of the different species

Silver(I) and N,N'-bis(2-hydroxyethyl)dithiooxamide (H₂X) in acidic solutions, form a series of water-soluble polynuclear complexes. These complexes can be represented by two general formulae: H_2qAg_pX_q and H_2q-1Ag_pX_q with q=1,2,3 and p=q, q-1.     

Spectroscopic evidence for the intermediacy of a 6-R-2,4-diazabicyclo-[3.1.0]hex-2-(3)ene in the photoisomerization of 4-R-1,4-(3,4)-dihydropyrimidines

¹H- and ¹³C-nmr spectroscopy is presented showing that photolysis of 4-(p-trifluoromethylphenyl)-1,4-(3,4)-dihydropyrimidine yields 6-(p-trifluoromethylphenyl)-2,4-diazabicyclo[3.1.0]hex-2-(3)ene.     

The determination of all the platinum group elements and gold in rocks and ore by neutron activation analysis after preconcentration by a nickel sulphide fire-assay technique on large samples

The noble metals are inhomogeneously distributed in sulphide ores and their host rocks. It is therefore necessary to analyse large sample sizes to obtain representative analyses. A nickel sulphide fire assay technique has been adapted to extract the noble metals from a large sample size (50 g) into a nickel sulphide button. Subsequently the fire assay button is dissolved in hydrochloric acid and the solution is filtered. The noble metal residue retained on the filter paper is analysed quantitatively by i.n.a.a. techniques. This method is rapid, relatively inexpensive and has better sensitivities for all the noble metals than other analytical techniques.     

Toward an accurate and efficient semiclassical surface hopping procedure for nonadiabatic problems

The derivation of a semiclassical surface hopping procedure from a formally exact solution of the Schrodinger equation is discussed. The fact that the derivation proceeds from an exact solution guarantees that all phase terms are completely and accurately included. Numerical evidence shows the method to be highly accurate. A Monte Carlo implementation of this method is considered, and recent work to significantly improve the statistical accuracy of the Monte Carlo approach is discussed.     

Investigations on the conditional kinetic and thermodynamic stability of aquatic humic substance-metal complexes by means of EDTA exchange, ultrafiltration and atomic spectrometry

The conditional metal availability and the kinetic stability of humic substance-metal species in humic-rich waters (e.g. bog water) was characterized by means of EDTA exchange. For this purpose a combined procedure consisting of time-controlled ligand exchange by EDTA, species differentiation by a fast single-stage tangential-flow ultrafiltration (TF-UF) technique (cut-off 1 kDa) and sensitive atomic spectrometry methods (e.g. AAS, ICP-OES, TXRF) was developed. The kinetics and the yield of the EDTA exchange served as operational parameters for assessing the kinetic stability and EDTA availability of HS-metal species, respectively. Considerable fractions of natural HS-metal species studied were shown to be EDTA-inert (e.g. 31% of the total Fe, 44% of the total Al) even after long reaction times (48 h), in contrast to artificial ones formed in solutions of isolated HS. Moreover, the conditional thermodynamic stability of HS-metal complexes formed by successive loading of an aquatic reference HS (HO14) with a number of heavy metal ions (e.g. Cr(III), Cu(II), Fe(III), Mn(II), Zn(II)) was also evaluated discriminating the free metal concentrations by means of TF-UF. In addition, from the loading isotherms obtained conditional complexation capacities could be derived for the studied HS exhibiting the order Fe(III)>Cu(II)>Cr(III)>Co(II)>Mn(II).     

The molecular structure of cis- and trans-1,2-difluoro-ethene: An electron diffraction study

The molecular structure of cis- and trans-1,2-difluoroethene was studied in the gas phase by electron diffraction, using the sector-microphotometer technique. The molecules are planar. For trans-difluoroethene the geometrical parameters are: C-F bond: 1.338(0.003) Å; CC bond: 1.320(0.009) Å; C-H bond: 1.088(0.004) Å, ∠CCF 119.8°(0.2°); ∠CCH: 125° (1.2°). For cis-difluoroethene: C-F bond: 1.332(0.003) Å; CC bond: 1.311(0.008) Å; C-H bond: 1.100(0.003) Å; ∠CCF: 122.5°(0.2°); ∠CCH:127.0 °(2.3°).     

Purification of adipoyl-7-amino-3-deacetoxycephalosporanic acid from fermentation broth using stepwise elution with a synergistically adsorbed modulator

Multicomponent adsorption data of a fermentation broth containing adipoyl-7-amino-3-deacetoxycephalosporanic acid (adipoyl-7-ADCA), a cephalosporin precursor for 7-ADCA, and two key impurities, alpha-hydroxyadipoyl-7-ADCA and alpha-aminoadipoyl-7-ADCA were obtained from batch equilibrium and frontal chromatography tests. Amberlite XAD-1600 was chosen as the resin. A rate model was applied to simulate the chromatograms. An alkaline buffer, which by itself has no affinity for the resin, was used as the eluent. The widely used reversed-phase modulator model is inaccurate in explaining the stepwise elution data. A new model, the induced competition model, has been developed to account for apparent retention of the buffer in the presence of adsorbed species. Close agreement between the simulations and the data was achieved with the new model.     

Stable gaseous aziridinium ions

Gaseous protonated aziridine ions are produced at the threshold from β-phenoxyethylamine molecular ions. The evidence for this is collisional activation spectra, using various precursors (including labelled analogues) under electron impact and field ionization conditions. Partial conversion to the acyclic \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} {\rm CH = }\mathop {\rm N}\limits^ + {\rm H}_{\rm 2} $\end{document} isomer occurs at higher electron energies and is rationalized by means of a potential energy surface constructed from energetic data.