Oxygen Attack Research Articles

Formation and selective rupture of 1,4-anhydroseptanoses

Formation 1,4-anhydroseptanose sugars via intramolecular attack of a benzyl ether oxygen onto an anomeric electrophile is reported. The regioselectivity of the cyclization was confirmed by NMR spectroscopy and an X-ray structure of a glucose-derived 1,4-anhydroseptanose. The transformation is relatively general and was applied to septanoses derived from glucose, mannose, xylose, and galactose. Regioselective opening of the 1,4-anhydro species to give septanose glycosides in preference to furanoses is also demonstrated.

Resistance of POSS Polyimide Blends to Hyperthermal Atomic Oxygen Attack.

Copolymers of polyhedral oligomeric silsesquioxane (POSS) and polyimide (PI) have shown remarkable resistance to atomic oxygen (AO) attack and have been proposed as replacements for Kapton on the external surfaces of spacecraft in the harsh oxidizing environment of low Earth orbit (LEO). POSS PI blends would be an economical alternative to the copolymers if they also resisted AO attack. Thus, blends of trisilanolphenyl (TSP) POSS and PI with different weight percentages of the Si7O9 POSS cage were cast into films and exposed to a hyperthermal AO beam, and they were characterized in terms of their recession, mass loss, surface morphology, and surface chemistry. In order to compare the AO resistance of the blends with POSS PI copolymers, samples of previously studied copolymers were also investigated in parallel with the blends. For all POSS PI materials, the AO resistance increased with increasing AO fluence and with increasing POSS cage loading. At similar POSS cage loadings and exposure conditions, the TSP POSS PI blends showed comparable erosion yields to the POSS PI copolymers, with specific samples of blends and copolymers achieving erosion yields as low as 0.066 × 10-24 cm3 atom-1 with an AO fluence of 5.93 × 1020 O atoms cm-2. SEM and XPS analyses indicated that passivating SiOx layers were formed on the surfaces of all POSS-containing polymers during AO exposure. Thus, a TSP POSS PI blend is proposed as a low-cost variant of a POSS polyimide for use in extreme oxidizing environments, such as LEO.

Comparison of Isothermal with Cyclic Oxidation Behavior of “Cr-Aluminide” Coating on Inconel 738LC at 900 °C

A two-step pack cementation technique was used to cover nickel-based IN-738LC superalloy with a Cr-aluminide coating layer. The IN-738LC/Cr-aluminide samples were oxidized at 900 °C for 350 and 700 h, isothermally, and 175 and 350 h, cyclically. Each cycle lasted for 25 h. Effects of the isothermal and the cyclic oxidation on (a) morphology, (b) new-phase formation, (c) weight gain, and (d) oxidation reactions were investigated by scanning electron microscopy, electron dispersive spectroscopy, X-ray diffraction, and weight gain measurements. From results, it was concluded that the oxygen attack was more severe during the cyclic oxidation than the isothermal procedure. Kinetics of the reactions indicated a parabolic rate law for the isothermal process, while the cyclic oxidation firstly followed a similar parabolic rate law which was trailed later on by a severe surface spallation.

Mechanism of the Glutathione Persulfide Oxidation Process Catalyzed by Ethylmalonic Encephalopathy Protein 1

Ethylmalonic encephalopathy protein 1 (ETHE1) is a β-lactamase fold-containing protein, which is related to the increased cellular levels of hydrogen sulfide. ETHE1 is essential for the survival of a range of organisms and catalyzes the oxidation of glutathione persulfide (GSSH). Currently, the catalytic mechanism of ETHE1 still remains unclear, despite a catalytic cycle that has been suggested from the crystal structure and a proposal for the mechanistically related cysteine dioxygenase (CDO). In this Article, we performed a series of quantum mechanical/molecular mechanical (QM/MM) calculations on the substrate GSSH oxidation by human ETHE1. Our calculation results reveal that the ground state of the iron(II)-superoxo reactant is quintet, which can be described as GSS+•–Fe(II)–O2•, and the most feasible reaction channel was found to start from the cleavage of dioxygen and a concerted attack of distal oxygen on the sulfur atom of the substrate, forming the metal-bound activated oxygen and a sulfite interm...

