Nitration of Durene and Pentamethylbenzene with Nitronium Salts in Nitromethane and Acetonitrile

Nature and proportions of the products formed in the nitrations mentioned in the title are affected by the process of mixing. Pentamethylnitrobenzene, formed initially in the nitration of pentamethylbenzene, is attacked by a nitronium ion at a position meta to the nitro-group. In the σ-complex formed, loss of a proton from a methyl group leads to production of by-products and release of nitrite ions. The nitrous acid formed is protonated, and water is displaced from the nitrous acidium ion by the anion, PF₆^?, giving the nitrosonium ion which forms a 1:1 symmetric molecular complex with pentamethylbenzene. A similar complex is formed in the reaction between durene and nitronium salts. The results are consistent with the view that no σ-bond is formed between the aromatic compound and the nitrosonium ion.     

Configurational studies of complexes of various tea catechins and caffeine in crystal state

Crystals of the complexes of (+)-catechin (CA) of non-galloylated catechin and (-)-catechin-3-O-gallate (Cg) of galloylated catechin with caffeine were prepared, and their stereochemical structures and intermolecular interactions were determined by X-ray crystallographic analysis. CA formed a 1 : 1 complex with caffeine by intermolecular hydrogen bonds, whereas Cg formed a 1 : 2 complex with caffeine, which was formed by face-to-face and offset π-π interactions and intermolecular hydrogen bonds. A solution of two kinds of non-galloylated catechin, CA and (-)-epicatechin (EC), and caffeine (molar ratio 1 : 1 : 2) in water afforded a 1 : 1 : 2 complex, the crystal structure of which had two layers, one layer in which CA and caffeine formed alternate lines and an other layer in which EC and caffeine formed alternate lines. The 1 : 1 : 2 complex was formed by offset π-π and CH-π interactions and intermolecular hydrogen bonds.     

Does double electron capture lead to the formation of biradicals? An ECD-SORI-CID study on lacticin 481

We studied lacticin 481, a small lantibiotic with three lanthionine bridges, by electron capture dissociation (ECD) in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Following electron capture, very little fragmentation was observed, but species formed by nondissociative single and multiple electron capture were abundant. Ions formed by double electron capture were subjected to sustained off resonance irradiation collision induced dissociation (SORI-CID) to determine whether stable biradicals were formed. In the SORI-CID spectra of the ions formed by double electron capture, some, but minor, H* radical loss was observed, which was not observed at all for regularly protonated ions. A small part of the ions formed by double electron capture are thus long-lived biradicals. Apart from the observed H* loss, the SORI-CID spectra of ions that captured two electrons was similar to that of regularly protonated ions and quite different from the SORI-CID spectra of radical ions formed by single electron capture. This implies that recombination of the two radical sites is the dominant process in biradical lacticin 481 ions, at least on the time scale of our SORI-CID experiments.     

Evidence for ionization-related conformational differences of peptide ions in a quadrupole ion trap

The differences in boundary-activated dissociation (BAD) onsets have been investigated for peptide ions that were generated by two different ionization techniques, nanoflow electrospray ionization (nanoESI) and liquid secondary-ion mass spectrometry (LSIMS). BAD onsets of these ions were determined to compare the relative internal energies of the ions. Protonated peptide ions formed by nanoESI had lower BAD onsets than ions formed by LSIMS. The BAD onsets of peptides derivatized to have a fixed charge on the N-terminus also were lower for those generated by nanoESI than those generated by LSIMS. The BAD onsets of ions formed by nanoESI did not change with the variation of collisional cooling periods after gating ions into the ion trap and after isolating them prior to dissociation, indicating that the ions formed by the two ionization techniques would not adopt the same energy distributions. It is proposed that the ions formed by the two techniques differ in secondary structure, and the LSIMS ions are collisionally cooled to a lower local minimum along the potential energy surface than the nanoESI ions. Ions formed by both techniques show the same dissociation patterns, so assuming the absolute energy required for dissociation is the same, the LSIMS ions possess a higher critical energy of dissociation. This leads to the observation of the higher BAD onset.     

Tribological properties of nanoparticle-laden ultrathin films formed by covalent molecular assembly

The tribological properties of ultrathin films containing nanoparticles encapsulated in immobilized dendrimers are investigated. The films were formed by covalent molecular assembly in supercritical carbon dioxide, and the Au nanoparticles were formed in aqueous solution. End-capping of the terminal amine groups of the dendrimer by fluorinated species resulted in a reduction in the size of the nanoparticles formed. The resulting film structure displayed a lower coefficient of friction when the nanoparticles were formed after fluorination. The observed improvement in the tribological properties is attributed to the reduction in agglomeration of the nanoparticles due to the presence of the fluorine moieties.     

