- Tunneling chemical reactions in solid parahydrogen: A case of CD3+H2→CD3H+H at 5 K
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Ultraviolet photolysis of CD3I in solid parahydrogen at 5 K gives CD3 radical, which decreases in a single exponential manner with a rate constant of (4.7±0.5)×10-6 s-1. Concomitantly, CD3H is formed, which is accounted for by the quantum tunneling reaction CD3+H2→CD3H+H. Under the same conditions. CH3I yields CH3 radical, but the corresponding reaction between CH3 and H2, expected to give CH4+H, does not proceed measurably at 5 K. The difference between the two systems is attributed to the difference in the zero point energy change.
- Momose, Takamasa,Hoshina, Hiromichi,Sogoshi, Norihito,Katsuki, Hiroyuki,Wakabayashi, Tomonari,Shida, Tadamasa
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Read Online
- The inversion mechanism for the reaction H + CD4 --> CD3H + D
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The reaction H+CD4CHD3+D is shown to take place by an inversion mechanism.The evidence is as follows.When the H atom has an anisotropic (perpendicular) velocity distribution is also perpendicular.For a relative energy near 2 eV, the reaction cross section for H+CD4 is 0.084+/0.014A2 and for H+CH3D is 0.040+/-0.015A2.At the same H atom energy, when CH3CD3 is substituted for CD4, no D atoms can be detected.Finaly, around 80percent of the initial H atom kinetic energy is released as kinetic energy of the D atom showing that the reaction is nearly vibrationally adiabatic.
- Chattopadhyay, Arun,Tasaki, Shintaro,Bersohn, Richard,Kawasaki, Masahiro
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Read Online
- Kinetics and Thermochemistry of the CH3, C2H5, and i-C3H7. Study of the Equilibrium R + HBr R-H + Br
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The kinetics of the reactions between CH3, C2H5, and i-C3H7 with HBr were studied in a tubular reactor coupled to a photoionization mass spectrometer.Rate constants were measured as a functio of temperature to determine Arrhenius parameters.The following rate constant expressions were obtained (units of the preexponential factors are cm3 molecule-1 s-1 and those of the activation energies are kJ mol-1; the temperature range covered in each study is also indicated): CH3 + HBr ; C2H5 + HBr ; i-C3H7 + HBr .These results were combined with determinations of the reverse rate constants to obtain equilibrium constants for the reactions R + HBr R-H + Br.Second-law-based analyses yielded heats of formation and entropies of CH3, C2H5, and i-C3H7 that are in close agreement with recent determinations of heats of formation in prior investigations of dissociation-recombination equilibria and calculations of entropies.The observed negative activation energies for R + HBr reactions (and negative activation energies inferred for R + I2 reactions from the current results) provide the basis for a detailed explanation for the disparities that currently exist between heats of formation of alkyl radicals that have been obtained from studies of bromination and iodination kinetics and those that are derived from kinetic studies of other reactions.A complex mechanism for R + HBr reactions that is consistent with the observed kinetic behavior of these reactions is discussed.
- Russell, John J.,Seetula, Jorma A.,Gutman, David
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- Dynamics of formation of products D2CN+, DCN +, and CD3+ in the reaction of N+ with CD4: A crossed-beam and theoretical study
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The formation of D2CN- in the reaction of N - (3P) with CD4 was studied using the crossed beam technique at collision energies of 3.66 and 4.86 eV. The experiments were complemented by calculations of stationary points on the triplet hypersurface of the system. The scattering data snowed that the reaction proceeds by the formation of two intermediate complexes having different lifetimes: a long-lived statistical intermediate and a short-lived complex (mean lifetime about one period of an average rotation) with more energy in translation than the statistical complex. Comparison with theoretical calculations suggests that the long-lived complex leads the CDND+ isomer of the product ion, whereas the short-lived complex leads prevailingly to the CD2N- isomer. The product DCN+ results from further decomposition of the primary product D2CN-, whereas CD3- is formed both by a hydride-ion transfer and a long-lived complex decomposition.
- Zabka, Jan,Roithova, Jana,Spanel, Patrik,Herman, Zdenek
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Read Online
- Platinum catalyzed c-h activation and the effect of metal-support interactions
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Catalytic C-H bond activation of methane and ethane on a series of silica supported platinum catalysts (Pt/SiO2) was studied by using hydrogen/deuterium (H/D) exchange. Kinetic experiments demonstrate that under the reaction conditions studied, the rate of C-H bond activation shows approximate first order dependence in alkane and inverse first order dependence in D2. The rate of C-H activation is affected by the presence of sodium on the silica support, where sodium-free supports have the fastest rates of C-H activation, as assessed by H/D exchange. CO adsorption and FTIR studies indicate that the Pt particles on the sodium-free support are more electron-deficient, having the most blue-shifted linear CO stretch, while sodium-containing supports are more electron-donating, having the most red-shifted linear CO stretch. It is proposed, based on the results described in this article and previous work in the literature, that more electron-donating supports cause the Pt particles to be more electron-rich and to adsorb D? (or H*) more strongly, thereby stabilizing the ground state and resting state of the catalyst, resulting in a decreased rate of C-H activation.
- Sattler, Aaron,Paccagnini, Michele,Lanci, Michael P.,Miseo, Sabato,Kliewer, Chris E.
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p. 710 - 720
(2020/01/02)
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- Photocatalytic halohydrocarbon dehalogenation conversion method
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The invention provides a photocatalytic halohydrocarbon dehalogenation conversion method which comprises the following steps: adding a photocatalyst quantum dot/rod into a solvent to obtain a solutionA; adding halohydrocarbon and an electronic sacrificial body into the solution A to obtain a solution B; utilizing a light source to irradiate the solution B and catalyzing the solution B to performhalohydrocarbon dehalogenation conversion. According to the photocatalytic halohydrocarbon dehalogenation conversion method disclosed by the invention, a nano quantum dot and a nano quantum rod are applied to dehalogenation conversion reaction of alkyl halide, alkenyl halide and alkyne halide for the first time; the reaction conditions are moderate, visible light is utilized as driving energy, a product is hydrocarbon compound, and the whole process has the advantages of environmental protection, conciseness and high efficiency. In addition, higher hydrocarbon of carbon chain growth can be generated after dehalogenation reaction, so that the method has potential application in preparation of higher hydrocarbon. According to the method disclosed by the invention, halohydrocarbon dehalogenation conversion and deuteration marking processes are jointly performed; hydrocarbon deuteration marking can be finished when a halohydrocarbon dehalogenation process is finished. The invention furtherprovides a method for performing deuteration marking on hydrocarbon.