The initial oxidation behaviors of uranium nitride UNx (x = 0, 0.23, 0.68, 1.66) films

The initial oxidation behaviors of uranium nitride films with different N/U ratios have been focused in the present work. Uranium nitride films with different nitrogen content, UNx (x = 0, 0.23, 0.68, 1.66), have been prepared on the Si substrate by radio frequency magnetron sputtering method. The experimental results showed that the UNx (x = 0, 0.23, 0.68, 1.66) films were fine and dense. The initial oxidation processes of uranium nitride films were investigated in an ultra-high vacuum chamber of Auger electron spectroscopy. After 105 L oxygen exposure, the oxide layer of UO2 were formed on the surface of U, and UN0.23, UN0.68 films formed UO2 with little UNxOy, while the UN1.66 film generated UNxOy oxide layer. UN1.66 film exhibited the thinnest oxide layer and provided the best considerable protection against oxygen attack. Also, the AES transition lines of UNxOy were identified for the first time.

Singlet Oxygen Attack on Guanine: Reactivity and Structural Signature within the B‐DNA Helix

Oxidatively generated DNA lesions are numerous and versatile, and have been the subject of intensive research since the discovery of 8-oxoguanine in 1984. Even for this prototypical lesion, the precise mechanism of formation remains elusive due to the inherent difficulties in characterizing high-energy intermediates. We have probed the stability of the guanine endoperoxide in B-DNA as a key intermediate and determined a unique activation free energy of around 6 kcal mol(-1) for the formation of the first C-O covalent bond upon the attack of singlet molecular oxygen ((1) O2 ) on the central guanine of a solvated 13 base-pair poly(dG-dC), described by means of quantum mechanics/molecular mechanics (QM/MM) simulations. The B-helix remains stable upon oxidation in spite of the bulky character of the guanine endoperoxide. Our modeling study has revealed the nature of the versatile (1) O2 attack in terms of free energy and shows a sensitivity to electrostatics and solvation as it involves a charge-separated intermediate.

Theoretical study on photooxidation mechanism of ruthenium complex [Ru(II)-(bpy)2 (TMBiimH2 )](2+) with molecular oxygen.

Photoinduced reactions of ruthenium complexes with molecular oxygen have attracted a lot of experimental attention; however, the reaction mechanism remains elusive. In this work, we have used the density functional theory method to scrutinize the visible-light induced photooxidation mechanism of the ruthenium complex [Ru(II)-(bpy)2 (TMBiimH2 )](2+) (bpy: 2, 2-bipyridine and TMBiimH2 : 4, 5, 4, 5-tetramethyl-2, 2-biimidazole) initiated by the attack of molecular oxygen. The present computational results not only explain very well recent experiments, also provide new mechanistic insights. We found that: (1) the triplet energy transfer process between the triplet molecular oxygen and the metal-ligand charge transfer triplet state of the ruthenium complex, which leads to singlet molecular oxygen, is thermodynamically favorable; (2) the singlet oxygen addition process to the S0 ruthenium complex is facile in energy; (3) the chemical transformation from endoperoxide to epidioxetane intermediates can be either two- or one-step reaction (the latter is energetically favored). These findings contribute important mechanistic information to photooxidation reactions of ruthenium complexes with molecular oxygen. © 2016 Wiley Periodicals, Inc.