Application of pulsating overpotential regime on the formation of copper deposits in the range of hydrogen co-deposition

Electrodeposition of copper by pulsating overpotential (PO) regime in the range of hydrogen co-deposition was examined by scanning electron microscopy. It was found that the increase of the pause-to-pulse ratio produced a strong effect on the morphology of electrodeposited copper. Honeycomb-like copper structures were formed with the pause-to-pulse ratios up to 5. Up to this value of the pause-to-pulse ratio, the diameter of the holes formed by attached hydrogen bubbles was decreasing, while their number was increasing by the application of PO regime. The compactness of the formed honeycomb-like structures was also increasing with the increasing pause duration. The increase of the pause-to-pulse ratio suppressed a coalescence of neighboring hydrogen bubbles. Copper dendrites in the interior of the holes and at their shoulders were formed with the higher pause-to-pulse ratios. The size of the formed dendrites, as well as their number, increased with the increasing pause duration. Depth of holes was decreasing with the increasing pause duration. The increased compactness of the obtained structures was explained by the use of a set of equations describing the effect of square-wave PO on electrodeposition process.     

Scaling analysis of polyacrylamide gel surfaces synthesized in the presence of surfactants

Surfaces of polyacrylamide hydrogels synthesized in the presence of surfactants were imaged by atomic force microscopy (AFM), and the surface morphology was studied by numerical scaling analysis. The gels were formed by polymerizing acrylamide plus a cross-linker in the presence of surfactants, which were then removed by soaking in distilled water. Gels formed in the presence of over 20% surfactant (by weight) formed clear, but became opaque upon removal of the surfactants. Other gels formed and remained clear. The surface morphology of the gels was studied by several one- and two-dimensional numerical scaling methods. The surfaces were found to be self-affine on short length scales, with a roughness (Hurst) exponent in the range from 0.85 to 1, crossing over to a constant root-mean-square surface width at long scales. Both the crossover length between these two regimes and the saturation value of the surface width increased significantly with increasing surfactant concentration, coincident with the increase in opacity. We propose that the changes in the surface morphology are due to a percolation transition in the system of voids formed upon removal of the surfactants from the bulk.     

Mass spectrometry of 4-amino-4′-nitroazobenzene compounds

The mass spectra of 32 substituted 4-amino-4′-nitroazobenzene compounds have been recorded and the most intense peaks have been used to characterize these spectra. It was found that the spectra of 4-amino-4′-nitroazobenzene compounds are characterized by peaks due to: (1) molecular ions, (2) fragment ions formed by cleavage of one of the carbon-nitrogen bonds adjacent to the azo linkage with the positive charge remaining with the amine fragment, (3) ions formed by cleavage alpha to the amine nitrogen with the charge remaining with the amine substituent, (4) ions formed by cleavage beta to the amine nitrogen with the loss of the amine substituent fragment, (5) secondary ions formed by cleavage beta to the amine nitrogen with the loss of the amine substituent fragment from the primary amine fragment (2), and (6) ions formed by loss of NO from the molecular ion. This work shows that 4-amino-4′-nitroazobenzene compounds exhibit fragmentation which is dependent in a consistent manner on the types of substituents. This work provides a basis for a systematic approach to the identification of 4-amino-4′-nitroazobenzene compounds.     

The evolution of nonequilibrium laser plasma of metals with the formation of structures

Processes in a plasma formed by the evaporation of material by middle power laser radiation on a metallic surface are analyzed. In the regime considered the laser radiation is not absorbed by a metallic plasma and the radiation energy is spent in the main for the atomic evaporation. As a result of the plasma relaxation, hard metallic particles of sizes of the order of 10 nm are formed. They are joined in fractal aggregates of micron sizes, and fractal aggregates may be joined in aerogel-like structures of centimeters sizes. Such a laserplasma is an example of a plasma whose evolution leads to fractal object formation. A fireball is considered as a fractal object formed as a result of the evolution of an evaporated plasma. The considered object with aerogel-like structure that is formed in a gas has both scientific and practical interest.     