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Paragraph 0101; 0102
(2019/04/02)
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- Catalytic Activation of Unstrained, Nonactivated Ketones Mediated by Platinum(II): Multiple C-C Bond Cleavage and CO Extrusion
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The complexes [Pt(tolpy)Cl(L)] (tolpy = 2-(4-tolyl)pyridyl; L = dmso, dms, py, PPh3, CO) are precursors for the catalytic cleavage of C-C bonds and extrusion of CO from a series of unactivated ketones such as cyclohexanone; deuterium labeling experiments demonstrate the involvement of a transfer hydrogen step in the mechanism.
- Sarju, Julia P.,Dey, Debashish,Torroba, Javier,Whitwood, Adrian C.,Redeker, Kelly,Bruce, Duncan W.
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supporting information
p. 4539 - 4542
(2019/11/29)
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- Accessing the Nitromethane (CH3NO2) Potential Energy Surface in Methanol (CH3OH)-Nitrogen Monoxide (NO) Ices Exposed to Ionizing Radiation: An FTIR and PI-ReTOF-MS Investigation
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(D3-)Methanol-nitrogen monoxide (CH3OH/CD3OH-NO) ices were exposed to ionizing radiation to facilitate the eventual determination of the CH3NO2 potential energy surface (PES) in the condensed phase. R
- Góbi, Sándor,Crandall, Parker B.,Maksyutenko, Pavlo,F?rstel, Marko,Kaiser, Ralf I.
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p. 2329 - 2343
(2018/03/21)
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- High-temperature Shilov-type methane conversion reaction: Mechanistic and kinetic studies
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Traditional Shilov reactions (performed in aqueous solution with a PtCl2 catalyst) for methane conversion suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low conversion rates have been achieved. In this paper, we show that Shilov-type C-H activations are achievable at much higher temperatures (~200 °C) by addition of concentrated aqueous solutions of Cl- to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt catalyst at mild reaction temperatures (~140 °C). Under high-pressure conditions (> 25.5 MPa), achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates (> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution. Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple C-H activations occurred. A comprehensive network reaction that included all the chain reactions was set up to clarify the reactivities and product selectivities of the methane activation reactions. The reaction network consisted of a series of parallel first-order reactions, which can be described by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies, and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C-H bonds in a methane molecule are equivalent, multiple substitutions during methane conversion cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopologue generations have similar activation energies, suggesting that the highest mono-substitution selectivity cannot be greater than 50%.
- Kang, Shujuan,Ma, Qisheng,Chen, Weiqun,Chen, Guanyi,Tang, Yongchun
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p. 1777 - 1784
(2015/10/20)
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- Highly efficient and stable photocatalytic reduction of CO2 to CH4 over Ru loaded NaTaO3
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An efficient and stable photocatalytic activity was obtained over NaTaO3 by introducing an electron donor (H2) into the CO2 reduction process with water. Ru/NaTaO3 demonstrated the best activity (CH4 51.8 μmol h-1 g-1) and product selectivity in converting CO2 to CH4.
- Li, Mu,Li, Peng,Chang, Kun,Wang, Tao,Liu, Lequan,Kang, Qing,Ouyang, Shuxin,Ye, Jinhua
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supporting information
p. 7645 - 7648
(2015/06/16)
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- Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr2O3(0001)
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The reaction of CH2Cl2 over the nearly-stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH2Cl2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as the rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH2Cl2 deactivate the surface by simple site-blocking of surface Cr3 + sites. A comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr2O3.
- Dong, Yujung,Brooks, John D.,Chen, Tsung-Liang,Mullins, David R.,Cox, David F.
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- On-stream regeneration of a sulfur-poisoned ruthenium-carbon catalyst under hydrothermal gasification conditions
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Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S-poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S-poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X-ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon-to-gas conversion and methane selectivity. Don't preach, bleach! Dilute hydrogen peroxide effects the reactivation of a sulfur-poisoned ruthenium catalyst under hydrothermal conditions. This mild oxidative treatment efficiently removes adsorbed sulfur from the ruthenium surface and restores the catalytic activity without corroding the catalyst support. Copyright
- Dreher, Marian,Steib, Matthias,Nachtegaal, Maarten,Wambach, Joerg,Vogel, Frederic
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p. 626 - 633
(2014/03/21)
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- Metal ions do not play a direct role in the formation of carbon-carbon triple bonds during reduction of trihaloalkyls by CrII or V II
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Carbyne radicals: Reactions of trihaloalkyl compounds with Cr2+ or V2+ in aqueous solutions yield alkynes and other products. Stepwise halogen abstractions from the trihaloalkyls form alkyl carbyne triradicals in solution. These radicals undergo coupling reactions, producing triply bonded alkyne molecules (see scheme). This process is not metal-assisted and does not occur in the coordination sphere of the metal ions.
- Levy, Ophir,Bino, Avi
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supporting information
p. 15944 - 15947
(2013/02/23)
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- Hydrogen-deuterium exchange of methane on nickel and potassium promoted nickel prepared by the reduction of nickel oxide
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The hydrogen-deuterium exchange reactions of methane in a deuterium stream were studied by a pulse experiment over a reduced nickel (Ni1373 prepared from nickel oxide calcined at 1373 K, and Ni773 prepared from nickel oxide calcined at 773 K) and K2O promoted reduced nickel (K-Ni1373 and K-Ni773). Ni1373 had a higher exchange activity than Ni773. The effects of the addition of K2O resulted in the decrease in the exchange activity due to the inhibition of step defect sites and/or the promotion of the change of nickel crystal structure. High activity of Ni1373 could be due to that the Ni1373 had the surface of a higher Ni(1 0 0)/Ni(1 1 1) ratio.