Reactive force field simulation studies on the combustion behavior of n-octanol

n-Octanol is a new promising fuel which is considered as an alternative to conventional diesel. In order to understand the combustion characteristics of n-octanol, reactive molecular dynamics (ReaxFF MD) simulation of n-octanol has been carried out at temperatures from 2000 to 4000K with equivalence ratios ranging from 0.5 to 2.0. Ethylene and formaldehyde have been found to be major intermediates in the oxidation of n-octanol which is in agreement with the available experimental data. Reaction mechanisms have been proposed for the formation of major intermediates (C2H4 and CH2O) and final products (CO and CO2) from where it is revealed that intermediates are mainly consumed by the build-up of hydroxyl and hydroperoxyl radicals. It was observed that the CO and CO2 formation from formaldehyde and ethylene starts with the attack of molecular oxygen or hydroxyl radical or hemolytic OH bond cleavage. Overall, a good qualitative agreement with the available experimental data in terms of product distribution has been found. The results indicate that ReaxFF MD simulations can give a detailed description of the reaction mechanism and product distribution for the combustion for oxygenated fuel such as octanol.

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism.

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of β-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a (syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. Mechanistic studies by both experiments and density functional theory (DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism—nucleophilic attack of enolate oxygen on Palladium followed by C–C bond-forming [3,3']-reductive elimination.

The mechanism research on the reaction HCNO + HO2: a theoretical investigation

The mechanism of reaction HCNO + HO2 is investigated by means of CCSD(T)/6-311 + G(d,p)//B3LYP/6-311 + G(d,p) method, in which Pi with the i = 1, 2, 3, …, 7 are involved, to determine a more reasonable pathway. Among four possible entrance patterns, the attack of oxygen to carbon is the most energetically feasible leading to complex a1. Starting from the a1, the rupture of O2–O3 bond to form P3 (HC(O)NO + OH) is the most favorable pathway. Alternatively, the a1 undergoes N–O1 bond rotation to form the isomer a2. A concerted O2–O3 bond and C–N bond cleavage leading to P4 (OH + HCO + NO) is the secondary feasible pathway. Other products are not energetically accessible because of higher barrier-consumed and/or complicated processes. We hope that the present work would be helpful to experimentally identify the product distributions and deeply understand the mechanism of the title reaction.

Mechanical Properties, Surface Structure, and Morphology of Carbon Fibers Pre-heated for Liquid Aluminum Infiltration

To efficiently produce carbon fiber-reinforced aluminum on a large scale, we developed a special high-pressure die casting process. Pre-heating of the fibers is crucial for successful infiltration. In this paper, the influence of heating carried out in industrial conditions on the mechanical properties of the fibers was investigated. Therefore, polyacrylonitrile-based high-tensile carbon fiber textiles were heated by infrared emitters in an argon-rich atmosphere to temperatures between 450 and 1400 °C. Single fiber tensile tests revealed a decrease in tensile strength and strain at fracture. Young’s modulus was not affected. Scanning electron microscopy identified cavities on the fiber surface as the reason for the decrease in mechanical properties. They were caused by the attack of atmospheric oxygen. The atomic structure of the fibers did not change at any temperature, as x-ray diffraction confirmed. Based on these data, the pre-heating for the casting process can be optimized.

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting

Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.

Role of Water in the Puzzling Mechanism of the Final Aromatization Step Promoted by the Human Aromatase Enzyme. Insights from QM/MM MD Simulations.

