Selective adsorption and reactivity of dipeptide stereoisomers in clay mineral suspension

Preferential adsorption of dipeptide diastereomers (dialanine, Val-Ala) on clay mineral surfaces was observed. Significantly higher adsorption of dipeptides composed from only one type of enantiomer of amino acid units in comparison to those containing both L- and D-type of amino acid units in their molecules, was experimentally proven. This selectivity was explained in terms of different hydrophobic properties of diastereomers, which are probably controlled by intramolecular interactions between nonpolar and polar parts of dipeptide molecules affected by their stereochemistry. A significantly higher reactivity of stereoisomers composed from the same type of amino acid enantiomers to form amide bonds was proven as well. Theoretical study could distinguish different properties of the diastereomers. The results of the calculations indicate possible effects of molecular stability in the stereoselectivity during the adsorption and reactions of Ala(2) diastereomers.     

Stereochemistry,tautomerism, and reactions of acridinyl thiosemicarbazides in the synthesis of 1,3‐thiazolidines

Acridin‐9‐yl hydrazine upon treatment with various isothiocyanates (RNCS, R = methyl, allyl, phenyl, p‐methoxy phenyl, and p‐nitro phenyl) yielded the corresponding thiosemicarbazides with acridine substituted on the carbazide‐type side. The alkyl‐substituted compounds were present in solution as equilibria consisting of the major H‐10, H‐12 tautomer (either E or Z or both about the C₁₃‐N₁₄ bond) and the minor H‐10, SH tautomer (either E or Z or both). The major species for the aromatic‐substituted compounds was the H‐10, H‐12 E tautomer, with the evident minor species being the H‐10, H‐12 Z tautomer. The thiosemicarbazides were each quantitatively converted into the analogous semicarbazides upon treatment with mesitylnitrile oxide wherein all structures were present in solution as the H‐10 tautomers with Z conformation about the C₁₃‐N₁₄ bond. Methylation of the compounds with methyl iodide yielded S‐methylated compounds wherein the Z configuration dominated in each case over the E configuration along the N₁₂‐C₁₃ double bond. Treatment of the thiosemicarbazides with methyl bromoacetate resulted in the formation of 1′,3′‐thiazolidin‐4′‐ones wherein the Z configuration predominated in each case over the E configuration along the N₁₂? C₁₃ double bond. With bromoacetonitrile as the bifunctional electrophile, the initial 1′,3′‐thiazolidin‐4′‐imines that formed spontaneously underwent Dimroth‐type rearrangement to the regiosiomeric 1′,3′‐thiazolidin‐4′‐imines.     

Influence of the broken symmetry of defect state distribution at the a-Si:H/c-Si interface on the performance of hetero-junction solar cells

Taking into account the fact that the distribution of defect states at the interface does not have strictly symmetrical shape, we present a simulation study of a-Si:H(n)/c-Si(p) and a-Si:H(p)/c-Si(n) structures with regard to the defect states at the interface, band offsets and doping concentration of the emitter. The presented results suggest for a-Si:H(n)/c-Si(p) solar cells a strong influence of the introduced broken symmetry between acceptor and donor defect states on the open-circuit voltage, whereas the a-Si:H(p)/c-Si(n) structure benefits from inherent favorable band alignment and remains unaffected.     

The fluoroformyloxyl radical geometry and ground-state rotational spectra of the free FC18O2· radical

The rotational spectra of the isotopically substituted free fluoroformyloxyl radical FC(18)O(2·) were measured using the Prague millimeter-wave high-resolution spectrometer. More than 110 rotational-fine-hyperfine transition lines were observed and assigned to appropriate quantum numbers in the frequency range of 235-270 GHz. The obtained transition frequencies were analyzed with standard effective Hamiltonians to acquire a set of precise rotational, centrifugal-distortion, fine, and hyperfine structure molecular constants. Merging the new FC(18)O(2·) isotopologue molecular parameters with those previously obtained for the ordinary FC(16)O(2)[middle dot] radical, the substitution molecular geometry in the ground vibronic state was evaluated. The molecular parameters for both radical isotopologues were also calculated by several quantum chemistry methods and their calculated mutual ratios are in remarkable agreement with the experimental FC(16)O(2·)/FC(18)O(2·) parameter ratios. The measurements, assignments of the 18-oxygen isotopologue FC(18)O(2·) radical millimeter-wave transitions, as well as the derivation of the fluoroformyloxyl radical ground-state geometry have been carried out for the first time.     

