Ion chromatographic determination of traces of some oxoanions with direct spectrophotometric detection

Investigation of the use of a single analytical procedure using the non-suppressed ion chromatographic method with direct spectrophotometric detection capable of determining eight oxoanions simultaneously is presented in this paper. Potassium phosphate was found to be the most suitable eluent for UV absorbance detection at 205 nm. Oxoanions AsO³⁻₃, SeO²⁻₃, AsO³⁻₄, VO⁻₃, SeO²⁻₄, WO²⁻₄, MoO²⁻₄ and CrO²⁻₄ were detected at ng ml⁻¹ levels with well separated peaks at retention time < 25 min. The working range is in the range ng ml⁻¹ to 50 μg ml⁻¹. The method is sufficiently sensitive to determine As (V), V(V), Mo(VI) and Cr(VI) anions (and NO⁻₃) directly in a river water sample. The accuracy of these results was established by comparison with conventional atomic absorption methods.     

Magnetic circular dichroism assignment of the longest wavelength band of a series of tetrahedral d OXY- and Thioanions

Room temperature magnetic circular dichroism spectra have been obtained for MoS²⁻₄, MoO²⁻₄, WS²⁻₄, ReS⁻₄, VS³⁻₄, VO³⁻₄ and OsO₄ through their first and second charge-transfer bands. These measurements demonstrate that the band at longest wavelength (ν₁ band) must be assigned to a t₁ → 2e transition for all the compounds investigated.     

The stereochemistry of quaternary N-methyl sparteinium cations, their derivatives, stereoisomers and analogues : Part II. Molecular and crystal structure, and IR spectra of methiodide and methperchlorate of 2-oxosparteine

The crystal and molecular structures of two quarternary salts of 2-oxosparteine (II), the methiodide (IICH⁺₃ • I⁻) and the methperchlorate (IICH⁺₃ • ClO⁻₄) have been determined on the basis of X-ray and IR data. The studies were performed by analogy to previously investigated quaternary salts of sparteine (I), the methiodide (ICH⁺₃ • I⁻) and the methperchlorate (ICH⁺₃ • ClO⁻₄). As expected, the configurations and conformations of cationic parts within the two pairs of quaternary salts are identical, except for the structure of their A/B fragments, which in ICH⁺₃ cations have the character of tertiary amines, but in IICH⁺₃ that of lactams.

On the basis of accumulated X-ray and IR data the similarities and differences in the modes of interaction of perchlorate and iodide anions with quaternary cations, and especially with their N⁺---CH₃ groups are discussed. In this discussion are also included the methiodide and methperchlorate of -isosparteine: IIICH⁺₃ • X⁻ (X⁻ = I⁻ or ClO⁻₄) where N⁺---CH₃ groups are cisoidally oriented to the basic nitrogen atoms. The most interesting observations are as follows: (i) When N⁺---CH₃ groups are easily accessible for direct quasi hydrogen bonding interactions with counter anions and when other positive charged groups, for instance lactam groups, are absent in quaternary cations, perchlorate anions interact more strongly than the iodide anions and in consequence introduce conformational changes into the ring with N⁺---CH₃ group as well as into further rings. (ii) Perchlorate and iodide anions interact with N⁺---CH₃ groups similarly and very weakly if at all, when the N⁺---CH₃ groups are for steric reasons inaccessible to counter anions or when in quaternary cations there are additional groups which attract the counter anions electrostatically. The last mechanism operates in both quaternary salts of 2-oxosparteine and this is the reason why their monocrystals are isosteric and IR spectra almost identical. (iii) The sterically hindered N⁺---CD₃ groups in both IIICD⁺₃ • X⁻ salts give rise in their IR spectra to two doubles of sharp, well resolved bands which indicate the presence of two different rotamers stabilized by two modes of weak intramolecular hydrogen bonds with basic N atoms. (iv) In IIICH⁺₃ • X⁻ and IIICD⁺₃ • X⁻ salts the perchlorate and iodide anions do not interact at all with the rotating and vibrating N⁺---CH₃ (N⁺---CD₃) group but the structures of these salts are not isosteric since the perchlorate anions interact more strongly than iodide anions with the A/B fragment of the IIICH⁺₃ cations. This is visible from the shapes and intensities of the so-called “trans” band in the IR spectra of both salts.     

