Specific features of synthesis,chemical composition,and thermal stability of anodic borosilicate coatings on silicon

Thickness and resistivity of anodic borosilicate coatings synthesized on n-type silicon were studied in relation to the conditions in which the final forming potentials are reached and to the content of boric acid in an ethylene glycol solution. A method for synthesis of the coatings for use in nanoelectronics was suggested on the basis of data on the thermal stability of their chemical composition.     

Specific detection of acetyl-coenzyme A by reversed-phase ion-pair high-performance liquid chromatography with an immobilized enzyme reactor

A selective chromatographic detection system for the determination of acetyl-coenzyme A (CoA) is reported. The short-chain acyl-CoA thioesters were separated by reversed-phase ion-pair high-performance liquid chromatography (HPLC), and then acetyl-CoA was selectively detected on-line with an immobilized enzyme reactor (IMER) as a post-column reactor. Thio-CoA liberated enzymatically from acetyl-CoA was determined spectrophotometrically after reaction with Ellman's reagent in the reagent stream. The IMER with phosphotransacetylase had a substrate specificity sufficient to determine acetyl-CoA and was active and stable in the mobile phase containing methanol and the ion-pair reagent. The calibration graph was linear between 0.2 and 10 nmol, with a detection limit of 0.05 nmol. This HPLC system with detection by IMER allows the selective identification and determination of acetyl-CoA in a mixture of acetoacetyl-CoA and 3-hydroxy-3-methylglutaryl-CoA, which are difficult to separate with ion-pair HPLC.     

A biomimetic approach to the chemical inactivation of chrysotile fibres by lichen metabolites

Some lichens were recently reported to modify the surface state of asbestos. Here we report some new insight on the physico-chemical modifications induced by natural chelators (lichen metabolites) on two asbestos samples collected in two different locations. A biomimetic approach was followed by reproducing in the laboratory the weathering effect of lichen metabolites. Norstictic, pulvinic and oxalic acid (0.005, 0.5 and 50 mM) were put in contact with chrysotile fibres, either in pure form (A) or intergrown with balangeroite, an iron-rich asbestiform phase (B). Mg and Si removal, measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), reveals an incongruent dissolution for pure chrysotile (A), with Mg removal always exceeding that of Si, while chrysotile-balangeroite (B) follows a congruent dissolution pattern in all cases except in the presence of 50 mM oxalic acid. A much larger removal of Mg than Si in the solutions of 0.5 and 50 mM oxalic acid with chrysotile (A) suggests a structural collapse, which in the case of chrysotile-balangeroite (B) only occurs with 50 mM oxalic acid; in these cases both samples are converted into amorphous silica (as detected by X-ray diffraction (XRD)). Subsequent to incubation, some new phases (Fe(2)O(3), CaMg(CO(3))(2), Ca(C(2)O(4)) x H(2)O and Mg(C(2)O(4))2 x H(2)O), similar to those observed in the field, were detected by XRD and micro-Raman spectroscopy. The leaching effect of lichen metabolites also modifies the Fenton activity, a process widely correlated with asbestos pathogenicity: pure chrysotile (A) activity is reduced by 50 mM oxalic acid, while all lichen metabolites reduce the activity of chrysotile-balangeroite (B). The selective removal of poorly coordinated, highly reactive iron ions, evidenced by NO adsorption, accounts for the loss in Fenton activity. Such fibres were chemically close to the ones observed in the field. Chrysotile-rich rocks, colonised by lichens, could be exposed to a natural bioattenuation and considered as a transient environmental hazard.     

Large stabilization of a DNA duplex by the deoxyadenosine derivatives tethering an aromatic hydrocarbon group

Novel deoxyadenosine derivatives tethering a phenyl or naphthyl group by means of an amido linker have been synthesized, and these derivatives, stacking on the 5' end of a DNA duplex, provide free energy contributions equal to or greater than that of the Watson-Crick A/T base pair.     

A new approach to the synthesis of lactams of muramic,isomuramic and normuramic acids via intramolecular O-alkylation: Stereochemical features of the intramolecular nucleophilic substitution

The title compounds were synthesized from the selectively protected N-acylated d-glucosamine derivatives, containing α-halo carboxylic acid moieties, via intramolecular 3-O-alkylation. It was found that if the starting compound contains asymmetric electrophilic center, isomuramic acid derivatives were mainly formed, regardless of the configuration of the electrophilic carbon atom. An explanation for the observed stereochemical results was proposed on the basis of the analysis of steric interactions in the molecules of the starting compounds, as well as using the concept of anchimeric assistance. It was shown that N-acetylation of the obtained lactam derivatives and subsequent methanolysis under mild conditions led to the selective cleavage of δ-lactam ring resulting in the formation of the corresponding ester derivatives of N-acetylmuramic acid or its analogues in high yields.     