On-line preconcentration of perfluorooctanoic acid and perfluorooctanesulfonic acid by nonaqueous capillary electrophoresis

Separation of major environmental pollutants as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) by capillary electrophoresis is reported for the first time. It is not possible to resolve the solutes in an aqueous media. However, the use of methanol and acetonitrile as the background electrolyte (BGE) solvents allowed their rapid separation in an uncoated capillary. A major effort was put into BGE optimization in respect to both separation efficiency and detection for further on‐line preconcentration. 5 mmol.L^?1 naphthalene‐1‐sulfonic acid and 10 mmol.L^?1 triethylamine dissolved in ACN/MeOH (50:50 v/v) provided best separation and detection conditions. Next, the large‐volume sample stacking and the field‐amplified sample injection were applied and compared. Large‐volume sample stacking improved limits of detection (LODs) with regard to the standard injection by 69 times for PFOA and 143 times for PFOS with LODs of 280 and 230 nmol.L^?1, respectively. Field‐amplified sample injection improved LODs 624 times for PFOAand 806 times for PFOS with LODs 31 and 40 nmol.L^?1, respectively. Both preconcentration methods showed repeatabilities of migration times less than 1.2% RSD intraday and 6.6% RSD interday. The method was applied on PFOA and PFOS analysis in a sample of river water treated with solid‐phase extraction, which further improved LOD toward 5.6 × 10^?10 mol.L^?1 for PFOS and 6.4 × 10^?10 mol.L^?1 for PFOA and allows the method to be used for river water contamination screening or decomposition studies.     

Binary and ternary recombination of para-H3(+) and ortho-H3(+) with electrons: state selective study at 77-200 K

Measurements in H(3)(+) afterglow plasmas with spectroscopically determined relative abundances of H(3)(+) ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H(3)(+) ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H(3)(+) recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.     

Highly fluorescent colloids based on rhodamine 6G, modified layered silicate, and organic solvent

Synthetic layered silicate saponite was modified with dodecyltrimethylammonium (C12), octadecyltrimethylammonium (C18), and dioctadecyldimethylammonium (2C18) cations for use as sorbents of the laser dye, rhodamine 6G (R6G). Via solvent exchange, transparent colloids in xylene were prepared and investigated using absorption and fluorescence spectroscopies. Molecular aggregation and partial quenching of the fluorescence were observed for the colloids based on 2C18 cations. Maximal fluorescence yields were observed for the colloids with C12 and C18 cations. Transparent gels without an apparent loss of luminescent efficiency could be prepared by concentrating the colloids. These highly fluorescent colloids and gels represent new types of materials with interesting optical properties.     

X‐Treme beamline at SLS: X‐ray magnetic circular and linear dichroism at high field and low temperature

X‐Treme is a soft X‐ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with École Polytechnique Fédérale de Lausanne. The beamline is dedicated to polarization‐dependent X‐ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end‐station has a superconducting 7 T–2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 × 10¹² photons s^?1, are presented. Scientific examples showing X‐ray magnetic circular and X‐ray magnetic linear dichroism measurements are also presented.     

Photoinitiation. V. Effect of medium polarity on the acrylonitrile polymerization photoinitiated by naphthalene

The heterogeneous polymerization of acrylonitrile photoinitiated by naphthalene is influenced by the polarity of the reaction medium. The rate of initiation increases with the increasing dielectric strength of the reaction medium. A similar trend is observable for Stern–Volmer constants of naphthalene fluorescence quenching by acrylonitrile. The ratio k_p/k_t^1/2 of the rate constant for propagation and termination reactions is not influenced by a change in the polarity of the reaction medium. The effect of viscosity on the value of k_p/k_t^1/2 known for polymerization in a homogeneous medium was not observed in the reaction systems studied.