Attack Of Atom Research Articles

Reductive amination of tertiary anilines and aldehydes

An unprecedented oxidant-mediated reductive amination of tertiary anilines and aldehydes without external reducing agents was developed via the nucleophilic attack of the oxygen atom of the carbonyl group to in situ generated iminium ions, in which tertiary anilines were used as both nitrogen source and reducing agent for the first time.

Ab initio study of unzipping processes in carbon and boron nitride nanotubes under atomic oxygen impact

We apply first principles calculations to compare the carbon and boron nitride nanotube unzipping under atomic oxygen impact. We show that the attack of several oxygen atoms can cause bond breaking in nanotubes, but the structure of boron nitride nanotubes is less damaged than the structure of carbon ones. With increasing diameter, the structural damage of nanotubes reduces.

Spectral Evidence for the One-Step Three-Electron Oxidation of Phenylsufinylacetic Acid and Oxalic Acid by Cr(VI)

The co-oxidation of a mixture of phenylsulfinylacetic acid (PSAA) and oxalic acid (OxH2) by Cr(VI) in 20% acetonitrile-80% water (v/v) medium follows third order kinetics, first order, each with respect to PSAA, OxH2 and Cr(VI). The reaction involves nucleophilic attack of sulfur atom of PSAA on chromium of the oxidizing species, Cr(VI)-OxH2 to form a ternary complex, Cr(VI)-OxH2-PSAA followed by a one-step three-electron reduction of Cr(VI) to Cr(III) and simultaneous oxidation of both the substrates. The reaction is catalysed by Mn2+ ion while retarded by Al3+ ion. Electron releasing substituents in the meta- and para-positions of the phenyl ring of PSAA enhance the rate of co-oxidation while electron withdrawing substituents retards the reaction. The Hammett plots at different temperatures exhibit excellent correlation with negative ρ values. The reaction series obey isokinetic relationship and the observed isokinetic temperature is lying below the experimental range of temperature.

Images of Survival, Stories of Destruction: Nuclear War on British Screens from 1945 to the Early 1960s

This article discusses a range of depictions and discussions of nuclear war, which appeared on British screens in the first half of the Cold War, in order to understand the changing way nuclear weapons were viewed within British culture. Using such screened images to understand how nuclear war was constructed and represented within British culture, the article argues that the hydrogen bomb, not the atomic bomb, was the true harbinger of the nuclear revolution that transformed cultural understandings of warfare and destruction. Although the atomic bomb created a great deal of anxiety within British popular culture, representations of atomic attack elided atomic destruction with that experienced in 1939–45, emphasising the ‘survivability’ of atomic war. In the thermonuclear era, the Second World War could not undertake the same symbolic work. The image of the city-destroying bomb was an imaginative as well as technological step-change. Screened representations stressed that a thermonuclear war would literally end the world. As such, they preceded, and indeed provided the cultural climate for, the rise of the Campaign for Nuclear Disarmament (CND). The Campaign exploited and further popularised this idea of the apocalyptic nuclear war as a key aspect of its political and moral standpoint. The article concludes, however, that the cultural hegemony of this vision of nuclear war equally helped underpin notions of nuclear deterrence. The basic assumptions about the nature of nuclear war constructed and circulated on British screens therefore formed part of CND's ‘cultural’ victory but the article also explains why this did not translate into the political realm.

Identification of C4H5, C4H4, C3H3 and CH3 radicals produced from the reaction of atomic carbon with propene: Implications for the atmospheres of Titan and giant planets and for the interstellar medium