- Osawa, Tsutomu,Futakuchi, Takashi,Imahori, Tomoharu,Lee, I-Yin Sandy
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experimental part
p. 68 - 71
(2010/06/20)
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- Kinetic and theoretical study of the hydrodechlorination of CH 4- xClx (x = 1-4) compounds on palladium
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The reaction kinetics of hydrodechlorination (HDCl) for a series of CH 4-xClx (x = 1-4) compounds were measured on a Pd/carbon catalyst. The rate of HDCl correlated with the C-Cl bond energy, suggesting scission of this bond in the molecularly adsorbed molecule is rate-determining. The measured reaction kinetics of the CH4-xClx compounds support a previously proposed Langmuir-Hinshelwood type reaction mechanism. Kinetic and isotope exchange experiments demonstrated the following: gas phase H2 and HCl are in equilibrium with surface H and Cl; adsorbed Cl is the most abundant surface intermediate; and irreversible scission of the first C-Cl bond is rate-determining. The overall hydrodechlorination reaction rate can be written as kKR-Cl[R-Cl]/(1 + KHCl[HCl]/KH2 1/2[H2]1/2). The activation energy of the rate-determining step was related linearly to the dissociation energy of the first C-Cl bond broken in a Broensted-Evans-Polanyi relationship. This behavior is in agreement with a previous study of CF3CF 3-xClx compounds. During the reaction of CH3Cl, CH2Cl2, and CHCl3 with deuterium, H-D exchange occurred in only 2%, 6%, and 9% of products, respectively. The increasing H-D exchange with Cl content suggests the steps which determine selectivity in these multipath, parallel reactions. The density functional theory (DFT)-calculated activation energies for the dissociation of the first C-Cl bond in the family of chlorinated methane compounds are in good agreement with the values extracted from kinetic modeling, suggesting that parameters estimated from DFT calculations may be used to estimate the reactivity of a particular chlorinated compound within a family of chlorocarbons.
- Chen, Nan,Rioux, Robert M.,Barbosa, Luis A. M. M.,Ribeiro, Fabio H.
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experimental part
p. 16615 - 16624
(2011/10/30)
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- Decarbonylation of ethanol to methane, carbon monoxide and hydrogen by a [PNP]Ir complex
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The putative three-coordinate Ir(i) PNPPri(PNPPri = [N{2-P(CHMe2)2-4-MeC6H3} 2]-) pincer complex decarbonylates ethanol to yield methane, hydrogen and [PNPPri]Ir(CO). The mechanism involves the isolable trans-[PNPPri]Ir(H)(Me)(CO), which is susceptible to photochemical reductive elimination of methane.
- Melnick, Jonathan G.,Radosevich, Alexander T.,Villagran, Dino,Nocera, Daniel G.
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supporting information; experimental part
p. 79 - 81
(2010/04/01)
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- Isothermal pyrolysis of iodomethanes in gases
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The fact was established that the pyrolysis of gaseous iodomethanes RI yields methane and non traces of recombination products R2. A pyrolysis mechanism was proposed and rate constants of limiting stages of the pyrolysis of iodomethane, trideuteroiodomethane, and diiodomethane over the range of 500-1500 K were determined. Pleiades Publishing, Ltd., 2009.
- Skorobogatov,Khripun,Rebrova
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scheme or table
p. 2641 - 2651
(2010/06/14)
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- Kinetics of reactions of CN with chlorinated methanes
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The kinetics of reactions of CN with the chlorinated methanes CH3Cl, CH2Cl2, CHCl3 and CCl4 were investigated over the temperature range 298-573 K, using laser induced fluorescence (LIF) spectroscopy. At 298 K, rate constants of 9.0 ± 0.3 × 10-13, 8.8 ± 0.4 × 10-13, 9.0 ± 0.5 × 10-13 and 4.3 ± 0.6 × 10-13 cm3 molecule-1 s-1 were measured, respectively. A small positive temperature dependence was observed, as well as kinetic isotope effects of kH/kD ~ 2.14-2.25. These data along with product detection experiments strongly suggest that hydrogen abstraction dominates these reactions.
- Samant, Vaishali,Hershberger, John F.
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experimental part
p. 64 - 67
(2009/02/04)
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- Steam reforming of biomass based oxygenates-Mechanism of acetic acid activation on supported platinum catalysts
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The activation of acetic acid during steam reforming reactions over Pt-based catalysts has been probed by decomposing CH3COOD over Pt/C. The product mixture contained CO2, CH4 and its D-analogs (CH4 - x Dx
- Matas Gueell,Babich,Seshan,Lefferts
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body text
p. 229 - 231
(2009/02/07)
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- H/D isotope exchange between methane and magic acid (HSO 3F-SbF5): An in situ NMR study
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The kinetics of hydron exchange between methane and a series of DSO 3F-SbF5 superacids were measured by in situ 2H decoupled 1H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG# (30°C) decreasing from 97 to 84 kJ mol -1 over the range of concentration 19 to 49 mol % SbF5. The constant free enthalpy of activation ΔH# (ca. 65 kJ mol-1) and the decreasing entropy of activation ΔS# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.
- Walspurger, Stephane,Goeppert, Alain,Haouas, Mohamed,Sommer, Jean
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p. 266 - 269
(2007/10/03)
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- Effects of rhodium dispersion on catalytic behavior of Rh/active-carbon catalysts for H/D exchange reaction between and CH4 and D2
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The H/D exchange reaction between CH4 and D2 was carried out over Rh/active-carbon catalysts, which were prepared from RhCl3 and Rh(NO3)3. In the case of the catalysts prepared from RhCl3, Rh species were homogeneously dispersed on the support from external surface to the inside of pores. Metallic particles of Rh were found to be the predominant species on the catalysts prepared from Rh(NO3)3 in the low Rh-loading region of 2 wt.%. The reaction rate per unit gram of catalyst and the product distribution in methane reflected well the Rh-dispersion on the catalysts. The catalysts which contained the highly dispersed Rh species as predominant species were found to be more active for the H/D exchange reaction than the catalysts with relatively large metal particles of Rh. On the former, the ratio of CH3D/CD4 was observed to be much higher than that on the latter.