Interplay of intramolecular hydrogen bonds,OH orientations,and symmetry factors in the stabilization of polyhydroxybenzenes

Polyhydroxybenzenes are the parent compounds of large classes of derivatives, many of which exhibit biological activities. The study of derivatives highlights the importance of the conformation stabilizing factors of the parent compounds. To identify these factors, a systematic comparative study of polyhydroxybenzenes was carried out through a computational study of all possible structures and conformers in vacuo and in three solvents differing by their polarities and by the types of interactions with the solute molecule (water, chloroform, and acetonitrile); the results in solution are complemented by the study of adducts with explicit water molecules and, for the simpler structures, also with explicit acetonitrile molecules. The greatest conformation stabilizing effect pertains to intramolecular hydrogen bonds, with preference for consecutive H‐bonds. Uniform orientation of the phenol OH is a stabilizing factor for structures with meta OH groups. Preference for structures with meta OH and with greater symmetry increases as the medium polarity increases. The coexistence of intramolecular H‐bonds and solute–solvent intermolecular H‐bonds in water and acetonitrile solution narrows the solvent‐effect difference between conformers with and without intramolecular H‐bonds. Comparison of results from different calculation methods (HF, MP2, and DFT/B3LYP, with 6‐31G(d,p) and 6‐31++G(d,p) basis sets) shows consistency of the identified trends. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Rheological and thermal behavior of PLA modified by chemical crosslinking in the presence of ethoxylated bisphenol A dimethacrylates

Crosslinking structures can be partly introduced into PLA by melt mixing in a twin screw extruder with dicumyl peroxide (DCP) and ethoxylated bisphenol A dimethacrylates (Bis‐EMAs) as a crosslinking coagent. The study of DCP and Bis‐EMA contents on the melt rheology, thermal properties, dynamic mechanical properties and morphology of the reactive extruded pellets is presented. The results show that PLA with a DCP content higher than 3 phr exhibits increases in both the melt modulus and complex viscosity as compared with PLA. The introduction of DCP into PLA improved the thermal stability of the PLA. PLAs with various Bis‐EMA contents showed the optimum storage modulus and complex viscosity to occur at 5 phr Bis‐EMAs. Moreover, the glass transition, cold crystallization and melting temperature of PLAs decreased with increasing Bis‐EMA content. The crystallinity of the partly crosslinked PLAs was lower than that of PLA. Similar to the rheological results, the thermo‐mechanical properties showed that the storage modulus and loss modulus of the partly crosslinked PLAs increased with increasing Bis‐EMA contents up to 5 phr. In addition, these partly crosslinked PLAs showed rough surface or sea island‐like structure. Copyright © 2016 John Wiley & Sons, Ltd.     

Specific C-H...N intramolecular interaction in 1-vinyl-2-cyanoethylthiobenzimidazole with the formation of a nine-membered intramolecular ring,according to1H and13C NMR data

In the¹H NMR spectrum of 1-vinyl-2-cyanoethylthiobenzimidazole, an abnormal downfield shift has been found in the signal of the -hydrogen atom of the vinyl group, and also an increase in the corresponding direct¹³C-¹H spin-spin coupling constant, in comparison with other 2-substituted benzimidazoles. This indicates a specific C-H...N intramolecular interaction in this molecule between the -hydrogen atom and the cyano nitrogen atom, forming a nine-membered intramolecular ring.Translated from Translated from Izvestiya Akademii Nauk SSSR No. 10, pp. 2285–2288, October, 1991.     

Conformational analysis,tautomeric preference,intramolecular hydrogen bonding,and solvent effect on dinitrosamine: A quantum chemical study

HF, B3LYP, and MP2 methods with the standard basis set, 6‐311++G(d,p), were used to study various aspects of dinitrosamine. These results were compared with the outcomes of G2 and CBS‐QB3 methods. First, the conformational analysis and characterization of equilibrium conformations, especially global minima, were performed. On the basis of relative energies, we found that the dinitroso tautomers are more stable than the nitroso‐hydroxy (NH) ones. This preference is well‐interpreted in terms of tautomerization process and nitrosamine resonance. Furthermore, the nature of O? H···O intramolecular hydrogen bond (IMHB), in chelated forms of NH (NH‐11 and NH‐13) was comprehensively studied to evaluate the effect of hetero atoms (N) on the characteristic of IMHB systems. According to the results of isodesmic reaction method, the hydrogen bond energy of NH‐11 is greater than the malonaldehyde (MA) and NH‐13, whereas the electron density analysis and energy‐geometry correlation methods clearly predict that the hydrogen bond of NH‐11 is weaker than the MA. Additionally, the geometrical, atoms in molecules (AIM) and natural bond orbital's (NBO) parameters also emphasize on the MA as a chelated form with the strongest hydrogen bond. Finally, the solvent effects on the relative stability of selected dinitrosamine conformers are evaluated by different continuum (polarizable‐continuum model, isodensity polarizable continuum model, and self‐consistent isodensity polarizable continuum model), discrete and mixed solvent models. Theoretical results readily show that the potential energy surface of dinitrosamine, especially global minima, is strongly affected by the solvent. © 2012 Wiley Periodicals, Inc.     