We observed the products C4H5, C4H4, C3H3 and CH3 of the C(3P)+C3H6 reaction using product time-of-flight spectroscopy and selective photoionization. The identified species arise from the product channels C4H5+H, C4H4+2H and C3H3+CH3. Product isomers were identified via measurements of photoionization spectra and calculations of adiabatic ionization energy. Product C4H5 probably involves three isomers HCCCHCH3, H2CCCCH3 and H2CCCHCH2. In contrast, products C4H4 and C3H3 involve exclusively HCCCHCH2 and H2CCCH, respectively. Reaction mechanisms are unraveled with crossed-beam experiments and quantum-chemical calculations. The 3P carbon atom attacks the π orbital of propene (C3H6) to form a cyclic complex c-H2C(C)CHCH3 that rapidly opens the ring to form H2CCCHCH3 followed by decomposition to HCCCHCH3/H2CCCCH3/H2CCCHCH2+H and H2CCCH+CH3; the corresponding branching ratios are 7:5:10:78 predicted with RRKM calculations at collision energy 4kcalmol−1. Nascent C4H5 with enough internal energy further decomposes to HCCCHCH2+H. Ratios of products C4H5, C4H4 and C3H3 are experimentally evaluated to be 17:8:75. This work provides a comprehensive look at product channels of the title reaction and gives implications for the formation of hydrocarbons in extra-terrestrial environments such as Titan and carbon-rich interstellar media. We suggest that the title reaction, hitherto excluded in any chemical networks, needs to be taken into account at least in the atmosphere of Titan and carbon-rich molecular clouds where rapid neutral–neutral reactions are dominant and carbon atoms and propene are abundant.

Thermal decomposition of CH3CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy

A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.

Illuminating the Mechanism of the Borane‐Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane

The reduction of C=O groups with silanes catalyzed by electron-deficient boranes follows a counterintuitive mechanism in which the Si-H bond is activated by the boron Lewis acid prior to nucleophilic attack of the carbonyl oxygen atom at the silicon atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a silicon-stereogenic silane, but applying the same technique to the related reduction of C=N groups was inconclusive due to racemization of the silicon atom. The present investigation now proves by the deliberate combination of our axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity-determining hydride-transfer step. These experiments became possible after the discovery that our axially chiral C(6)F(5)-substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.

Gold-catalyzed direct cycloketalization of acetonide-tethered alkynes in the presence of water

A methodology for the direct preparation of bridged acetals from acetonide-tethered alkynes under gold catalysis in the presence of water has been developed. The bicyclic ring structures bearing a bridged five-membered ring arise from the regioselective bis-oxycyclization by initial attack of the oxygen atom to the internal alkyne carbon.

Dimethylbut-2-ynedioate mediated esterification of acids via sp3 C–N bond cleavage of benzylic tertiary amines

A straightforward synthetic transformation of tertiary amines to the esterification of equimolar amounts of acids was demonstrated in this manuscript. The present protocol can be regarded as a straightforward synthetic transformation of tertiary amines to the esterification of equimolar amounts of acid. Moreover, the reaction is fundamentally new chemistry for the sp3 C–N bond cleavage coupled with esterification. The esterification reaction conditions are very mild and functional group-tolerated, such as hydrosilanes, alcohols, phenol, and amino acids. A mechanism is proposed in which the tertiary amine reacts with dimethyl but-2-ynedioate to form zwitterionic salt, and then the zwitterionic intermediate subsequently accept hydrogen to assist the nucleophilic attack of oxygen atom of carboxyl group at the benzyl group of tertiary amine, while a separate reaction pathway leads to esterification.

Synthesis of Enantiopure Tertiary Skipped Diynes via One-Pot Desymmetrizing TMS-Cleavage

Enantiopure tetrasubstituted skipped diynes were readily synthesized from N-protected amino esters upon addition of lithium TMS-acetylide which was found to be desymmetrizing through one-pot selective TMS-cleavage. The deprotection of the TMS group was realized through a one-pot silicon atom attack by the liberated methoxide, which was diastereoselective due to a conformational favorable chelate.

