Contribution to cation–π interaction of the univalent silver cation with racemic [7]helicene in the solid state

Structural Chemistry - Yellow crystals of the complex [7]helicene–silver perchlorate (i.e., C30H18–AgClO4 or more accurately [Ag(η4-C30H18)]ClO4, respectively), crystallizing in...     

Colored graphs without colorful cycles

A colored graph is a complete graph in which a color has been assigned to each edge, and a colorful cycle is a cycle in which each edge has a different color. We first show that a colored graph lacks colorful cycles iff it is Gallai, i.e., lacks colorful triangles. We then show that, under the operation mon ≡ m + n − 2, the omitted lengths of colorful cycles in a colored graph form a monoid isomorphic to a submonoid of the natural numbers which contains all integers past some point. We prove that several but not all such monoids are realized. We then characterize exact Gallai graphs, i.e., graphs in which every triangle has edges of exactly two colors. We show that these are precisely the graphs which can be iteratively built up from three simple colored graphs, having 2, 4, and 5 vertices, respectively. We then characterize in two different ways the monochromes, i.e., the connected components of maximal monochromatic subgraphs, of exact Gallai graphs. The first characterization is in terms of their reduced form, a notion which hinges on the important idea of a full homomorphism. The second characterization is by means of a homomorphism duality. The first author would like to express his thanks for support by project LN 00A056 of the Ministry of Education of the Czech Republic. The second author would like to express his thanks for support by project LN 00A056 of the Ministry of Education of the Czech Republic, by the NSERC of Canada and by the Gudder Trust of the University of Denver.     

1,8,9-Substituted anthracenes, intramolecular phosphine donor stabilized metaphosphonate and phosphenium

A series of 1,8,9-tris(phosphorus) substituted anthracenes was synthesised and fully characterised. Their molecular geometry is such that the middle phosphorus atom is in a highly congested space, with terminal phosphorus atoms in close proximity. The middle phosphorus atom accepts electron density from none, one or both terminal phosphorus atoms to form P-P bonds. Thus phosphino (repulsively interacting), metaphosphonato (single donor stabilised) as well as phosphenium (doubly phosphine donor stabilised) environments around the phosphorus atom in position 9 were synthesized and structurally characterised. Observed P[dot dot dot]P distances range from 2.22 to 3.27 A.     

Mechanism of anti-inflammatory action of liquorice extract and glycyrrhizin

The antiradical activity, protective effect against lipid peroxidation of liposomal membrane, and inhibitory effect on whole blood reactive oxygen species (ROS) liberation of Glycyrrhiza glabra crude extract and glycyrrhizin, its major compound, were assessed. The liquorice extract showed significant activity in all the three assay systems used in a dose dependent manner. It displayed remarkable reactivity with free stable 1,1'-diphenyl-2-picrylhydrazyl (DPPH) radical, inhibitory efficacy in peroxidatively damaged unilamellar dioleoyl phosphatidylcholine (DOPC) liposomes, and inhibition of ROS chemiluminescence, generated by whole blood, induced by both receptor-bypassing stimuli (PMA) and receptor operating stimuli (Opz) in the ranking order of stimuli PMA> Opz. These activities may be attributed to phenolic antioxidants involving isoflavan derivatives, coumarins and chalcones. Nonetheless, triterpene saponin glycyrrhizin exhibited no efficacy in the system of DPPH reaction and peroxidation of liposomal membrane, and negligible inhibition of chemiluminescence generated by inflammatory cells. These results indicate that the mechanism of anti-inflammatory effect of glycyrrhizin most probably does not involve ROS and this major constituent is not responsible for the inhibition effects of liquorice extract on neutrophil functions.     