- Sakagami,Ogata,Takahashi,Matsuda
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p. 1930 - 1934
(2007/10/03)
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- H/D exchange between CH4 and CD4 catalysed by a silica supported tantalum hydride, (?SiO)2Ta-H
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The silica supported tantalum hydride (?SiO)2Ta-H 1, catalyses the H/D exchange reaction between CH4 and CD4 at 150 °C producing the statistical distribution of all methane isotopomers.
- Lefort, Laurent,Coperet, Christophe,Taoufik, Mostafa,Thivolle-Cazat, Jean,Basset, Jean-Marie
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p. 663 - 664
(2007/10/03)
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- Structure and reactivity studies of CoHNO+ in the gas phase
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CoHNO+ was generated in the gas phase by collision-induced dissociation (CID) or sustained off-resonance irradiation (SORI)-CID of CoH(CO)3NO+, formed from chemical ionization of Co-(CO)3NO with CH5+. Among the three isomeric structures, HCo(NO)+ (1), Co(HNO)+ (2), and Co(NOH)+ (3), density functional calculations found that 2 was the global minimum on the potential energy surface with 1 and 3, 14.1 and 15.4 kcal/mole higher in energy. Co(HNO)+ reacted with methane by dehydrogenation to yield CoCH3NO+, while CID of CoCH3NO+ yielded competitive formation of CoCH3+ and CoNO+, SORI-CID of CoCH3NO+ yielded CoCH3+ exclusively. These results suggested CoCH3NO+ had structure CH3CoNO+ rather than Co(CH3NO)+. A mechanism was proposed for the reaction with methane, where a four-centered transition state is involved for C-H bond activation as opposed to direct C-H insertion.
- Chen, Huiping,Jacobson, Denley B.,Freiser, Ben S.
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p. 10884 - 10892
(2007/10/03)
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- Methane hydrogenation and confirmation of CHx intermediate species on NaY encapsulated cobalt clusters and Co/SiO2 catalysts: EXAFS, FTIR, UV characterization and catalytic performances
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The adsorption of Co2(CO)8 onto dehydrated NaY powder under an N2 atmosphere predominantly yielded supported Co4(CO)12. The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in situ IR, extended X-ray absorption fine structure (EXAFS) and diffuse reflectance spectroscopies. The IR spectrum assigned to the species Co4(CO)12/NaY is shifted significantly from that observed for externally supported analogues on NaY and for this cluster in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes, which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeolitic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C2+ hydrocarbons in comparison with Co/SiO2 catalysts. On the basis of IR (νC-H 2960, 2880 cm-1 and δC-H 1520, 1393 cm-1) spectroscopy, mass spectrometry and reaction studies, one can conclude that CHx (x = 0, 1 or 2) surface carbonaceous species were generated by CH4 dissociation on the activated cobalt catalyst; the CH2 species was quite reactive, and propagated higher hydrocarbons.
- Shen, Guo-Cheng,Ichikawa, Masaru
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p. 1185 - 1193
(2007/10/03)
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- Effect of Pt Particle Size on H/D Exchange of Methane over Alumina- and Zeolite-supported Catalysts
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Deuterium exchange in methane was studied over a series of Pt/Al2O3 and Pt/mazzite catalysts.The reaction proceeds by a simple stepwise mechanism with CH3D being the primary product.Large and small metal particles located in zeolite and alumina catalysts have different catalytic activity with respect to deuterium exchange in methane.The rate of reaction per Pt atom was the same on the catalysts investigated for particles >15-20 Angstroem and considerably lower than the rate of reaction on samples with smaller Pt clusters.The activity of small clusters of Pt (diameter 15-20 Angstroem) depended on the nature of the support.
- Khodakov, Andrei,Barbouth, Nisso,Berthier, Yves,Oudar, Jacques,Schulz, Philippe
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p. 569 - 574
(2007/10/02)
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- Influence of Strong Metal-Support Interaction on Exchange with Deuterium and other Reactions of Hydrocarbons. Part 1. - Studies with Rh/TiO2 and Rh/SiO2
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The changes in the catalytic properties of Rh/TiO2 caused by raising the reduction temperature from 473 to 773 K have been investigated for four reactions, the exchange with deuterium of methane and cyclopentane, and the hydrogenolysis of 2,2-dimethylpropane and methylcyclopentane.The so-called strong metal-support interaction (SMSI) brought about by the high-temperature reduction had least effect on the exchange of methane but reduced the rate of hydrogenolysis of 2,2-dimethylpropane by factors of 1E4 or 1E5.The SMSI was reversible and its influence on each of the reactions was eliminated by oxidation of the catalyst followed by low-temperature reduction.No marked changes in the catalytic behaviour of Rh/SiO2 resulted from increasing the reduction temperature.The results provide further evidence for the existence of a number of kinds of catalytic sites on Rh/TiO2.The sites for the more structure-sensitive reactions tend to be more seriously affected by SMSI and probably involve more metal atoms than the sites responsible for methane exchange.
- Costa Faro, Arnaldo da,Kemball, Charles
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p. 741 - 748
(2007/10/02)
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- Guided Ion Beam Studies of Scattering Dynamics and Energy Disposal: The CD3(1+)+C2H6 and the CD3(1+)+C3H8 Case
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The gas-phase ion-molecule reactions of CD3(1+)+C2H6 and of CD3(1+)+C3H8 were investigated in a guided ion beam apparatus.Several basic reaction mechanisms such as hydride, methylene, and ethylene transfer as well as eliminative and double-eliminative association were observed.Absolute integral cross sections were measured at collision energies ranging from 0.1 to 5 eV.The branching ratios are in good agreement with recent ion cyclotron resonance results.Velocity distributions of products (especially of very slow ones) were recorded by the guided ion beam time-of-flight (GIB-TOF) method.Thereby, direct type of reaction and reaction via a long-lived complex are distinguished.In contrast to previous crossed beam experiments, the hydride transfer has been identified to proceed via a direct mechanism, even at low collision energies.Partitioning of the total available energy into translational energy and internal excitation is estimated as a function of total energy.