Kinetics of H atom attack on unsaturated hydrocarbons using spectral uncertainty propagation and minimization techniques

Unsaturated hydrocarbons are an important component of hydrocarbon fuels and intermediates in their oxidation. Under rich conditions, H atom attack is one of the principal pathways of the decomposition of these unsaturated compounds. Consequently, it is critical to understand the H atom attack mechanisms as part of chemical model development. Previous studies have examined the kinetics of H atom attack on various unsaturated hydrocarbons in single pulse shock-tubes. These studies have noted that there are multiple pathways by which H atom attack can proceed, so it is straightforward to measure relative rates but absolute rates are more difficult to estimate. In addition, there is a confounding influence from secondary chemistry. A multiparameter optimization and uncertainty minimization technique is used to constrain a chemical model for the oxidation of H2/CO/C1–C4 hydrocarbons against a range of measurements of the H atom attack process on toluene, trimethylbenzene (TMB), propyne, and propene. The recommended rate constant expressions, with 2σ uncertainties, are as follows:k(CH4+H↔CH3+H2)=108.81±0.21s-1cm3mol-1K1.6T1.6exp(-5400±500K/T)k(C2H2+CH3↔p-C3H4+H)=1011.17±0.18s-1cm3mol-1K0.6T0.6exp(-7200±400K/T)k(p-C3H4+H↔a-C3H4+H)=1018.20±0.21s-1cm3mol-1KT-1exp(-6100±500K/T)k(C3H6+H↔C2H4+CH3)=1024.80±0.20s-1cm3mol-1K3T-3exp(-7400±700K/T)k(C3H6+H↔a-C3H5+H2)=105.14±0.19s-1cm3mol-1K-2.5T2.5exp(-1300±200K/T)k(C6H5CH3+H↔C6H6+CH3)=106.23±0.19s-1cm3mol-1K-2.2T2.2exp(-2000±400K/T)k(C6H5CH3+H↔C6H5CH2+H2)=1014.17±0.25s-1cm3mol-1exp(-4500±500K/T)k(TMB+H↔m-xylene+CH3)=1014.07±0.18s-1cm3mol-1exp(-3900±400K/T)k(TMB+H↔(CH3)2)=1014.52±0.25s-1cm3mol-1exp(-4300±600K/T)In addition, we quantify the effect of secondary chemistry on these rate estimates and the contribution to their uncertainty. Furthermore, we demonstrate how the detailed measurements constrain the model’s predictions of global properties such as ignition delay time in propene oxidation.

Formation of (Alkenylphosphonio)phenylruthenium Complexes from Diphenylacetylene and a [CpRu(dppm)] Cation: Experimental Evidence for the Equilibrium between η1-Disubstituted Vinylidene and η2-Internal Alkyne

The reaction of diphenylacetylene at a cationic ruthenium complex with a dppm (Ph2PCH2PPh2) ligand, [CpRu(dppm)]+, has been studied to reveal essentially for the first time the existence of an equilibrium between an η2-internal alkyne and η1-disubstituted vinylidene at a transition metal center. The reaction mixture at 70 °C for 69 h unexpectedly afforded a coupling product of diphenylacetylene and the dppm ligand, an (alkenylphosphonio)phenyl complex [CpRu{Ph2PCH2P(C6H4)Ph(η2-C(Ph)═CHPh)}]+ with the extremely rare coordination mode of κ1P,η1C,η2C,C′. The (alkenylphosphonio)phenyl complex further undergoes inversion of the coordination face of the alkene moiety at p-xylene reflux temperature. Both isomers of (alkenylphosphonio)phenyl complexes as well as the intermediary η2-internal alkyne and η1-disubstituted vinylidene complexes were fully characterized spectroscopically and crystallographically. By considering the structure of the (alkenylphosphonio)phenyl complexes obtained, the coupling reaction has been proposed to involve the attack of a phosphorus atom on the coordinated diphenylacetylene, which is in equilibrium with an η1-diphenylvinylidene ligand, and the C–H bond activation of a phenyl group on the cationic phosphorus atom leading to the products. Theoretical calculations support the proposed mechanism.