Time-dependent stokes shifts of fluorescent dyes in the hydrophobic backbone region of a phospholipid bilayer: combination of fluorescence spectroscopy and ab initio calculations

We explored the time-dependent Stokes shifts of fluorescent dyes containing an anthroyloxy chromophore (2-AS, 9-AS, and 16-AP) in bilayers composed of palmitoyl-oleoyl-phosphatidylcholine. The obtained data revealed a nontrivial solvation response of these dyes, which are located in the backbone region of the bilayer with a gradually increasing depth. For comparison, steady-state emission spectra in the neat solvents of various polarities and viscosities were also recorded. The results indicate that on the short picosecond time scale the AS dyes undergo complex photophysics including formation of states with a charge-transfer character. This observation is supported by ab initio calculations of the excited states of 9-methylanthroate. The slower nanosecond part of the relaxation process can be attributed to the solvation response of the dyes. A slowdown in solvent relaxation is observed upon moving toward the center of the bilayer. A mechanism similar to preferential solvation present in the mixture of a polar and nonpolar solvent is considered to explain the obtained data.     

Simple palladium(II) precatalyst for Suzuki-Miyaura couplings: efficient reactions of benzylic, aryl, heteroaryl, and vinyl coupling partners

trans-PdBr(N-Succ)(PPh3)2 (1) is a universally effective precatalyst for Suzuki-Miyaura cross-couplings of benzylic halides with aryl- or heteroarylboronic acids. Substituted aryl halides and halogenated cyclic enones can be cross-coupled with aryl- or vinylboronic acids in excellent yields. Catalyst recycling is also demonstrated.     

Determination of fluoroquinolone antibiotics in surface waters from Mondego River by high performance liquid chromatography using a monolithic column

A novel LC-fluorescence detection method based on the use of a monolithic column for the determination of norfloxacin, ciprofloxacin, and enrofloxacin antibiotic residues in environmental waters was developed. Fluoroquinolones (FQs) were isocratically eluted using a mobile phase consisting of 0.025 M phosphoric acid solution at pH 3.0 with tetrabutylammonium and methanol (960:40, v/v) through a Chromolith Performance RP-18e column (100x4.6 mm) at a flow rate of 2.5 mL/min and detected at excitation and emission wavelengths of 278 and 450 nm, respectively. After acidification and addition of EDTA, water samples were extracted using an Oasis HLB cartridge. Linearity was evaluated in the range of 0.05 to 1 microg/mL and correlation coefficients of 0.9945 for norfloxacin, 0.9974 for ciprofloxacin, and 0.9982 for enrofloxacin were found. The limit of quantification was 25 ng/L for the three FQs. The recovery of FQs spiked into river water samples at 25, 50, and 100 ng/L fortification levels ranged from 76.5 to 91.0% for norfloxacin, 78.5 to 97.2% for ciprofloxacin, and 79.4 to 93.6% for enrofloxacin. This method was successfully applied to the analysis of water samples from the Mondego River, and ciprofloxacin and enrofloxacin residues were detected in eight water samples.     

Water Interaction with Iron Oxides

We present a mechanistic study on the interaction of water with a well‐defined model Fe₃O₄(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol⁻¹) is considerably higher than all previously reported values obtained by indirect desorption‐based methods. By employing ¹⁸O‐labeled water molecules and an Fe₃O₄ substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non‐dissociated water molecule creating a thermodynamically very stable dimer‐like complex.     

Origin‐independent sum over states simulations of magnetic and electronic circular dichroism spectra via the localized orbital/local origin method

Although electronic and magnetic circular dichroism (ECD, MCD) spectra reveal valuable details about molecular geometry and electronic structure, quantum‐chemical simulations significantly facilitate their interpretation. However, the simulated results may depend on the choice of coordinate origin. Previously (?těpánek and Bou?, J. Comput. Chem. 2013, 34, 1531), the sum‐over‐states (SOS) methodology was found useful for efficient MCD computations. Approximate wave functions were “resolved” using time‐dependent density functional theory, and the origin‐dependence was avoided by placing the origin to the center of mass of the investigated molecule. In this study, a more elegant way is proposed, based on the localized orbital/local origin (LORG) formalism, and a similar approach is also applied to generate ECD intensities. The LORG‐like approach yields fully origin‐independent ECD and MCD spectra. The results thus indicate that the computationally relatively cheap SOS simulations open a new way of modeling molecular properties, including those involving the origin‐dependent magnetic dipole moment operator. © 2015 Wiley Periodicals, Inc.