- Photoinduced ethane formation from reaction of ethene with matrix-isolated Ti, V, or Nb atoms
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The reactions of matrix-isolated Ti, V, or Nb atoms with ethene (C 2H4) have been studied by FTIR absorption spectroscopy. Under conditions where the ethene dimer forms, metal atoms react with the ethene dimer to yield matrix-isolated ethane (C2H6) and methane. Under lower ethene concentration conditions (~1:70 ethene/ Ar), hydridic intermediates of the types HMC2H3 and H2MC 2H2 are also observed, and the relative yield of hydrocarbons is diminished. Reactions of these metals with perdeuterioethene, and equimolar mixtures of C2H4 and C2D 2, yield products that are consistent with the production of ethane via a metal atom reaction involving at least two C2H4 molecules. The absence of any other observed products suggests the mechanism also involves production of small, highly symmetric species such as molecular hydrogen and metal carbides. Evidence is presented suggesting that ethane production from the ethene dimer is a general photochemical process for the reaction of excited-state transition-metal atoms with ethene at high concentrations of ethene.
- Thompson, Matthew G.K.,Parnis, J. Mark
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- Differential Reduction of CO2 by Molybdenum and Vanadium Nitrogenases
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The molybdenum and vanadium nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, it was demonstrated that the Vnitrogenase was nearly 700 times more active than its Mo counterpart in reducing CO to hydrocarbons. Herein, a similar discrepancy between the two nitrogenases in the reduction of CO2 is reported, with the Vnitrogenase being capable of reducing CO2 to CO, CD4, C2D4, and C2D6, and its Mocounterpart only capable of reducing CO2 to CO. Furthermore, it is shown that the Vnitrogenase may direct the formation of CD4 in part via CO2-derived CO, but that it does not catalyze the formation of C2D4 and C2D6 along this route. The exciting observation of a Vnitrogenase-catalyzed C-C coupling with CO2 as the origin of the building blocks adds another interesting reaction to the catalytic repertoire of this unique enzyme system. The differential activities of the V and Monitrogenases in CO2 reduction provide an important framework for systematic investigations of this reaction in the future.
- Rebelein, Johannes G.,Hu, Yilin,Ribbe, Markus W.
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- Intermolecular methyl group exchange and reversible P-Me bond cleavage at cobalt(III) dimethyl halide species
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The cobalt(III) dimethyl halide complexes cis,mer-(PMe3) 3Co(CH3)2X (X = Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer of the methyl ligands during isotopic labeling experiments. Extensive mechanistic studies exclude radical, methyl iodide elimination, and disproportionation/comproportionation pathways for exchange of the methyl groups between metals. A related cobalt(III) dimethyl complex supported by the tridentate phosphine ligand MeP(CH2CH 2PMe2)2 showed dramatically slower methyl ligand transfer, indicative of a mechanism for intermetallic exchange with a requisite phosphine dissociation. Crossover experiments between cobalt(III) dimethyl halide complexes supported by PMe3 and MeP(CH 2CH2PMe2)2 are consistent with a dicobalt transition structure in which only one cobalt center requires phosphine dissociation prior to methyl transfer. An additional methyl group scrambling process between cis,mer-(PMe3)3Co(CH3) 2I and free PMe3 was also identified during the investigation and originates from reversible P-CH3 bond cleavage.
- Xu, Hongwei,Williard, Paul G.,Bernskoetter, Wesley H.
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- Mechanistic considerations for C-C bond reductive coupling at a cobalt(III) center
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The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe 3)3Co(CH3)2I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe 3)3CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) × 10-5 s -1 (50 °C) and activation parameters of ΔH a = 31.4(8) kcal/mol and ΔS a = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C-C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s-1 for phosphine loss from cis,mer-(PMe3)3Co(CH3)2I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C-C bond formation. An unusual intermolecular exchange of cobalt-methyl ligands was also observed by isotopic labeling.
- Xu, Hongwei,Bernskoetter, Wesley H.
