Fractional damping enhances chaos in the nonlinear Helmholtz oscillator

Nonlinear Dynamics - The main purpose of this paper is to study both the underdamped and the overdamped dynamics of the nonlinear Helmholtz oscillator with a fractional-order damping. For that...     

Spectroscopic characterization of core and shell regions in monolayer protected gold nanoparticles

We report studies on the structure of the metallic core and the alkyl cap layer in monolayer protected gold nanoparticles having sizes down to 1.6 nm. These particles are obtained by laser ablating gold targets in alkane-thiol solutions at different concentrations. The electronic structure of gold core and the vibrational properties of the capping hydrocarbon chains reveal effects connected with the nanosized nature of the particles.     

Thermal decomposition products of copoly(arylene ether sulfone)s characterized by direct pyrolysis mass spectrometry

Pyrolysis products with mass of up to 850 Da were detected by direct pyrolysis mass spectrometric (DPMS) analysis of a series of copoly(arylene ether sulfone)s (PES-PPO) synthesized by nucleophilic condensation of either 4,4′-dichlorodiphenylsulfone (CDPS) or 4,4′-bis-(4-chlorophenyl sulfonyl) biphenyl (long chain dichloride, LCDC) with different molar ratios of hydroquinone (HQ) or dihydroxydiphenylsulfone (HDPS). Pyrolysis products retaining the repeating units of the initial copolymers were formed at temperatures ranging from 420 °C to 470 °C (near the initial decomposition temperature). At temperatures higher than 450 °C were observed products containing biphenyl units, formed by the elimination process of SO₂ from diphenyl sulfone bridges. Products having biphenyl and dibenzofuran moieties were detected in the mass spectra recorded at temperatures above 550 °C. These units were formed by loss of hydrogen atom from diphenyl ether bridges. Although the EI (18 eV) mass spectra of the pyrolysis products of the samples investigated were very similar, it was found that the relative intensity of some ions reflects the molar composition of the copolymers analysed. Cyclic and linear oligomers with very low molecular mass, present in the crude copolymers, were also detected by DPMS. Thermogravimetric analysis also showed their excellent thermal stability below 400 °C. It indicates that the copolymers yield a char residue of 40-45% at 800 °C, which increases with the PPO mole fraction in the samples.     

Solid-state photodimerization of steroid enones

Androst-4-ene-3,17-dione (1) and 17alpha-methyltestosterone (2) are dimerized in the solid-state by UV radiation. These substances were selected by a search in the CSD among the steroid enones presenting in the crystalline state an intermolecular short contact between a hydrogen alpha to a carbonyl group and the oxygen of an enone system. Dimerization occurs by transfer of the hydrogen to the oxygen and connection between the two involved carbons. Androst-4-ene-3,17-dione (1) affords dimer 3 and trimer 4, both formed by connection of the C-16 of a molecule with the C-3 of a near one. Irradiation of 17alpha-methyltestosterone (2) gives the isomeric trienones 5 and 6. These compounds are reasonably formed by dehydration of unisolated intermediate products 7 and/or 8 obtained by coupling of two molecules through a linkage between the C-2 and the C-3' carbons. The formation mechanisms of the photoproducts are satisfactory explained on the basis of the molecular arrangement of the monomers in the crystal state. Modeling of the dimeric molecules was done using molecular mechanics calculations. A single-crystal X-ray of the dimer of androst-4-ene-3,17-dione confirms the structural interpretation of spectral data. The conformer found in the solid-state agrees well with the results of molecular mechanics calculations.     

Simultaneous high-performance liquid chromatographic determination of antazoline phosphate and tetrahydrozoline hydrochloride in ophthalmic solution

A method for the determination of 2-[(N-phenyl)benzylaminomethyl]-2-imidazoline X H3PO4 (antazoline phosphate) and 2-(1,2,3,4-tetrahydro-1-naphthyl)-2-imidazoline X HCl (tetrahydrozoline hydrochloride) in ophthalmic solution is described. The pharmaceutical preparation is analysed directly by reversed-phase ion-pair high-performance liquid chromatography and the method is very rapid, selective and simple.     

Dimer asymmetry in superoxide dismutase studied by molecular dynamics simulation

Summary Molecular dynamics (MD) simulations of 100 ps have been carried out to study the active-site behaviour of the Cu,Zn superoxide dismutase dimer (SOD) in water. The active site of each subunit was monitored during the whole simulation by calculating the distances between functional residues and the catalytic copper. The results indicate that charge orientation is maintained at each active site but the solvent accessibility varies. Analysis of the MD simulation, carried out by using the atomic displacement covariance matrix, has shown a different intra-subunit correlation pattern for the two monomers and the presence of inter-subunit correlations. The MD simulation presented here indicates an asymmetry in the two active sites and different dynamic behaviour of the two SOD subunits.     

Mechanism of flame retardant action of red phosphorus in polyacrylonitrile

The mechanism of flame retardant (FR) action of red phosphorus in polyacrylonitrile combustion was investigated by thermogravimetry, flash-pyrolysis GC-MS, and combustion methods. Red phosphorus was found to increase the thermal stability in air of polyacrylonitrile and to induce a char residue increment on this substrate. Both these effects disappeared when pyrolysis was carried out under nitrogen flow. Flash-pyrolysis GC-MS experiments showed that red phosphorus does not alter the pyrolysis product distribution of polyacrylonitrile, which implies that there is no specific interaction between polyacrylonitrile and red phosphorus. These data also showed that polymeric red phosphorus decomposes to volatile white phosphorus (P₄) during pyrolysis. These observations allow us to propose a simple model for the mechanism of FR action of red phosphorus on polyacry-lonitrile at the molecular level. Combustion data for polyacrylonitrile-red phosphorus mixtures are in agreement with the proposed mechanism of FR action.     

The fragmentation of 5- and 3-substituted thiophene-2-carboxamides under electron impact

The 70 eV electron impact mass spectra of twelve 5- and 3-substituted thiophene-2-carboxamides are discussed with the aid of exact mass measurements and labelling experiments. All mass spectra exhibit pronounced molecular ions. Some isomeric 5- and 3-substituted title compounds can be differentiated by mass spectrometry. The fragmentation is influenced by a strong ‘ortho-effect’ which activates the NH₃ elimination. In the other cases the most important fragmentation is NH₂˙ loss, followed by CO elimination.