- Mark, Stefan,Schellhammer, Christoph,Niedner-Schatteburg, Gereon,Gerlich, Dieter
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p. 15587 - 15594
(2007/10/02)
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- Reactions of Hydrocarbons over Ru/SiO2: Exchange with Deuterium and the Onset of Hydrogenolysis
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Exchange reactions with deuterium of a number of hydrocarbons, chosen to provide evidence about the nature of the intermediates, have been followed mass spectrometrically over a ruthenium-silica catalyst.The selected hydrocarbons were methane, propane, 2-methylpropane (2MP), 2,2-dimethylpropane (DMP), and 2,2,3,3-tetramethylbutane (TMB).Products from the reaction of propane and 2MP were analysed by deuterium NMR spectroscopy.The extent of multiple exchange increased with temperature and mechanisms involving αα- or αδ-adsorbed intermediates were favoured.The hydrogenolyses of DMP and TMB were followed at higher temperature with emphasis on the selectivity of the reaction and the change of the pattern of products with contact time.Subsidiary experiments to learn more about the course of the reactions with TMB were carried out with 2,2,3-trimethylbutane (TriMB) and 2,3-dimethylbutane (23DMB).There was evidence for a strong hydrocarbon-ruthenium interaction leading to a sequence of events with increase of temperature, relatively easy exchange but with self-poisoning, followed by moderately easy hydrogenolysis, again subject to self-poisoning.The sequence suggested the formation in turn of reversibly adsorbed hydrocarbon intermediates, more strongly adsorbed species leading to hydrogenolysis and finally carbonaceous residues.
- Brown, Ronald,Kemball, Charles
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p. 585 - 594
(2007/10/02)
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- Coenzyme F430 from Methanogenic Bacteria: Mechanistic Studies on the Reductive Cleavage of Sulfonium Ions Catalyzed by F430 Pentamethyl Ester
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Mechanistic questions regarding the reductive cleavage of sulfonium ions by the NiI form of coenzyme F430 pentamethylester (F430M) were adressed in a series of kinetic studies and isotope labeling experiments.In neat DMF, methane formation from Dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h.In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo-first-order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol.From the temperature dependence of the reaction rate, an estimate for the activation parameters ΔH(excit.) = 49 kJ mol-1 and (apparent) ΔS(excit.) = -114 J K-1 mol-1 was derived.The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H-entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved.In contrast to the reaction with the homogeneous one-electron reductant sodium naphthalide, the F430M-catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of Me-S over that of alkyl-S (alkyl Me) bonds.Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.
- Lin, Shu-Kun,Jaun, Bernhard
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p. 1478 - 1490
(2007/10/02)
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- Structure and Catalytic Activity of Alumina-Supported Pt-Co Bimetallic Catalysts. 2. Chemisorption and Catalytic Reactions
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A series od Pt1-xCox/Al2O3 bimetallic catalysts have been characterized by temperature-programmed reduction (TPR), chemisorption of hydrogen and CO, deuterium exchange using both methanol and methane, and activity for the CO/H2 reaction.A Pt-assisted reduction mechanism over the entire range of composition was established by the TPR studies as well as by the chemisorption results.An enhanced metallic dispersion for the Pt-rich catalyst and formation of bimetallic particles on the Co-rich side was also indicated.In the CO hydrogenation over the Pt-rich catalysts the predominant products are methanol and dimethyl ether whereas on the Co-rich samples hydrocarbons and higher alcohols are produced.The mechanisms of deuterium exchange with methane and methanol are significantly different, the former being catalyzed solely by metallic sites while the latter utilizes both oxide and metallic sites for stepwise and multiple exchange, respectively.On the basis of the XPS data (preceding article) as well as the chemisorption results reported here, a surface model is introduced for interpretation of the catalytic results.
- Guczi, Lazlo,Hoffer, Tamas,Zsoldos, Zoltan,Zyade, Souad,Maire, Gilbert,Garin, Francois
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p. 802 - 808
(2007/10/02)
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- Kinetics of the Reaction of Alkyl Radicals with HBr and DBr
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Time-resolved resonance fluorescence detection of Br atom appearance following laser flash photolysis of RI (R = CH3, CD3, C2H5, t-C4H9) or Cl2/RH (R = CH3, C2H5) has been employed to study the kinetics of the reactions CH3 + HBr (1), CD3 + HBr (2), CH3 + DBr (3), C2H5+ HBr (4), C2H5 + DBr (5), t-C4H9 + HBr (6), and t-C4H9 + DBr (7) as a function of temperature (257-430 K) and pressure (10-300 Torr of N2).The rates of all reactions are found to increase with decreasing temperature; i.e., activation energies are negative, and 298 K rate coefficients for reactions 1 and 3-7 are found to be significantly faster than previously thought.Arrhenius expressions for reactions 1, 3, 4, and 6 in units of 10-12 cm3 molecule-1 s-1 are k1 = (1.36 +/- 0.10) exp, k3 = (1.07 +/- 0.17) exp, k4 = (1.33 +/- 0.33) exp, and k6 = (1.07 +/- 0.34) exp; errors are 2? and represent precision only.Normal kinetic isotope effects are observed (kHBr > kDBr), although the ratio kHBr/kDBr decreases in magnitude with decreasing activation energy; i.e., kHBr/kDBr is largest for R = CH3 and smallest for R = t-C4H9.Combining our results with the best available kinetic data for the reverse reactions (Br + RH) yields the following 298 K alkyl radical heats of formation in units of kcal mol-1: CH3, 35.3 +/- 0.6; C2H5, 29.1 +/- 0.6; t-C4H9, 12.1 +/- 0.8; errors are 2? and represent estimates of absolute accuracy.
- Nicovich, J. M.,Dijk, C. A. van,Kreutter, K. D.,Wine, P. H.
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p. 9890 - 9896
(2007/10/02)
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- Synthesis of Alkan-2-ones by Dirhodium-mediated Four Carbon Coupling
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Alkan-2-ones (RCH2COCH3) are obtained by C-C-C-C coupling of the four ligands in the dirhodium complexes, + (R=CH3, 13CH3, CD3, C2H5); the reaction is intramolecular.
