Strain in strain-free benzenoid hydrocarbons: the case of fibonacenes

In a recent paper (Radenkovic et al. Chem Phys Lett 625:69–72, 2015), a new method for quantifying the strain energy in benzenoid molecules, resulting from the repulsion between the bay H-atoms was elaborated. In this work, we present a modified procedure, capable of estimating the strain energy in a single-step calculation. Strain energies were obtained at the B3LYP/def2-TZVP level of density functional theory. It was found that in benzenoid molecules with a single bay region, the strain energy is essentially constant, equal to around 7.3 kJ/mol. On the other hand, in the case of the first four members of the fibonacene series, the strain energy is found to be linearly proportional to the number of bay regions.     

Interactions of nitrogen-donor bio-molecules with dinuclear platinum(II) complexes

Substitution reactions of the dinuclear Pt(II) complexes, [{Pt(en)Cl}₂(μ-pz)]²⁺ (1), [{Pt(dach)Cl}₂(μ-pz)]²⁺ (2) and [{Pt(dach)Cl}₂(μ-4,4?-bipy)]²⁺ (3), and corresponding aqua analogs with selected biologically important ligands, viz. 1,2,4-triazole, L-histidine (L-His) and guanosine-5?-monophosphate (5?-GMP) were studied under pseudo-first-order conditions as a function of concentration and temperature using UV–vis spectrophotometry. The reactions of the chloride complexes were followed in aqueous 25 mmol L^?1 Hepes buffer in the presence of 40 mmol L^?1 NaCl at pH 7.2, whereas the reactions of the aqua complexes were studied at pH 2.5. Two consecutive reaction steps, which both depend on the nucleophile concentration, were observed in all cases. The second-order rate constants for both reaction steps indicate a decrease in the order 1 > 2 > 3 for all complexes. Also, the pK_a values of all three aqua complexes were determined. The order of the reactivity of the studied ligands is 1,2,4-triazole > L-His > 5?-GMP. ¹H NMR spectroscopy and HPLC were used to follow the substitution of chloride in the dichloride 1, 2, and 3 complexes by guanosine-5?-monophosphate (5?-GMP). This study shows that the inert and bridging ligands have an important influence on the reactivity of the studied complexes.     

Intercalator-DNA interactions revealed by NIR surface-enhanced Raman spectroscopy

Using 1064 nm excited surface-enhanced Raman spectroscopy (SERS) a well known intercalator, ethidium bromide (EB), and a structurally related compound, 4-methyl-2,7-diamino-5,10-diphenyl-4,9-diazapyrenium hydrogensulfate (ADAP), have been studied. Concentration dependent SERS spectra of both aromatic species (1 × 10⁻⁷-5 × 10⁻⁵ M) indicated existence of dimeric associates at high concentration and an equilibrium shift towards monomers with a concentration decrease. Interactions of the intercalating molecules with DNA have been studied for various intercalator/DNA (base pair) molar ratios ranging from 10/1 to 1/10. In colloidal samples containing an intercalator in excess relative to DNA binding sites (from 10/1 to 2/1) enhancement of the Raman scattering gradually weakened, indicating a decrease in a number of free molecules adsorbed on the metal surface due to binding with DNA. At the drug/DNA ratios of 1/2 and 1/5 weaker but observable SERS bands indicated insertion of the drug molecules between the base pairs (intercalation strongly diminished interaction of the drug molecules with metal surface) as well as non-intercalative binding of the drug molecules able to stay in closer contact with a metal surface. A total intercalation of EB and ADAP molecules (intercalator/DNA of 1/7 and 1/10) resulted in almost complete loss of the SERS signal. Intensity of the SERS spectra of the intercalator/DNA complexes relative to the SERS intensity of the free intercalating molecules diminished to a lesser degree for ADAP/DNA than for EB/DNA. The obtained difference was attributed to a larger aromatic surface of the ADAP molecules which, although intercalated, could be positioned near the enhancing nanoparticles, unlike the smaller EB molecules which were deeply inserted within the DNA helix.     