The enzyme human aromatase (HA) catalyzes the conversion of androgens to estrogens via two hydroxylation reactions and a final unique aromatization step. Despite the great interest of HA as a drug target against breast cancer detailed structural and spectroscopic information on this enzyme became available only in the past few years. As such, the enigmatic mechanism of the final aromatization step is still a matter of debate. Here, we investigated the final step of the HA enzymatic cycle via hybrid quantum-classical (QM/MM) metadynamics and blue-moon ensemble simulations. Our results show that the rate-determining step of the aromatization process is the nucleophilic attack of the distal oxygen of a peroxo-ferric species on the formyl carbon of the enol-19-oxo-androstenedione, which occurs with a free energy barrier (ΔF(#)) of ∼ 16.7 ± 1.9 kcal/mol, in good agreement with experimental data. This reaction is followed by a water mediated 1β-hydrogen abstraction (ΔF(#) = 7.9 ± 0.8 kcal/mol) and by the formation of a hydroxo-ferric moiety. This latter may be finally protonated by a hydrogen delivery channel involving Asp309 and Thr310, both residues pointed out as crucial for HA activity. In the absence of the catalytic water in the active site the substrate does not assume a position suitable to undergo the nucleophilic attack. Our data not only reveal a novel possible mechanism for the aromatization process consistent with some of the spectroscopic and kinetic data available in the literature, complementing current knowledge on the mechanism of this enzyme, but also point out a remarkable influence of the level of theory used on the calculated free energy barriers. The structural information obtained in this study may be used for the rational structure-based drug design of HA inhibitors to be employed in breast cancer therapy.

Novel degradation mechanism for triarylmethane dyes: Acceleration of degradation speed by the attack of active oxygen to halogen groups

We studied triarylmethane dyes using LC/MS and UV/VIS spectrometry and demonstrated that they undergo several degradation mechanisms depending on their chemical structures, thus leading to specific degradation rates and colour fading. In particular, we found that Basic Blue 1 (BB1) shows significantly fast decolourization. We suggest a new specific degradation mechanism for BB1 that arises from photogenerated ·OH radicals from oxygen in air.

저궤도 우주환경하의 실란처리된 그래핀 첨가 나노 복합재료의 물성특성

이 논문은 저궤도 우주환경하에서 그래핀 충진 고분자 나노복합재료에 대한 물성 변화를 고찰하였다. 고분자 기지재와의 적합성과 원자산소 공격에 대한 탄소재료의 저항성을 향상시키기 위해 유기실란으로 그래핀 옥사이드의 표면을 실란처리를 수행하였다. 실란이 그래핀 옥사이드 표면에 결합여부를 XPS 장비를 사용하여 분석하였다. 실란 처리된 그래핀 옥사이드와 실란 처리하지 않은 그래핀 옥사이드를 강화재로 사용한 나노복합재료를 제작하여 순수 에폭시와 인장물성을 비교하였으며, 모사된 저궤도 우주환경에서의 물성 변화를 관찰하였다. 그 결과 실란처리를 하여 그래핀 나노 복합재료를 제작할 경우 우주환경요소 중 원자 산소에 저항성을 가지는 것을 확인하였으며, 실란처리를 하지 않은 경우에는 오히려 성능이 감소함을 확인할 수 있었다. This study investigates the property of graphene filled polymer nanocomposites in LEO(Low Earth orbit) environment conditions. In order to improve compatibility with polymer matrices and resistance of carbon material against AO(Atomic oxygen) attack, silanization of graphene oxide with organosilane was carried out. The corresponding moieties were characterized through X-ray photoelectron spectroscopy (XPS). Graphene oxide filled nanocomposites were prepared using solution based processing methods. The sets of specimen series were tested in an accelerated LEO simulated space environment facility. Graphene oxide and silane treated graphene oxide reinforced nanocomposites were compared with neat epoxy. The comparison revealed that the silane treated graphene filled polymer composite shows inherent resistance against atomic oxygen attack while the lack of silane treatment resulted in a reduction in performance.

Mechanism Switch in Mannich‐Type Reactions: ELF and NCI Topological Analyses of the Reaction between Nitrones and Lithium Enolates

AbstractThe mechanism of the addition of lithium enolates derived from esters, ketones and aldehydes to nitrones (Mannich‐type reaction) has been studied using DFT methods. While the reactions with α‐methoxy and α‐methyl enolates takes place through a stepwise mechanism, consisting of an initial nucleophilic attack of the enolate to the nitrone carbon followed by a second nucleophilic attack of the nitrone oxygen to the formed carbonyl group, the reaction with α‐unsusbtituted enolates takes place through a one‐step mechanism. The IRC analysis shows the presence of a hidden intermediate in agreement with one kinetic step two stages process. The topological analysis of the electronic localization function (ELF) confirms that only when the first C–C bond is formed, does the C–O bond formation begin. The NCI analyses, are also in agreement with the formation of intermediates for α‐methoxy and α‐methyl enolates and a highly asynchronous one‐step process in the case of α‐unsusbtituted enolates.

