Silicate Xerogels with Dopant-Induced Chirality

A series of silicate xerogels with entrapped chiral amino acids have been obtained via sol-gel technology. The transparent, glassy samples obtained exhibit chirality in the bulk due to the presence of the entrapped asymmetric molecules. Measurements of the optical activity of the doped xerogel samples revealed that the entrapment did not significantly influence the optical activity observed for liquid solutions of the amino acids. Thus, the sol-gel method enables the preparation of amorphous optical materials exhibiting properties of strictly spatially defined molecular systems. Apart from the obvious optical applications, such porous materials with asymmetric centers might find interesting applications in chiral chemical syntheses and separations.     

Is planarity of pyridin-2-yl- and pyrazin-2-yl-formamide thiosemicarbazones related to their tuberculostatic activity? X-ray structures of two pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones

Crystal structures of two title compounds and several their relatives known earlier reveal conservative and characteristic features, which may be related to their tuberculostatic activity. The molecules are predominantly planar due to conjugation through five successive bonds in the zwitterionic fragment S⁻–C(sp²)–N–NH⁺–C(sp²)–NH₂ and intramolecular hydrogen bonds, which prevent rotation of the adjacent pyrazine (or pyridine) ring. It has been suggested that in spatial sense such planar molecules resemble acridines intercalating with nucleic acids and that similar process may be responsible for tuberculostatic activity of the title pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones.     

Crystal Structures of 1,5,9,18,22,26-Hexaaza[11.11]-p-cyclophane Adducts; Two-dimensional Supramolecular Networks

Two newly identified supramolecular structures arise from self-assembly of the macrocyclic 1,5,9,18,22,26- hexaaza[11.11]-p-cyclophane salts with o-nitrophenol (C₂₈H₅₀N₆)⁴⁺·4(C₆H₄NO₂O)⁻ (1) and with HCl (C₂₈H₅₂N₆)⁶⁺·6Cl^-·4H₂O (2). In both cases two-dimensional supramolecular sheets are formed.     

Low temperature crystal and molecular structure of nitrobenzene

The crystal and molecular structure of nitrobenzene was determined at 103 K by X-ray diffraction, yieldingR=0.034 and a highly precise geometry of the molecule (esds of bond lengths 0.001 Å and bond angles 0.1°). The bond angles in the ring agree very well with additive scheme based on angular substituent parameters. X-X electron density maps support the view that the C-N bond does not exhibit-character, and, hence, the nitro group interacts with the ring mostly by inductive effects.     

Multicomponent analysis with a computerized flow injection system using LED photometric detection

An original computer-controlled solid-state photometer has been developed and its potential for simultaneous multicomponent flow-injection analysis of binary and ternary mixtures is demonstrated. The device is simple in construction. Its principle of operation is based on rapid sequential measurements of the absorbance of the complexes formed by the analytes with chromogenic reagents at the wavelengths corresponding to the emission maxima of three light emitting diodes (563, 580 and 638 nm).     

Stimuli-responsive surface crystallization of phospholipids from bimodal colloidal particles

These studies focus on the effect of phospholipids in the presence of ionic surfactants on the behavior of poly(methylmethactrylate/n-butyl acrylate) (p-MMA/nBA) colloidal particles during film formation. With the presence of two surfactants, it is possible to obtain particles that exhibit two distinct particle sizes. The presence of hydrogenated soybean phosphatidylcholine (HSPC) and sodium dioctyl sulfosuccinate (SDOSS), which stabilize these bimodal colloidal dispersions, has a significant effect on the mobility of individual components during coalescence. Specifically, the presence of HSPC inhibits migration of SDOSS to the film-air (F-A) interface. Furthermore, the presence of electrolyte species such as aqueous CaCl2 has a very pronounced effect on film formation. When the Ca2+/HSPC ratio is 0.1/1.0, SDOSS is released to the F-A interface during coalescence. At 2.0/1.0 Ca2+/HSPC, HSPC diffuses to the F-A interface and crystalline domains consisting of HSPC are formed. This stimuli-responsive behavior is confirmed using IRIR imaging that ultimately exhibits different surface morphologies. These studies illustrate for the first time that it is possible to control the release of two different surface-active species during coalescence that form crystalline domains.     

Electronic spectrum of dibiphenylene-ethene radical anion

The electronic absorption spectrum for the dibiphenylene-ethene radical anion ( ), generated electrochemically in dimethylsulfoxide solutions, is reported. Based on calculations by the LCAO-MO method in theHückel approximation and on the analysis of the band positions it is suggested that in the molecule two planar fluorenylidene fragments are twisted (60°) about the central C=C bond.

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Das Elektronenabsorptionsspektrum des Dibiphenylen-Radikalanions (Kurze Mitteilung)

Zusammenfassung Das Dibiphenylenethen-Radikalanion ( ) wurde elektrochemisch hergestellt und das Elektronenabsorptionsspektrum aufgenommen. Auf Grund von LCAO-MO Berechnungen in derHückel-Annäherung und der Analyse der Bandenlagen wird nahegelegt, daß im -Molekül zwei planare Fluorenyliden-Fragmente ca. 60° um die zentrale C=C-Bindung verdrillt angeordnet sind.

     

Photoinitiation by radical-cationic mechanism in the polymerization of isobutylene with lewis acids

ESR spectra of Lewis acids (VCl₄, TiCl₄, TiBr₄, SnCl₄, and AlBr₃) and their mixtures with isobutylene were investigated in a n-heptane solution in the dark and under irradiation at 400–480 nm at ?80 to ?150°C. A signal was observed only upon irradiating mixtures of VCl₄, TiCl₄, or TiBr₄ and isobutylene. The signal was identified as an isobutylene radical-cation by comparison with a simulated spectrum. A signal indicating the presence of peroxy radicals were recorded in measurements carried out in the presence of oxygen; these radicals originated from reaction of the isobutylene radical-cation with oxygen. Radical-cation initiation by visible light is indicated by the polymerization of isobutylene by VCl₄, TiCl₄, and TiBr₄ and by ESR spectra. The inhibiting effect of oxygen in photochemically initiated polymerization of isobutylene was also elucidated.     

Transient Radicals Formed by Electron Transfer between inorganic ions and excited aromatic molecules in polar solvents

Fluorescence quenching of aromatic molecules by inorganic anions has been the subject of many investigations, yet the nature of the quenching mechanism is not fully understood. The fluorescence-quenching rate constants correlate with electrochemical data, but the radicals expected to form upon transfer of an electron to the excited aromatic molecules have escaped observation. We report the first observation of radical-ion species formed by electron-transfer quenching with inorganic anions in acetonitrile. A decisive step leading to formation of separated radical ions is the trapping of the primary charge-transfer complex by a second inorganic ion.     

Flow-injection single-point titration of acids with biamperometric detection at polarized platinum electrodes

Flow-injection single-point titration of acids is based on biamperometric measurement of iodine formed quantitatively in the reaction of iodide with iodate. Results are presented for determination of sulphuric, hydrofluoric, monochloroacetic, formic and acetic acids. The slopes of the calibration plots, which are linear for about 1–10 rum acid, depend on the strength of the acid, decreasing from sulphuric acid to acetic acid. With microcomputer on-line data processing, relative standard deviations were about 0.2%.