- Sunley, Glenn J.,Saez, Isabel M.,Gulliver, David J.,Williams, Peter S.,Maitlis, Peter M.
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p. 193 - 195
(2007/10/02)
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- Construction of heterobimetallics bridged by (oxyalkyl)phosphines: Syntheses of trans-Me2Ta(μ-CH2)(μ-OCMe2CH 2Ph2P)2PtMe and (TMEDA)Ta(μ-CH2)(μ-Me)(μ-OCMe2CH 2Ph2P)2Ni
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Precursors to potentially bridging (oxyalkyl)phosphine ligands, μ-OCR2(CH2)nPh2P (R = tBu, n = 1; R = Me, n = 1, 2), were synthesized through the addition of LiCH2PPh2·TMEDA and LiPPh2 to tBu2C=O and Me2CCH2O. Treatment of ZrCl4 with 2.0 equiv of LiOCtBu2CH2Ph2P·xTHF provided (Ph2 PCH2CtBu2O)2ZrCl2 (1), which could be alkylated with MeLi to generate pseudo-Oh trans-(Ph2PCH2CtBu2O) 2ZrMe2 (2) and with tBuCH2Li to give pseudo-Td (Ph2PCH2CtBu2O) 2Zr(CH2tBu)2 (3). The fluxional, five-coordinate tris(alkoxide) (Ph2PCH2CtBu2O) 2Zr(OCtBu2CH2Ph2P)Cl (4), prepared from 1 and LiOCtBu2CH2Ph2P·xTHF, was methylated to afford four-coordinate (Ph2PCH2CtBu2O)3ZrMe (5). No heterobimetallics were produced when 1-5 were exposed to several substitutionally labile late-metal complexes. The tBu groups apparently place severe conformational contraints on the potential bridging ligands. Alcoholysis of Zr(CH2Ph)4 by HOCMe2(CH2)2Ph2P provided (Ph2P(CH2)2CMe2O) 2Zr(CH2Ph)2 (6) and (Ph2P(CH2)2CMe2O)4Zr (7), which was shown to conproportionate with Zr(CH2Ph)4 to give 6. Complex mixtures were obtained when 6 was utilized in the preparation of heterobimetallics. Metathesis of Me3TaCl2 with 2 equiv of LiOCMe2CH2Ph2P afforded (PPh2CH2CMe2O)2TaMe3 (8). Treatment of (COD)PtMe2 with 8 produced trans-Me2Ta(μ-CH2)(μ-OCMe2CH 2Ph2P)2PtMe (10, 21%) via thermolysis of an intermediate oligomer, ([-(Me)3Ta(μ-OCMe2CH2Ph 2P)Pt(Me)2PPh2CH2CMe 2O-]n)1/n (9). Similarly, (TMEDA)NiMe2 and 8 yielded (TMEDA)Ta(μ-CH2)(μ-Me)(μ-OCMe2CH 2Ph2P)2Ni (11, 30%). During the formation of 10 and 11, bimetallic Me/Me exchange reactions were prevalent, as shown via labeling experiments and isotopic shifts observed in the 195Pt{1H} NMR spectra of 10-dn, a mixture of isotopomers prepared from (CD3)3TaCl2 (8-d9). Plausible mechanisms rationalizing the generation of the μ-CH2 and μ-CH3 ligands of 10 and 11 are also discussed.
- Baxter, Steven M.,Wolczanski, Peter T.
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p. 2498 - 2509
(2008/10/08)
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- The Oxidative Coupling of Methane on Lithium Nickelate(III)
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Kinetic studies and isotopic exchange measurements (CH4-CD4 and 16O2-18O2) of the oxidative coupling of methane over stoichiometric LiNiO2 indicate a redox mechanism involving lattice oxygen atoms.The formation of C2 products is second-order in methane, both in the presence and absence of gaseous oxygen.Methane is dissociatively adsorbed on Ni3+-O2- sites, and the rate-determining step is the coupling of adsorbed CH3.Reduction of the catalyst by methane forms NiO, over which deep oxidation occurs.Adsorbed oxygen or gaseous oxygen is responsible for the deep oxidation.
- Hatano, Masaharu,Otsuka, Kiyoshi
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p. 199 - 206
(2007/10/02)
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- Elimination processes for alkyl, hydride, and hydroxy derivatives of permethyltungstenocene
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Elimination processes for alkyl, hydride, and hydroxy derivatives of permethyltungstenocene have been examined. The alkyl-hydride derivatives Cp*2W(R)H (Cp* = η5-C5Me5; R = CH3, CH2C6
- Parkin, Gerard,Bercaw, John E.
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p. 1172 - 1179
(2008/10/08)
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- Spectroscopic and chemical studies of nickel(II) hydrides
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The trans influence of X ligands on the spectroscopic properties of the Ni-H bond is reported for the series of square-planar nickel hydrides trans-HNi(X)(PCy3)2 (X = Me, Ph, CN, SCN, I, Br, Cl, SPh, S(p-tol), SH, OAc, O2CH, O2CPh, O2CCF3, OPh) prepared by oxidative addition of HX to [Ni(PCy3)2]2N2 or derivatives of subsequent products. The infrared-derived parameter v(Ni-H) shows a similar ligand dependence as the proton chemical shift of the hydride ligand, with more covalently bound ligands such as methyl or phenyl producing lower v(Ni-H) and smaller upfield chemical shifts as compared to those ligands that bind to Ni(II) with more electrostatic character such as anionic O-donor ligands. Comparisons with other ligand influenced, spectroscopic scales are made. Carbon dioxide and iodomethane were used as chemical probes of reactivity at the Ni-X or Ni-H bond. The derivatives with stronger Ni-H bonds (S and O donors) show no reactivity at the hydride while the C-bond derivatives exhibit CO2 insertion at the hydride. The Ni-H functionality is active toward iodomethane in the C-donor derivatives. A mechanism of hydrogen atom abstraction by methyl radicals is consistent with literature precedents as well as the Ni-H bond strengths determined by spectroscopies.
- Darensbourg, Marcetta Y.,Ludwig, Maria,Riordan, Charles G.