Regioselective intramolecular Pauson-Khand reactions of C60: an electrochemical study and theoretical underpinning

Suitably functionalized fulleropyrrolidines endowed with one or two propargyl groups at the C-2 position of the pyrrolidine ring (1,6-enynes) react efficiently and regioselectively with [Co2(CO)8] to afford the respective Pauson-Khand products with an unprecedented three (5 a-d, 7, and 24) or five (25) pentagonal rings, respectively, fused onto the fullerene sphere. Fulleropyrrolidines with 1,7-, 1,9-, 1,10-, or 1,11-enyne moieties do not undergo the PK reaction and, instead, the intermediate dicobalt complexes formed with the alkynyl group are isolated in quantitative yields. These differences in reactivity have been studied by DFT calculations with a generalized gradient approximation (GGA) functional and several important energy and structural differences were found for the intermediates formed by the interaction between the coordinatively unsaturated Co atom and the pi system of C60 in 1,6- and 1,7-enynes. The different lengths of the alkyne chains are responsible for the observed reactivities. Cyclic voltammetry reveals that the presence of the cyclopentenone's carbonyl group connected directly to the C60 core results in PK compounds with remarkable electron-accepting ability.     

13C chemical shifts of symmetrically substituted biphenyls: Unambiguous signal assignment for the carbons ortho and para to an aryl group

The natural abundance ¹³C NMR spectra of 2,2′-dimethyl-, 2,2′-dimethoxy- and 2,2′-dihydroxybiphenyls, and a series of 2,2′-dimethoxy-5,5′-disubstituted biphenyls were recorded. Unambiguous signal assignments of the carbons ortho and para to an aryl ring in biphenyls were made by selective deuteration and/or the graphical method for ¹H single frequency off-resonance decoupled spectra. Contrary to the reported assignments, it was shown that the signal for C-6 in 2,2′-dimethylbiphenyl clearly appears at lower field than that for C-4. The signals for the ortho carbons (C-6) of 2,2′-dimethoxy-5,5′-disubstituted biphenyls generally appeared at lower fields than those for the para carbons (C-4). The validity of applying deuterium isotope shifts to the assignments of ¹³C chemical shifts of di- and tetra-substituted biphenyls is also discussed.     

A Lewis acid initiated intramolecular cyclization of benzylidene acetal with an azide functional group: novel synthesis of oxazolines and oxazines

Stereoselective synthesis of 2-phenyl-oxazolines and 2-phenyl-oxazines via Lewis acid initiated intramolecular cyclization of benzylidene acetal with an azide functional group is described.     

Specific N-terminal protein labelling: use of FMDV 3C pro protease and native chemical ligation

We report an effective strategy for generating N-terminal cysteinyl proteins by proteolytic cleavage using the enzyme 3C pro, suitable for a wide range of applications via native chemical ligation.     

Synthesis of the aziridinomitosene skeleton by intramolecular Michael addition of alpha-lithioaziridines: an aromatic route featuring deuterium as a removable blocking group

A convergent synthetic route to the 1,2-aziridinopyrrolo(1,2-a)indole 34 has been developed. Key features of this route include the deuterium kinetic isotope effect to block undesired indole lithiation during tin-lithium exchange from 27a to 30a, the intramolecular Michael addition to generate the enolate 31a, and conversion into 34 by trapping with phenylselenenyl chloride. Reductive cleavage of the N-trityl group in 34 allows access to tetracyclic aziridinomitosenes containing the aziridine N-H subunit. Reduction of the C(9) ester in 34 with LAH gives the primary alcohol 35 with the correct C(9), C(9a), C(10) oxidation state corresponding to the aziridinomitosenes, and deprotection of 34 affords 37.     

Asymmetric synthesis of a (2Z,7E)-cyclononadiene by an intramolecular cycloalkylation and insight to its conformational properties

Here, we report a novel synthesis of a monosubstituted, enantio- and diastereomerically enriched trans-cyclononadienol. The reaction consists of an enantioselective (-)-sparteine-mediated allylic lithiation of an achiral 7-chlorononadienyl carbamate and a subsequent stereospecific intramolecular allyllithium-allyl chloride coupling. The stereochemical course of the cyclization has been determined, and the high configurative stability of the chiral nine-membered carbocycle has been investigated by kinetic measurements and rationalized by computational calculations.     

Sulfinyl group as an intramolecular nucleophile: synthesis of bromohydrins from beta-methyl-gamma,delta-unsaturated sulfoxides with high 1,2-asymmetric induction

Bromohydrins have been prepared from beta-methyl-gamma,delta-unsaturated sulfoxides with high regio- and stereoselectivity. The reaction proceeds via neighboring group participation of the sulfinyl moiety with inversion of sulfoxide configuration as proven by an (18)O labeling study and X-ray crystallography.