Mechanism of the activation step of the aminoacylation reaction: a significant difference between class I and class II synthetases

In the present work we report, for the first time, a novel difference in the molecular mechanism of the activation step of aminoacylation reaction between the class I and class II aminoacyl tRNA synthetases (aaRSs). The observed difference is in the mode of nucleophilic attack by the oxygen atom of the carboxylic group of the substrate amino acid (AA) to the αP atom of adenosine triphosphate (ATP). The syn oxygen atom of the carboxylic group attacks the α-phosphorous atom (αP) of ATP in all class I aaRSs (except TrpRS) investigated, while the anti oxygen atom attacks in the case of class II aaRSs. The class I aaRSs investigated are GluRS, GlnRS, TyrRS, TrpRS, LeuRS, ValRS, IleRS, CysRS, and MetRS and class II aaRSs investigated are HisRS, LysRS, ProRS, AspRS, AsnRS, AlaRS, GlyRS, PheRS, and ThrRS. The variation of the electron density at bond critical points as a function of the conformation of the attacking oxygen atom measured by the dihedral angle ψ (Cα–C′) conclusively proves this. The result shows that the strength of the interaction of syn oxygen and αP is stronger than the interaction with the anti oxygen for class I aaRSs. This indicates that the syn oxygen is the most probable candidate for the nucleophilic attack in class I aaRSs. The result is further supported by the computation of the variation of the nonbonded interaction energies between αP atom and anti oxygen as well as syn oxygen in class I and II aaRSs, respectively. The difference in mechanism is explained based on the analysis of the electrostatic potential of the AA and ATP which shows that the relative arrangement of the ATP with respect to the AA is opposite in class I and class II aaRSs, which is correlated with the organization of the active site in respective aaRSs. A comparative study of the reaction mechanisms of the activation step in a class I aaRS (Glutaminyl tRNA synthetase) and in a class II aaRS (Histidyl tRNA synthetase) is carried out by the transition state analysis. The atoms in molecule analysis of the interaction between active site residues or ions and substrates are carried out in the reactant state and the transition state. The result shows that the observed novel difference in the mechanism is correlated with the organizations of the active sites of the respective aaRSs. The result has implication in understanding the experimentally observed different modes of tRNA binding in the two classes of aaRSs.

Coupled Cluster and Density Functional Theory Calculations of Atomic Hydrogen Chemisorption on Pyrene and Coronene as Model Systems for Graphene Hydrogenation

Ab initio coupled cluster and density functional theory studies of atomic hydrogen addition to the central region of pyrene and coronene as molecular models for graphene hydrogenation were performed. Fully relaxed potential energy curves (PECs) were computed at the spin-unrestricted B3LYP/cc-pVDZ level of theory for the atomic hydrogen attack of a center carbon atom (site A), the midpoint of a neighboring carbon bond (site B), and the center of a central hexagon (site C). Using the B3LYP/cc-pVDZ PEC geometries, we evaluated energies at the PBE density functional, as well as ab initio restricted open-shell ROMP2, ROCCSD, and ROCCSD(T) levels of theory, employing cc-pVDZ and cc-pVTZ basis sets, and performed a G2MS extrapolation to the ROCCSD(T)/cc-pVTZ level of theory. In agreement with earlier studies, we find that only site A attack leads to chemisorption. The G2MS entrance channel barrier heights, binding energies, and PEC profiles are found to agree well with a recent ab initio multireference wave function theory study (Bonfanti et al. J. Chem. Phys.2011, 135, 164701), indicating that single-reference open-shell methods including B3LYP are sufficient for the theoretical treatment of the interaction of graphene with a single hydrogen atom.

Rhenium-Catalyzed Regio- and Stereoselective Addition of Imines to Terminal Alkynes Leading to N-Alkylideneallylamines

The reaction of terminal alkynes with imines using ReBr(CO)(5) as a catalyst results in the production of N-alkylideneallylamines and not the conventional propargylamines. The substituent on the imine nitrogen is important, and a diphenylmethyl group gave the best result. The catalytic cycle of this regioselective C-C bond forming reaction appears to involve the formation of an alkynyl rhenium species and subsequent nucleophilic attack of the alkynyl β-carbon atom on the imine carbon to give a vinylidene rhenium species.

H2TPP organocatalysis in mild and highly regioselective ring opening of epoxides to halo alcohols by means of halogen elements.