Ab initio structural trends and torsional sensitivity in n-hexane and comparisons with crystallographic structural results

The molecular structures of n-hexane were determined by RHF/4-21G ab initio geometry optimization at 30° grid points in its three-dimensional τ₁(C11–C8–C5–C1), τ₂(C14–C11–C8–C5), τ₃(C17–C14–C11–C8) conformational space. Of the resulting 12×12×12=1728 grid structures, 468 are symmetrically non-equivalent and were optimized constraining the torsions τ₁, τ₂, and τ₃ to the respective grid points, while all other structural parameters were relaxed without any constraints. From the results, complete parameter surfaces were constructed using natural cubic spline functions, which make it possible to calculate parameter gradients, |P|=[(∂P/∂τ_i)²+(∂P/∂τ_j)²]^1/2, where P is a C–C bond length or C–C–C angle. The parameter gradients provide an effective measure of the torsional sensitivity of the system and indicate that dynamic activities in one part of the molecule can significantly affect the density of states, and thus the contributions to vibrational entropy, in another part. This opens the possibility of dynamic entropic conformational steering in complex molecules; i.e. the generation of free energy contributions from dynamic effects of one part of a molecule on another. When the conformational trends in the calculated C–C bond lengths and C–C–C angles are compared with average parameters taken from some 900 crystallographic structures containing n-hexyl fragments or longer C–C bond sequences, some correlation between calculated and experimental trends in angles is found, in contrast to the bond lengths for which the two sets of data are in complete disagreement. The results confirm experiences often made in crystallography. That is, effects of temperature, crystal structure and packing, and molecular volume effects are manifested more clearly in bond lengths than bond angles which depend mainly on intramolecular properties. Frequency analyses of the τ₁, τ₂ and τ₃ torsional angles in the crystal structures show conformational steering in the sense that, if τ₁ is trans peri-planar (170°≤τ₁≤180°; −180°≤τ₁≤−170°), the values of τ₂ and τ₃ are clustered closely around the ideal gauche (±60°) and trans (±180°) positions. In contrast, when τ₁ is in the region (50°≤τ₁≤70°), there is a definite increase in the populations of τ₂ and τ₃ at −90 and −150°.     

Studies on the determination of dissolved oxygen in water

Studies on the decomposition rates of the Mn(III) complex of cyclohexanediaminetetra-acetate (DCTA) in light and in darkness have shown that this complex is more stable than the one derived from ethylenediaminetetra-acetate. The optimum pH range for the determination of dissolved oxygen by means of the Mn(III)-DCTA complex is found to be between 3 and 4. The absorbance of this complex is independent of the amount of DCTA used (in the range 0.2–1.0 g) with water samples containing a maximum of 3.2 ppm of dissolved oxygen. Significant interferences are caused by the presence of CO²⁻₃, HCO⁻₃, S₂O²⁻₃, PO³⁻₄, I⁻, NO⁻₂, SO²⁻₃, Ca²⁺, Fe²⁺ and Fe³⁺ at 500 times the oxygen concentration.     

Thermogravimetric study on perovskite-like oxygen permeation ceramic membranes

The oxygen permeation properties of mixed-conducting ceramics SrFeCo_0.5O_3−δ (SFCO), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), La_0.2Sr_0.8Co_0.8Fe_0.2O_3−δ (LSCFO) and Ba_0.95Ca_0.05Co_0.8Fe_0.2O_3−δ (BCCFO) were studied by thermogravimetric method in the temperature range 600–900 °C. The results show that the oxygen adsorption rate constants k_a of all material are larger than oxygen desorption rate constants k_d and both k_a and k_d are not strongly dependent on temperature in the studied temperature range. The oxygen vacancy contents δ(N₂) and δ(O₂) in nitrogen and oxygen and their difference Δδ = δ(N₂) − δ(O₂) play an important role in determining the temperature behavior of oxygen permeation flux J_O2.     