Functionalization of a Pyridine Framework through Intramolecular Reissert–Henze Reaction of N‐(Carbamoyloxy)pyridinium Salts and Unexpected Insertion of Ethereal Solvents

AbstractA new strategy for synthesis of the 2‐pyridyl carbamates is reported. This strategy exploits a new intramolecular transformation of an acyloxy group from N‐carbamoyloxypyridinium salts (Reissert–Henze‐type reaction) as the key step. Addition of a silver salt effectively accelerates the intramolecular attack of the carbonyl oxygen at the 2‐position of the pyridine ring. Additionally, a new rearrangement of the acyloxy group, combined with insertion of an ethereal solvent to give pyridine derivatives having a coordinating tether, is reported.

Effects of atomic oxygen exposure on the tribological performance of ZrO2-reinforced polyimide nanocomposites for low earth orbit space applications

The effects of atomic oxygen exposure on pure polyimide and nano-ZrO2 reinforced polyimide composites were investigated in a ground-based simulation facility. The experimental results indicated that the surface structure of both pure polyimide and ZrO2/polyimide composites were destroyed by atomic oxygen attack, but the addition of nano-ZrO2 particles in polyimide could obviously decrease the mass loss, which showed that ZrO2 could enhance the atomic oxygen resistance. The results of ZrO2/polyimide composites before and after atomic oxygen exposure showed that atomic oxygen irradiation aggravated the friction and wear of the ZrO2/polyimide composites. The wear mechanism was mainly abrasive particles wear arising from the ZrO2-rich layer on the surface of composites. The ZrO2/polyimide composites with 1 wt% nano-ZrO2 owns the lowest varying rate of the friction coefficient and wear rate before and after atomic oxygen exposure, which showed stable friction and wear properties and was expected to become a kind of potential tribological materials for practical spacecraft designation.

A QM/MM study of the initial steps of catalytic mechanism of nitrile hydratase

Four possible reaction paths of an Fe-containing nitrile hydratase (NHase) are examined using the method of hybrid quantum mechanics/molecular mechanics (QM/MM) to reveal the initial steps of the catalytic mechanism of NHases. The reaction barrier for a nucleophilic attack of the sulfenic oxygen of αCys114-SO− to the substrate is the lowest among the four paths, though another path, in which a water molecule activated by αCys114-SO− attacks the substrate, also has a low enough barrier. Under the protein environment, βArg56 has weaker proton donor ability and shows no proton transfer to αCys114-SO−, which was reported in model clusters.

Changes in the composition, structure and friction property of sputtered MoS2 films by LEO environment exposure

Radio frequency-sputtered MoS2 films had been exposed for 43.5h in real low earth orbit (LEO) space environment by a space environment exposure device (SEED) aboard China Shenzhou-7 manned spaceship. The composition, morphology, phase structure and friction property of the exposed films were investigated using X-ray photoelectron spectroscope (XPS), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray energy-dispersive spectroscopy (EDS) and ball-on-disk tribometer. XRD and EDS results revealed that the as-deposited MoS2 films were characterized by a MoSxOy phase structure, in which x and y values were determined to be ∼0.65 and 1.24, respectively. XPS analysis revealed that due to space atomic oxygen attack, the film surface was oxidized to MoO3 and MoSxOy with higher O concentration, while the partial S was lost. However, the affected depth was restricted within the surface layer because of protective function of the oxidation layer. As a result, the friction coefficient only exhibited a slight increase at initial stage of sliding friction.