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p. 1630 - 1634
(2008/10/08)
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- H-D COUPLING CONSTANTS AND DEUTERIUM ISOTOPE EFFECTS ON THE PROTON CHEMICAL SHIFTS IN PARTIALLY DEUTERIATED METHANES
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Very accurate frequency measurements on 500 MHz 1H NMR spectra of CH4, CH3D, CH2D2, and CHD3 are reported.There are relatively large (up to 10percent) solvent and temperature effects on the isotope shifts (2Δ=-13.998 to -15.555 ppb for CH3D), which are non-additive by ca 0.2 ppb/deuterium; 2JHD (-1.929 +/- 0.001 Hz, sign assumed, equivalent to 2JHH=-12.57 Hz) is almost independent of solvent, temperature, and the number of deuterons present.
- Anet, Frank A. L.,O'Leary, Daniel J.
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p. 2755 - 2758
(2007/10/02)
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- Mn(I)-Induced 1,6-Demethanation across the CC Triple Bond of Linear Alkynes in the Gas Phase. A Case for the Generation of Manganese Cycloalkynes?
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Complexes of Mn(alkynes)+ were generated in the gas phase and found to exhibit a reactivity which is even richer than that of the analogous Fe(alkyne)+ species.Among the many unimolecular dissociations, the Mn+-induced demethanation of 4-octyne is of particular interest.The study of isotopomers and the effects of alkyl chain lengths reveals the operation of an unprecedented 1,6-elimination mode across the CC triple bond, and the experimental results may be explained by invoking the intermediate generation of the as yet unknown metallacycloalkynes.The implications of the unexpected, rich gas-phase ion chemistry of Mn+ with regard to theoretical models are discussed.
- Schulze, Christian,Schwarz, Helmut
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- Coenzyme F430 from Methanogenic Bacteria: Methane Formation by Reductive Carbon-Sulphur Bond Cleavage of Methyl Sulphonium Ions Catalysed by F430 Pentamethyl Ester
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The nickel(I)-form of coenzyme F430 pentamethyl ester is an efficient catalyst for the reductive cleavage of methyl sulphonium salts to methane and thioesters.
- Jaun, Bernhard,Pfaltz, Andreas
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p. 293 - 294
(2007/10/02)
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- A Stepwise Mechanism for Gas-Phase Unimolecular Ion Decompositions. Isotope Effects in the Fragmentation of tert-Butoxide Anion
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Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an cyclotron resonance (ICR) spectrometer has been used to the mechanism of a gas-phase negative ion unimolecular decomposition.Upon irradiation with a CO2 laser (both high-power pulsed and low-power continous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane.The mechanism of this formal 1,2-elimination reaction was probed by measuring hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions.Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed.These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex.The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory.The implications of this study for several previously reported unimolecular ion decompositions are also discussed.
- Tumas, William,Foster, Robert F.,Pellerite, Mark J.,Brauman, John I.
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p. 961 - 970
(2007/10/02)
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- Isotopic Excange in the Sonolysis of Aqueous Solutions Containing D2 and CH4
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Water was insonated under an argon atmosphere which contained various amounts of a D2-CH4 (2:1 vol percent) mixture.Maximum yield for the formation of CH3D, CH2D2, CHD3 and CD4 was observed at 40 vol percent argon.Ethane, ethylene, acetylene, and many higher hydrocarbons were also produced with about one third of the deuterated methane yield.In addition, some H/D exchange took place between D2 and H2O, and H2 was formed from water.Carbon monoxide and carbon were produced, too.A mechanism involving free radicals and atoms is discussed to explain these observations.
- Hart, Edwin J.,Fischer, Ch.-Herbert,Henglein, Arnim
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p. 4166 - 4169
(2007/10/02)
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- Deceptive Similarities in the Reactions of Fe+ and Co+ with Linear Nitriles in the Gas Phase
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The gas-phase reactions of the transition-metal ions Fe+ and Co+ with linear C(4)-C(12) nitriles are reported.In spite of an overall similar reactivity pattern, a more detailed analysis, based on the study of labeled nitriles, reveals distinct differences with regard to the mechanisms of elimination of alkenes and alkanes.For both metal ions, hydrogen and alkenes are generated from linear C(4)-C(12) nitriles, and the intermediates are formed via oxidative addition to terminal and internal C-H bonds.For the RCN/Fe+ system insertion in an internal C-H bond commences at position C(8) of the nitrile; for the analogous RCN/Co+ system, the oxidative addition to an internal C-H bond starts at position C(7) of the nitrile.Similarly, alkane formation is different for the two transition-metal ions.For RCN/Fe+ the generation of alkanes is observed for nitriles having at least eight carbon atoms; in contrast, the elimination of alkanes from RCN/Co+ is already observed for C(6) nitriles.Alkane elimination seems to follow the conventional mechanism (i.e., oxidative addition to a C-C bond, β-hydrogen transfer, and reductive elimination) for the RCN/Co+ system, whereas for the RCN/Fe+ complex there exists an additional mechanism.This mechanism corresponds to the loss of H2 from an internal position of the alkyl chain followed by the elimination of an alkene.Some possible origins of the different behavior of Fe+ vs.Co+ are discussed.
- Lebrilla, Carlito B.,Drewello, Thomas,Schwarz, Helmut
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p. 5639 - 5644
(2007/10/02)
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- Radical-Based Dephosphorylation and Organophosphonate Biodegradation
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Products resulting from the degradation of organophosphonates by Escherichia coli are identified and used as a basis for evaluating mechanisms which may be the chemical basis of the biodegradation.One mechanistic hypothesis which is consistent with the biodegradation products involves radical-based dephosphorylation.Chemical modeling of this process is achieved by the reaction of alkylphosphonic acids with lead(IV) tetraacetate and electrochemical oxidation at a platinum anode.
- Frost, J. W.,Cordeiro, M. L.,Li, D.
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p. 2166 - 2171
(2007/10/02)
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- NOVEL OXIRANE FORMATION VIA 1-METALLA-2-OXACYCLOBUTANES
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1-Metalla-2-oxacyclobutane complex, which was formed by intramolecular γ-hydrogen elimination of alkoxy(alkyl)metal complex of Ni or Pd, yields mainly oxirane, while its Ti complex affords olefins resulting from C-O bond cleavage.