We found that elemental iodine and bromine are converted to trihalide nucleophiles (triiodine and tribromide anion, respectively) in the presence of catalytic amounts of meso-tetraphenylporphyrins (H2TPP). Therefore a highly regioselective method for the synthesis of β-haloalcohols through direct ring opening of epoxides with elemental iodine and bromine in the presence of H2TPPs as new catalysts is described. At room temperature a series of epoxide derivatives were converted into the corresponding halohydrins resulting from an attack of trihalide species anion atoms at the less substituted carbon atom. This method occurs under neutral and mild conditions with high yields in various aprotic solvents, even when sensitive functional groups are present.

Effect of transition metal oxides catalysts on oxidative desulfurization of model diesel

In this paper, model diesel is used to study the performance of oxidative desulfurization (ODS) system compared to hydrodesulfurization (HDS) process. A detailed parametric experimental study was performed to select the best technique for sulfur removal. The effects of solvent, oxidant, bimetallic oxide catalyst, dopant, dopant ratio and calcination temperature were investigated. Dimethylformamide (DMF) and tert-butyl hydroperoxide (TBHP) were found to be the best solvent and oxidizing agent for the removal of sulfur compounds in model diesel. Both solvent and oxidant were then applied to explore the applicability of various catalysts, such as iron, manganese, molybdenum, tin, zinc and cobalt in model diesel. The results showed that the catalytic activity was decreased in the order: Mo>Mn>Sn>Fe≈Co>Zn. Further investigation of doped molybdenum revealed that 4.35% WO3/16.52% MoO3/Al2O3 in the ratio of 10:90 with calcination temperature at 500°C was assigned as the best catalyst in this research. Under mild reaction condition, this catalyst showed high conversion with appreciable stability until 150hours and can be used as a reusable active catalyst in ODS treatment. Additionally, on the basis results obtained, a mechanistic proposal for this reaction was postulated, as an oxidation mechanism by nucleophilic attack of the sulfur atom on peroxo species of WO3/MoO3/Al2O3.

Exploration of regiospecificity phenomenon in [2+3] cycloaddition reactions between arylnitrones and trans-substituted nitroethenes on the basis of the reactivity indices theory

Article history: Received May 30, 2012 Received in Revised form July 31, 2012 Accepted 1August 2012 Available online 1 August 2012 Analysis of global electrophilicity and nucleophilicity power of the addends indicate polar character of [2+3] cycloaddition reactions between arylnitrones and trans-substituted nitroethenes. The regioselectivity of these reactions is determined by nucleophilic attack of oxygen atom from nitrone on activated β-position of nitroalkene. Interaction of this type leads to 4-nitroisoxazolidines, which are the only reaction products. © 2012 Growing Science Ltd. All rights reserved.

Gold-catalyzed bis-cyclization of 1,2-diol- or acetonide-tethered alkynes. Synthesis of β-lactam-bridged acetals: a combined experimental and theoretical study

2-Azetidinone-tethered alkyn-1,2-diols or alkynyl acetonides, readily prepared from imines of (R)-2,3-O-isopropylideneglyceraldehyde, were used as starting materials for the regio- and diastereospecific catalytic bis-oxycyclization reaction in the presence of a gold/acid binary system. Interestingly, in contrast to the gold-catalyzed reactions of N-tethered terminal alkynes, which lead to the corresponding 6,8-dioxabicyclo[3.2.1]octane derivatives (proximal adduct), the reactions of substituted alkynic diols and acetonides under identical conditions gave the 7,9-dioxabicyclo[4.2.1]nonane derivatives (distal adducts) as the sole products, through exclusive 7-endo/5-exo bis-oxycyclizations by initial attack of the oxygen atom to the external alkyne carbon. Moreover, the mildness of the method allowed the incorporation of a 1,3-diyne moiety as reactive partner, displaying exquisite chemoselectivity toward the internal alkynic moiety. In order to confirm the mechanistic proposal, labeling studies with deuterium oxide have been performed. Besides, density functional calculations were performed to gain insight into the mechanisms of the bis-oxycyclization reactions.

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC(6)H(4)CH=CHCO(+)/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH=CH(2)]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the C(α)-CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC(6)H(5), respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.