Oxidation of carbon monoxide and formic acid on bulk and nanosized Pt–Co alloys

Bulk Pt₃Co and nanosized Pt₃Co and PtCo alloys supported on high area carbon were investigated as the electrocatalysts for the CO_ads and HCOOH oxidation. Pt₃Co alloy with Co electrochemically leached from the surface (Pt skeleton) was employed to separate electronic from ensemble and bifunctional effects of Co. Cyclic voltammetry in 0.1 M HClO₄ showed reduced amount of adsorbed hydrogen on Pt sites on Pt₃Co alloy compared to pure Pt. However, no significant difference in hydrogen adsorption/desorption and Pt-oxide reduction features between Pt₃Co with Pt skeleton structure and bulk Pt was observed. The oxidation of CO_ads on Pt₃Co alloy commenced earlier than on Pt, but this effect on Pt₃Co with Pt skeleton structure was minor indicating that bifunctional mechanism is stronger than the electronic modification of Pt by Co. The HCOOH oxidation rate on Pt₃Co alloy was about seven times higher than on bulk Pt when the reaction rates were compared at 0.4 V, i.e., in the middle of the potential range for the HCOOH oxidation. Like in the case of CO_ads oxidation, Pt skeleton showed similar activity as bulk Pt indicating that the ensemble effect is responsible for the enhanced activity of Pt₃Co alloy toward HCOOH oxidation. The comparison of CO_ads and HCOOH oxidation on Pt₃Co/C and PtCo/C with the same reaction on Pt/C were qualitatively the same as on bulk materials.     

Non-isothermal gas flow in microchannel with equal wall temperatures

A non-isothermal two-dimensional compressible subsonic slip gas flows is analyzed in this paper. It is pressure-driven steady flow in microchannels with variable cross section (convergent, divergent, and constant height channel). Since the flows correspond to microspaces, the rarefaction effect is taken in account. The obtained gas temperature profiles are non-uniform. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Voltammetric Study of Reduction Processes in Rhodium(+3)–Selenium(+4) Acidic Mixtures

Using square‐wave voltammetry and linear scan voltammetry, some reduction processes, that take place on the hanging mercury drop electrode (HMDE) in rhodium(+3)–selenium(+4) mixtures, were studied in highly acidic sulfate, chloride and perchlorate solutions. The special attention was paid to the peak located at ?1.1 V, that reflects catalytic hydrogen evolution on the rhodium islets, and its changes under the influence of added selenium(+4), or selenium(?2) produced at the electrode. It can be concluded that addition of selenite to a dilute rhodium solution has the similar effect as the increase of rhodium(+3) concentration, making the catalytic hydrogen evolution more pronounced, probably by changing the properties of metal clusters on the mercury drop. If so, the effect could be of some practical importance in production of rhodium deposits on other surfaces, too.     

Electron spectroscopic investigations of CdS and CdTe electrochemically coated on glass

The electrodeposition of CdS and CdTe is investigated to improve the stoichiometric properties of CdS/CdTe layers on ITO-glass substrates for solar cell applications. X-ray photoelectron spectroscopy is utilized for the characterization of the CdS and CdTe layers. The influence of the electrodeposition potential, the pH and the thiosulfate concentration on the stoichiometry of CdS and CdTe layers are discussed.     

Two new heuristics for the location set covering problem

Summary In this paper we present two new greedy-type heuristics for solving the location set covering problem. We compare our new pair of algorithms with the pair GH1 and GH2 [Vasko and Wilson (1986)] and show that they perform better for a selected set of test problems.     

A study of the reactions of methionine- and histidine-containing peptides with palladium(II) complexes: The key role of steric crowding on palladium(II) in the selective cleavage of the peptide bond

¹H NMR spectroscopy was applied to study the reactions of palladium(II) complexes, cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ (dpa is 2,2′-dipyridylamine acting as a bidentate ligand) with the dipeptides methionylglycine (Met-Gly) and histidylglycine (His-Gly), and the N-acetylated derivatives of these dipeptides, MeCOMet-Gly and MeCOHis-Gly. All reactions were carried out in the pH range 2.0–2.5 with equimolar amounts of the palladium(II) complex and the peptide at two different temperatures, 25 and 60 °C. In the reactions of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ with Met-Gly and His-Gly, no hydrolysis of the peptide bond was observed. The final product in these reactions was the [Pd(dpa)₂]²⁺ complex. The square-planar structure of this complex was confirmed by X-ray analysis. The reaction of the cis-[Pd(dpa)(H₂O)₂]²⁺ complex with the MeCOHis-Gly and MeCOMet-Gly peptides under the previously mentioned experimental conditions was remarkably selective in the cleavage of the amide bond involving the carboxylic group of methionine in the side chain. The modes of coordination of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ in the reactions with the non-acetylated peptides and the total steric inhibition of the hydrolytic reaction between cis-[Pd(dpa)(H₂O)₂]²⁺ and MeCOHis-Gly can be attributed to the steric bulk of the palladium(II) complex. This finding should be taken into consideration in designing new palladium(II) complexes for the regioselective cleavage of peptides and proteins.