The enthalpy of mixing of liquid NaF and NaMgF3 from drop calorimetry

The enthalpy of mixing of three liquid mixtures of NaF and NaMgF₃ has been measured by drop calorimetry and was found to be negative. This energy release is attributed to a change in the equilibrium

Mg_1/4[MgF²⁻₄]_3/4+f⁻ å MgF²⁻₄ to the formation of complex MgF²⁻₄-ions. A ΔH^M diagram for the system NaF-MgF₂ up to 50 mol % MgF₂ has been constructed.     

Computational analysis of side-chain conformations in polyaspartates exhibiting reversible helical sense inversion in the solid state

Poly(β-phenylpropyl -aspartate), poly(β-phenylbutyl -aspartate), and poly(β-phenylpentyl -aspartate) exhibit a reversible transformation from a right-handed -helix (_R) to a left-handed ω-helix (ω_L) in the solid state. During this transition, the infrared (IR) dichroism of the side-chain ester group and the birefringence change drastically, showing that the side-chain conformations are different for these two helices. In the present study, for the purpose of elucidating the preferred side-chain conformation in each helix, we performed the computational analyses. The energy contours, the directions of the IR transition moments and the anisotropies in polarizability as functions of the first two dihedral angles of the side chain, χ₁ and χ₂ were calculated. Then, comparing them with the experimental IR dichroism and birefringence data, we elucidated the specific side-chain conformation preferred for each _R or ω_L skeleton. The preferred values of (χ₁, χ₂) were found to be (−75, −60°) for _R and (180, 45°) for ω_L.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.     

Luminescent properties of carbon-rich starburst gold(I) acetylide complexes. Crystal structure of [TEE][Au(PCy3)]4 ([TEE]H4=tetraethynylethene)

Two carbon-rich starburst gold(I) acetylide complexes [TEE][Au(PCy₃)]₄ (3, [TEE]H₄=tetraethynylethene) and [TEB][Au(PCy₃)]₃ (6, [TEB]H₃=1,3,5-triethynylbenzene) were prepared and their UV–vis absorption, emission and excitation spectra have been recorded. In fluid CH₂Cl₂ solutions, 3 exhibits prompt ¹(ππ*) fluorescence with λ_0–0 and λ_max at 413 and 428 nm, respectively, while 6 displays ³(ππ*) phosphorescence with λ_0–0 and λ_max at 446 and 479 nm, respectively. The crystal structure of 3·CH₂Cl₂ has been determined.     

Thermodynamic characterization of ternary solutions of uncharged polymers

The thermodynamic interactions in aqueous solutions of uncharged polymers were studied. Using a gel-deswelling method, the water activities (chemical potentials) in binary and ternary (two polymers in one solvent) solutions of methylcellulose (MC), polyvinyl alcohol (PVA) and polyvinyl pirrolidone (PVP), respectively were determined at various polymer volume fractions (1.0 × 10⁻² < v₂ < 1.0 × 10⁻¹). On the theoretical basis of the Flory–Huggins approximations, the relevant solvent–segment (χ₁₂ or χ₁₃) and segment–segment pair interaction parameters (χ₂₃) have been calculated.

The solvent activity curves (ln a₁ versus polymer volume fraction) can be well described by a polynomial of third-degree in both the binary and the ternary solutions of the polymers. The solvent–segment interaction parameters exhibit a slight dependence on the polymer concentration. For each binary solution, the χ₁₂−v₂ function can be fitted by a straight line wich has a small positive slope. In the mixtures of two polymers, the values of the segment–segment (χ₂₃) interaction parameters were close to zero or sligthly negative (χ₂₃ 0 ± 0.03), indicating that under the studied conditions, the polymers in the ternary solutions are compatible.     