Self-assembled monolayers of alkanoic acids on the native oxide surface of SS316L by solution deposition

Stainless steel 316L is a widely used biomaterial substrate whose biocompatibility could be improved by surface modification. As a first step in this process, self-assembled monolayers of octanoic acid, octadecylcarboxylic acid, 16-hydroxyhexadecanoic acid, 12-aminododecanoic acid, and 1,12-dodecane dicarboxylic acid have been formed on the native oxide surface of stainless steel 316L by a simple, one-step solution deposition method. The ordering, close-packing, and coverage of the monolayers formed were characterized by diffuse reflectance infrared spectroscopy, contact angle measurements, and atomic force microscopy. The same procedure was applicable for all long alkyl chain carboxylic acids. This process formed chemically and mechanically stable monolayers. These carboxylic acids formed a bidentate bond with the stainless steel substrate. Robust chemical attachment of the acids to stainless steel through a simple process provides a stepping stone to improving the biocompatibility of stainless steel 316L.     

Structure and phase equilibria of mixtures of the complex salt hexadecyltrimethylammonium polymethacrylate, water and different oils

This work reports on phase diagrams for mixtures of a complex salt formed by a cationic surfactant and an oppositely charged polyelectrolyte, hexadecyltrimethylammonium polymethacrylate, in binary mixtures with water and in ternary mixtures containing water and organic solvents of different polarity ('oils'): decanol, octanol, p-xylene and cyclohexane. The liquid crystalline structures formed were identified by small angle X-ray scattering measurements, which also provided information about changes in the size of the aggregates as a function of the system composition. These results are analysed in comparison with others previously reported [Bernardes et al., J. Phys. Chem. B 110 (2006) 10332-10340] for the analog complex formed with polyacrylate and, in general, reveal that the presence of an extra methylene group in the polymer chain does not produce significant changes in the complex phase diagrams nor in the structure of the liquid crystalline phases formed. Additionally, the obtained results confirm once more the approach used to analyze these kinds of systems formed by polymer and oppositely charged surfactant.     

Etudes photochimiques—XI: Reaction de photosubstitution de la quinoxaline en milieu neutre

Irradiated in ether, quinoxaline is substituted on C₂ according to scheme 3. Radical VIII, which is formed by a monophotonic process is the intermediate of the reaction. VIII is also formed in methanol and in ethanol. It has been caracterised by ESR by the second moment method. Spin densities of VIII and of similar radicals formed from 3-methyl quinoxaline and 2,3-dimethyl quinoxaline have been calculated and agree with experimental results.     

Adlayer of hydroquinone on Pt(111) in solution and in a vacuum studied by STM and LEED

Hydroquinone (HQ) adlayers were formed on Pt(111) in HF solution and in a vacuum. By using scanning tunneling microscopy (STM) in solution, it was revealed that HQ formed an ordered structure on Pt(111) with a strong attractive interaction between two adjacent hydroxyl groups in neighboring HQ molecules. After the sample was transferred into a vacuum, low-energy electron diffraction (LEED) measurement was performed, which showed that the (2.56 x 2.56)R16 degrees incommensurate structure of the HQ adlayer was formed in solution. The HQ adlayer on Pt(111) was formed also by vapor deposition, and the identical (2.56 x 2.56)R16 degrees adlayer structure was found by LEED and STM in a vacuum.     

The mechanistic puzzle of transition-metal-catalyzed skeletal rearrangements of enynes

Three pathways actually compete in metal-catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo-cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition-metal-catalyzed cyclization of some enynes, particularly, 1,7-enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn-cyclopropyl metal-carbenes formed in the syn pathway.     

Tubular gel fabrication and cell encapsulation in laminar flow stream formed by microfabricated nozzle array

We have used a laminar flow stream formed by a microfabricated nozzle array to prepare cell-encapsulated alginate gel micro-tubes, in which cells formed a cylindrical multi-cellular aggregate after cultivation for two weeks.     

Phase separation during synthesis of polyetherimide/epoxy semi-IPNs

The phase separation behavior and the morphology of polyetherimide (PEI)-modified diglycidyl ether of bisphenol A (DGEBA) epoxy resin were studied using scanning electron microscopy and light scattering. Reaction kinetics, cloud point and onset of gelation were determined by differential scanning calorimeter, optical microscope and physica rheometer, respectively. The mixture of partially cured epoxy and PEI showed bimodal upper critical solution temperature (UCST) behavior. For PEI content smaller than 10 wt%, the blends exhibited a sea-island morphology formed via nucleation and a growth mechanism. Above 25 wt% PEI content, the phase separation proceeded via a spinodal decomposition mechanism and a nodular structure was formed. With PEI content between 15 and 20 wt%, dual phase morphology was observed. This morphology was formed via primary spinodal decomposition and secondary phase separation within the dispersed phases and the matrix phases formed by the primary phase separation. This morphology was presumed to be formed in the reaction-induced phase separation mechanism with the mixture showing bimodal UCST behavior. The curing temperature had an effect on the final morphology, and the modulus of PEI-modified epoxy was influenced by the phase separation.     