- Miyashita, Akira,Ishida, Jun-ya,Nohira, Hiroyuki
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p. 2127 - 2130
(2007/10/02)
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- Reactions of Hydrocarbons on Alumina-supported Pt-Ir Bimetallic Catalysts - Part 1. - Exchange of Methane and Cyclopentane with Deuterium, and Hydrogenolysis of Butane, Pentane and Cyclopentane
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Exchange reactions with deuterium of cyclopentane (303 K) and of methane (473 K) and the hydrogenolysis of n-butane, n-pentane and cyclopentane have been studied on a series of alumina-supported Pt-Ir catalysts.Metal composition had little effect on the activity of the catalysts for the exchange reactions but influenced the character of the reactions in a regular manner.Rates of hydrogenolysis increased with iridium content, and the change was >100 for n-butane but only 10 for cyclopentane.The main reaction with cyclopentane was ring-opening, but the extent of further hydrogenolysis increased with iridium content in a manner that suggested the possible requirement of ensembles of three iridium atoms.
- Garden, David,Kemball, Charles,Whan, David A.
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p. 3113 - 3124
(2007/10/02)
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- Activation of C-H Bonds in Saturated Hydrocarbons. H-D Exchange between Methane and Benzene catalysed by a Soluble Iridium Polyhydride System
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The soluble iridium pentahydride (i-Pr3P)2IrH5 (activated by t-Bu-CH=CH2) catalyses H-D exchange between C6D6 and CH4 under mild conditions.
- Cameron, Charles J.,Felkin, Hugh,Fillebeen-Khan, Tauqir,Forrow, Nigel J.,Guittet, Eric
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p. 801 - 802
(2007/10/02)
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- Absolute Rate Coefficients for Methyl Radical Reactions by Laser Photolysis, Time-Resolved Infrared Chemiluminescence: CD3+HX->CD3H+X (X=Br, I)
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Absolut rate coefficients are reported for the room temperature reactions of deuterated methyl radicals with HI and HBr, CD3+HI (HBr)->CD3H+I (Br).Excimer laser photolysis of CD3I is used to generate methyl radicals, and time-resolved infrared chemiluminescence from the CH stretch of the CD3H products is detected to follow the time evolution of the reaction.The rate constants obtained in this manner are (7.7+/-0.7)*10-12 cm3 molecule-1 s-1 for CD3+HI and (4.7+/-0.4)*10-12 cm3 molecule-1 s-1 for CD3+HBr.These rate constants are considerably greater than earlier, indirectly measured values and indicate that the activation energy for these light-atom transfer reactions is lower than previously believed.
- Donaldson, D. J.,Leone, Stephen R.
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p. 936 - 941
(2007/10/02)
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- Proton Affinity and Heat of Formation of Silylene
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Using the techniques of Fourier transform ion cyclotron resonance spectroscopy, we determined the proton affinity of silylene to be 201 +/- 3 kcal*mol-1 from a study of the kinetics and thermochemistry of proton transfer from SiH3+ to a series of n-donor bases with well-established gas-phase base strengths.This value leads to ΔHfo298(SiH2) = 69 +/- 3 kcal*mol-1, which is 11 kcal*mol-1 higher than the previously accepted value (Wals, R.Acc.Chem.Res. 1981, 14, 246-252), but in excellent agreement with the recent theoretically recommended value of 68.1 kcal*mol-1 (Ho, P.; Coltrin, M.E.; Binkely, J.S.; Melius, C.F.J.Phys.Chem. 1985, 89, 4647-4654).
- Shin, Seung Koo,Beauchamp, J. L.
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p. 1507 - 1509
(2007/10/02)
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- Mechanistic and Kinetic Study of Alkane Activation by Ti+ and V+ in the Gas Phase. Lifetimes of Reaction Intermediates
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The reactions of Ti+ and V+ with several deuterium-labeled alkanes are studied by using an ion beam apparatus.The dominant reactions observed for both of these metal ions are single and double dehydrogenations.Alkane loss recations are also observed for Ti+ but may be due to electronically excited states.The dehydrogenation mechanisms are investigated by using partially deuterated alkenes.The results are consistent with 1,2-eliminations for both V+ and Ti+, where deuterium scrambling may occur in the latter case.It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti+ with n-butane.Although the dehydrogenation reactions of V+ and Ti+ appear to be similar to those of Ru+ and Rh+, there are some important differences in the reactivity of V+.Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V+ in comparison to Ti+, Ru+, and Rh+.Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti+ and V+ interact with n-butane.The measured upper limits to the unimolecular decomposition rates are 1.47 X 105 s-1 and 1.23 X 107 s-1 for V+ and Ti+ respectively.Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d10 is employed as the neutral reactant.The slower rate for V+ is suggested to arise from the inability of V+ to form two strong ? bonds due to the 3d4 electronic configuration of the ground-state ion.This renders C-H bond insertion energetically much less favorable for V+ than for the other metal ions and limits the excitation energy of reaction intermediates.
- Tolbert, M. A.,Beauchamp, J. L.
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p. 7509 - 7517
(2007/10/02)
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- Exchange Reactions of Hydrocarbons on Silica-supported Rh-Pt Bimetallic Catalysts
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A series of silica-supported rhodium-platinum catalysts containing 10E-1 mol metal g-1 silica have been shown to be well dispersed by temperature-programmed reduction.Exchange reactions with deuterium of methane (463 K), 2,2-dimethylpropane (323 K) and n-butane (311 K) were followed using most of the catalysts in the series.Two patterns of activity against composition curve were found.For methane and 2,2-dimethylpropane, i.e. molecules containing only primary C-H bonds which cannot form α,β-diadsorbed species, platinum was as effective if not more effective than rhodium.However, for the exchange of n-butane (and also propane) rhodium was some 20-30 times more active than platinum.The extent of multiple exchange increased with rhodium content.There was some evidence for the importance of electronic effects from the variations of activity and selectivity with metal composition.
- Oliver, James A.,Kemball, Charles,Brown, Ronald,Jamieson, Edward W.
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p. 1871 - 1882
(2007/10/02)
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