Untersuchungen zur atomspektroskopischen spurenanalyse in AB-halbleiter-mikroproben—II : Untersuchungen zur bestimmung von Ga-spuren in In und InAs und von In-spuren in Ga, GaAs und GaP durch AAS mit elektrothermischer atomisierung

The atomic-absorption determination of traces of Ga in In, P, As and InAs and of traces of In in Ga, P, As, GaAs and gap is described. The mechanism of the evaporation of the trace element according to the medium used (HCl or HNO₃) is discussed. Nitric acid medium is recommended for analytical determinations. The evaporation of the trace elements and matrices was investigated by means of absorbance vs. time curves. It is shown that a thermal fractionation of AsO³⁻₄ and PO³⁻₄ matrices and Ga and In traces is possible in the ashing step. In the case of Ga the thermal fractionation of the matrices (In³⁺, In³⁺/AsO³⁻₄) from the trace element is possible in the atomization step. The thermal fractionation of In traces from the matrices (Ga³⁺, Ga³⁺/AsO³⁻₄, Ga³⁺/Po³⁻₄) is impossible. The results and the reasons for the non-specific absorption are discussed. The absolute detection limits are Ga 45 pg, In 35 pg. The relative detection limits are 1 ppm in a 0.1 mg sample.     

Untersuchungen zur atomspektroskopischen spurenanalyse in AB-halbleiter-mikroproben—I : Verdampfbarkeit und unspezifische absorption von AB-verbindungen und ihren komponenten in der graphitrohrküvette

The evaporation of the species Ga³⁺, In³⁺, PO³⁻₄, AsO³⁻₄, Ga³⁺/AsO³⁻₄, Ga³⁺/PO³⁻₄ and In³⁺/AsO³⁻₄ in HCl and HNO₃ medium was investigated by measurement of the non-specific absorption at 250 nm, in a graphite cuvette, with a continuum light source. The maximum of the non-specific absorption is given for the ashing and atomizing phases as a function of the ashing temperatures. Ashing temperatures for analytical determinations can be derived from the results. The mechanisms of evaporation of the substances were investigated by means of extinction-time curves. Absorption spectra of the matrices and of the pure acids used were measured between 190 and 330 nm in the graphite cuvette at different ashing and atomizing temperatures. The InCl- and GaCl-bands of the C-system and new bands of GaO- and InO-molecules were found. The PO-band at 246 nm was detected. The results are discussed and can be applied for thermal fractionation and for background correction by the two-line method in trace analysis by electrothermal AAS.     

Turbidimetric determination of sulphate in waters employing flow injection and lead sulphate formation

An analytical flow-injection procedure based on PbSO₄ colloidal formation is proposed as a turbidimetric determination of sulphate in natural waters. Ethanol-water was used as a medium in order to improve the sensibility of the method. Both chemical and flow variables as well as interfering species were studied. A detection limit of 0.3 μg SO²⁻₄ ml⁻¹ was found, and the analytical range (according to Beer's law) was 2–20 μg SO²⁻₄ ml⁻¹. The precision was better than 3% R.S.D. and the sample throughput was ca. 35 h⁻¹. The method, when compared with a standard methodology, gave good results when applied to water analysis.     

Thermochemical study of the complex formation of ag(I) with some sulphur-containing aminopyridines

The enthalpies of formation of the complexes between the silver(I) ion and some sulphur-containing aminopyridines of general formula N(CH₂)_n−1-S-(CH₂)_m-NH₂ where n = 1, and m = 1,2; 1,3; 2,2; 2,3 have been determined by direct calorimetric titration at 25°C in 0.5 M (K)NO₃ solution. The corresponding entropy terms, ΔS, have been calculated using the obtained enthalpy values and the previously reported ΔG values.