Specific features of the growth,composition, and content of natural gas hydrates synthesized in inverted oil emulsions

A study of specific features of the growth, composition, and content of natural gas hydrates formed in a water-in-oil emulsion demonstrated that the process in which hydrates are formed in a water-oil emulsion occurs in stages and depends on the saturation of hydrate growth zones with the hydrate-forming gas via diffusion of natural-gas components across the oil phase. Hydrates enriched in methane are formed in water-oil emulsions, compared with the hydrates grown from distilled water, which is accounted for by the difference in solubility between natural-gas components in oil and water, and also by the presence of a surfactant layer on the surface of emulsified water drops. With increasing fraction of water in an emulsion, the content of hydrates decreases, and the mass of a hydrate being formed is independent of the composition of the water-oil emulsion.     

Influence du pH sur les proprietes chimiques et structurales des films d'oxyde formes sur l'acier 316L et les alliages 600 et 690 dans les milieux aqueux a haute temperature

Influence of pH on the chemical and structural properties of the oxide films formed on 316L stainless steel, alloy 600 and alloy 690 in high temperature aqueous environments. The oxide films formed on 316L stainless steel, alloy 600 and alloy 690 at 320°C in high temperature aqueous environments of different pH have been examined by glow discharge optical spectroscopy, scanning electron microscopy, atomic force microscopy and capacitance measurements. The analytical study reveals that the films formed at pH 5 are mainly composed of chromium oxides. When the pH increases the chromium concentration decreases and those of the other two elements (Ni and Fe) tend to increase. The films formed at pH 5 on 316L stainless steel and alloy 600 are thick and powder-like. The film formed at the same pH on alloy 690 is thin and is composed of a compact protective inner layer and a less-compact outer layer formed by crystals of mixed iron-nickel-chromium oxides. The morphological appearance of the thick films and that of the thin films is very different. However, equivalent morphologies can be observed for the relatively thin duplex films formed at pH8 and pH9.5 on the 316L stainless steel and nickel-base alloys. The evolution of the chemical composition of the films is accompanied by important changes from the point of view of their semiconductivity.     

Single-channel recordings of gramicidin at agarose-supported bilayer lipid membranes formed by the tip-dip and painting methods.

Agarose-supported BLMs were prepared by the tip-dip and painting methods, and single-channel recordings of gramicidin were examined for the development of an ion-channel sensor. The supported BLMs formed by the tip-dip method had an electric resistance of >1.0 x 10(11) omega and a longer lifetime as compared with unsupported ones, which enabled single-channel recordings of gramicidin. The supported BLMs formed by the painting method also enabled single-channel recordings, but the lifetime was shorter than that of unsupported planar BLMs formed by the monolayer folding method.     

Complexation of DNA with cationic surfactants as studied by small-angle X-ray scattering

The phase behaviors of the complex formed by didodecyldimethylammonium bromide(DDAB)and cetyltrimethylammonium bromide(CTAB)interacting with three different types of DNAs,salmon testes DNA(~2000 bp),21-bp double-stranded oligonucleotides(oligo-ds DNA),and 21-nt single-stranded oligonucleotides(oligo-ss DNA)were studied by synchrotron small-angle X-ray scattering.It was found that the DNA length and flexibility,together with the positive/negative charge ratio,determined the final structure.At higher charge ratios,the DNA length exhibited negligible effect.Both oligo-ds DNA and salmon DNA formed inverted hexagonal packing of cylinders with CTAB,as well as bilayered lamella with DDAB.However,at lower charge ratios,oligo-ds DNA formed a distorted hexagonal phase with CTAB and a new structure with DDAB,which was different from the behaviors of salmon DNA.The flexible oligo-ss DNA formed rich structures that were subject to environmental disturbance.Kinetic study also indicated that the structures of the complex formed by oligo-ss DNA took much longer to mature than the structures formed by oligo-ds DNA.We attributed this result to the conformational adjustment of oligo-ss DNA in the complex.