The crystal structure of LSbOF

LSbOF is orthorhombic, space group Pnma, with a = 8.873, b = 4.099 and c = 5.483 Å. Four anions are bonded to Sb³⁺, all to one side, and such polyhedra form chains by edge sharing along c. Evidence is given for an ordered arrangement of fluorine and oxygen anions. The lone pair of electrons is described as taking a certain volume, the centre of which is derived. Relationships with other structures are discussed.     

AB initio MO LCAO UHF calculations of magnetic hyperfine interactions in π-radicals. Isotropic and anisotropic couplings of ·CH3, ·NH3+, ·C2H5 and ·N2H4+

A series of π-electron radicals have been investigated with ab initio methods, using the unrestricted Hartree-Fock approximation. The geometries have been partially optimized, and magnetic hyperfine coupling constants have been evaluated. For the dipolar hyperfine couplings, the results obtained are in fair agreement with experiment, whereas the isotropic coupling constants show greater deviations. These values are also shown to be sensitive to annihilation of the quartet contamination of the wavefunction.     

Calculation of chlorine Kα1,2 X-ray emission shifts in molecules

Shifts of the chlorine Kα_1,2 X-ray emission line in gaseous molecules and in crystalline NaCl are correlated with effective atomic charges from CNDO/2 molecular orbital calculations made (i) for the neutral atomic ground states. and (ii) within the equivalent core approximation. For the latter case CNDO/2 parameters are derived for argon. The best agreement with experimental chemical shifts is obtained with the equivalent cores approximation. The results for NaCl depend strongly upon the CNDO/2 treatment of sodium.     

Ab initio MO LCAO UHF calculations of magnetic hyperfine interactions in σ radicals. Isotropic and anisotropic couplings of NO2 and CO2−

The magnetic hyperfine coupling constants in NO₂ and CO₂^? have been computed by an initio methods. Spin annihilation is found to be essential in order to obtain useful results for the dipolar couplings, but has much less influence on the isotropic couplings. The electric quadrupole coupling constants have also been evaluated, and are in good agreement with available experimental data.     

Determination of trace levels of sulphate in water using flow-injection and in-line preconcentration

Sulphate is preconcentrated on a strong anion-exchange resin and determined using the effect of sulphate ions on the complexation of methylthymol blue and barium. A computer-controlled flow-injection analyser is used to automate the whole procedure. The resin has two functions: it preconcentrates sulphate and also separates sulphate from divalent cations that may interfere in the determination step. The system can handle 30 samples per hour and has a working range from 25 to 1000 μg 1^?1 of sulphate. Lower detection limits can be obtained by changing the preconcentration conditions. The effect of both anionic and cationic interferents was studied.     

Microstructural Analysis of the Effects of Poly(ethylene glycol) on an Acid Catalyzed Sol-Gel Derived Ceramic Material

Ceramic materials have been derived from an acid catalyzed sol-gel process. The addition of different molecular weights and concentrations of polyethylene glycol (PEG) to the sol mixture modifies the phase behaviour of the sol-gel process. The resulting gel is burned at 973 K to make porous ceramic materials. Nitrogen adsorption-desorption isotherms are used to assess the effects of PEG on the internal structure of the burned ceramic material. These isotherms indicate an extensive pore network exists consisting of micropores and mesopores. In the micropore region of the isotherms, the _S-plot analysis reveals changes in specific primary micropore volumes, specific total pore volumes, specific external surface areas and specific SPE surface area when PEG is added in the sol-gel process. The average pore width and the overall mesopore size distribution curves shift to higher pore size values and ranges on addition of PEG to the sol-gel mixture. The presence of PEG during the sol-gel process leads to an apparent narrowing of the micropore size distribution. The results of this work clearly indicate that the molecular weight and the concentration of a polymer, such as PEG, influences the eventual internal structure of a ceramic after burning.     

Solvent effects on optically detected magnetic resonance in triplet spin labels

We have calculated solvent effects on the zero-field splitting (ZFS) constants induced by electron spin–spin coupling (SSC) in the low-lying triplet states of azaaromatic molecules in solutions using multiconfiguration self-consistent-field wave functions and the polarizable continuum model. The second-order spin–orbit coupling (SOC) contribution to the splitting of the ^3* states is found to be almost negligible, and the calculations therefore provide a good estimate of the ZFS parameters and their solvent dependence based only on the electron spin–spin coupling expectation values. The correlation between the shift in the ZFS and the phosphorescence frequency that has been observed in optically detected magnetic resonance experiments in low-temperature glasses is supported by our direct SSC calculations without taking SOC into account. This makes it possible to distinguish between the two theories that earlier were proposed to explain the inhomogeneous broadening of triplet state spectra, and discard the one that is exclusively based on the SOC-induced mixing of the singlet and triplet states.Contribution to the Jacopo Tomasi Honorary IssueAcknowledgments. This work was supported (B. M.) by the Swedish Royal Academy of Science (KVA). This work was also supported by the Norwegian Research Council through a grant of computer time from the Program for Supercomputing. We are grateful to B. Schimmelpfenning for his valuable assistance in the computations.     

Dissociative recombination of HeH+. I. Rovibrational spectrum of HeH Rydberg states

The repulsive ground electronic state X²Σ⁺ of HeH is strongly coupled to the Rydberg states at small interatomic distances. Such large couplings also occur between some of the Rydberg states. HeH⁺ ions that capture an electron in a Rydberg state end up in separated He and H atoms by indirect predissociation. This paper presents a study of potential functions and pertinent matrix elements involving the lowest electronic states: the ²Σ⁺ states, X, A, C, and D, and the ²Π states B and E. Individual transition rates as well as total radiative and non-radiative lifetimes have been computed for the lowest vibrational and rotational levels. Received: 22 June 1998 / Accepted: 21 August 1998 / Published online: 12 October 1998     

Use of disposable clean-up columns for selective removal of humic substances prior to measurements with a nitrate ion-selective electrode

Measurements of nitrate in natural waters with a nitrate ion-selective electrode are seriously affected by the presence of humic substances. These can be removed quantitatively by a clean-up procedure with cheap disposable adsorption columns packed with chemically-bonded alkylamino silica. The method is applied to natural water samples with high contents of humic substances. The nitrate concentrations found were in good agreement with determination by ion chromatography.