In acid medium (pH < 3) coordination occurs through the thioether group and the protonated species AgLH³⁺₂ and AgL₂H⁵⁺₄ are enthalpy-stabilized and entropy-destabilized. At higher pH values (pH &>; 3) additional chelation through the pyridine nitrogen is obvious by a marked increase in the enthalpy of formation of the complexes AgL₂H⁴⁺₃, AgL₂H³⁺₂ and AgLH²⁺. The last complex dimerizes into a cyclic dimer Ag₂L, H⁴⁺₂. At still higher pH values (pH &>; 6) the participation of the amino group in the dimeric chelates Ag₂L₂H³⁺ and Ag₂L²⁺₂ is revealed again by a marked increase in the heat of complexation.     

Antisymmetric resonance Raman scattering from the breathing mode of CuBr4

Circular polarization measurements establish a large antisymmetric component of the Raman scattering from the breathing vibration of CuBr²⁻₄ in acetonitrile when excited at 5145 Å in the first Br → Cu charge-transfer (π) absorption band. This is attributable to spin degeneracy and to the 700 cm⁻¹ spin-orbit splitting of the CT excited state.     

Gas-phase stability of cluster ions SF m + (SF6) n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SF⁺_m (SF₆)_n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF⁺_m (SF₆)₁ were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF⁺_m (SF₆)_n with m = 0−4. The structures of SF⁺₅, SF⁺ (SF₆)₁, SF⁺₃ (SF₆)₁, and SF⁺₅ (SF₆)₁ were investigated by ab initio molecular orbital calculations. For SF⁺₅, the D_3h geometry is found to be most stable andC_4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.     

Field investigation of major and minor ions along Summit (Central Greenland) ice cores by ion chromatography

As a part of the European EUROCORE and GRIP (Greenland Ice Core Project) operations aimed at recovering deep ice cores at Summit (Central Greenland), we have for the first time successfully performed ion chromatography measurements in the field and investigated in detail the soluble impurities, including Na⁺, NH⁺₄, K⁺, Mg²⁺, Ca²⁺, F⁻, CH₃COO⁻, CH₂ OHCOO⁻, HCOO⁻, CH₃SO₃⁻, Cl⁻, NO⁻₂, SO₄²⁻ and C₂O₄²⁻, trapped in ice deposited over some 200 000 years in Greenland.     

Studies on adducts of rhodium(II) tetraacetate and rhodium(II) tetratrifluoroacetate with some amines in CDCl3 solution using H, C and N NMR

¹H, ¹³C and ¹⁵N NMR spectroscopy has been applied for investigation of amine adducts with rhodium(II) tetraacetate dimer and rhodium(II) tetratrifluoroacetate dimer in CDCl₃ solution. Subsequent formation of two adducts, 1:1 and 2:1, was proved by NMR and VIS titration experiments, and by NMR measurements at reduced temperatures, from 233 to 273 K. The adduct formation shift, defined as Δδ=δ_adduct−δ_ligand and characterizing complexation reaction, varies from ca. 0 to +1.6 ppm for ¹H, from ca. −10 to +6 ppm for ¹³C and from −4.4 to −39 ppm for ¹⁵N NMR. Formation of N–Rh bond slows the inversiof on the nitrogen atom and generates, in the case of N-methyl-(1-phenylethyl)-amine, a nitrogenous chiral center in the molecule. VIS spectra of amine-dirhodium salt mixture contain two bands in the 532–597 nm spectral range, assigned to 1:1- and 2:1-adducts.     

A molecular mechanics study on novel Ln(0) π-arene compounds

To examine the steric effects on the stability of Ln(0) π-arene compounds, molecular mechanics (MMP2) calculations are performed on Gd(η-C₆H₆)₂ and Ln(η-Bu^t₃C₆H₃)₂ (where Ln is Gd, Yb and Y ). The small potential-well depth ( ≈ 2 kcal mol⁻¹) and the large Gd-C equilibrium distance ( > 3.3 Å) explains the instability of Gd(η-C₆H₆)₂, while the difference in the stability between Gd(η-Bu^t₃C₆H₃)₂ and Yb(η-Bu^t₃C₆H₃)₂ can be attributed to the difference in the van der Waalsradii of the two metals and the more contracted 5d orbitals on the Yb atom.