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- A NMR method for the analysis of mixtures of alkanes with different deuterium substitutions
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13C NMR at 125.76 MHz with 1H and 2H decoupling, 2H NMR at 76.77 MHz with 1H decoupling, and 1H NMR at 500.14 MHz with 2H decoupling were employed as analytical tools to study the complex mixtures of deuterated ethanes resulting from the catalytic H-D exchange of normal ethane with gas-phase deuterium in the presence of a platinum foil. Reference samples consisting of 1:1 binary mixtures of pure normal ethane and ethane-dn (n = 1-6) were used to identify the peak positions in the 13C, 2H, and 1H NMR spectra due to each individual isotopomer, and the effect of isotopic substitution on the chemical shifts was determined in each case. While the NMR of all three nuclei worked well for the identification of the individual components of the 1:1 standard mixtures, both 1H and 2H NMR suffered from inadequate resolution when studying complex reaction mixtures because of the broadening of the lines due to 1H-1H (1H NMR) and 2H-2H (2H NMR) couplings. 13C NMR was therefore determined to be the method of choice for the quantitative analysis of the reaction mixtures. Using the 13C NMR results, a correlation that takes into account the primary and secondary isotope substitution effects on chemical shifts was deduced. This equation was used for the identification of the individual components of the mixtures, and integration of the individual observed resonances was then employed for quantification of their composition. This study shows that 13C NMR with 1H and 2H decoupling is a viable procedure for studying mixtures of deuterated ethanes. Furthermore, the additivity of the isotopic effects on chemical shifts and the transferability of the values obtained with ethane to other molecules makes this approach general for the analysis of other isotopomer mixtures.
- Loaiza, Alfonso,Borchardt, Dan,Zaera, Francisco
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- Hydrogenation of Ethylene on Metal Electrodes. Part 5. Reduction of Light Ethylene on Pt in Deuteroperchloric Acid Solution and the Dual-pathway Mechanism
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Electroreduction of light ethylene on a platinum electrode was conducted in a heavy-water solution of deuteroperchloric acid.Deuterium-atom distributions in the product, ethane, support the previous conclusion that ethylene diffusion is rate-controlling at potentials less positive than ca. 100 mV, whereas the surface reaction is rate-controlling at more positive potentials where the Tafel line holds.The D-atom distribution in the latter potential region reveals double maxima at - and -ethanes.This distribution is explained by the dual-pathway mechanism which assumes two reaction rates for the step C2H4(a) + H(a) C2H5(a).The difference in the reaction rate will be attributed to the difference in the adsorption state of C2H4(a) but not of H(a), since only the weakly adsorbed hydrogen atoms are active in the hydrogenation.Reduction of light ethylene with D2 on platinum in deuteroperchloric acid solution gives the same results.A computer simulation based on the above mechanism can reproduce quantitatively not only the present distributions but also others given in the literature, even those observed for the gas-phase heterogeneous reduction.
- Fujikawa, Keikichi,Kita, Hideaki,Sato, Shinri
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- Alkylperoxy and Alkyl Radicals. 1. Infrared Spectra of CH3O2 and CH3O4CH3 and the Ultraviolet Photolysis of CH3O2 in Argon + Oxygen Matrices
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Methyl radicals, generated by the pyrolysis of azomethane and/or methyl iodide, were allowed to interact with matrices of Ar + 10percent O2 and the products isolated.IR spectra were obtained for species containing the following isotopically labeled groups: CH3, 13CH3, CD3, 16O2, 18O2 and 16O(18)O.From these spectra, the methylperoxy radical and its dimer, dimethyltetroxide, were identified and vibrational assignments made.Irradiation of CH3O2 at ca. 254 nm resulted in its photodissociation.The nature of this process in the matrix is discussed.
- Ase, P.,Bock, W.,Snelson, A.
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- Changes in ligand coordination mode induce bimetallic C-C coupling pathways
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Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd0/PdII catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a PdI/PdII redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a PdII-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a PdI dimer, a PdII monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.
- Blacquiere, Johanna M.,Boyle, Paul D.,Jackman, Kyle M. K.,Liang, Guangchao,Zimmerman, Paul M.
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supporting information
p. 3977 - 3991
(2022/03/31)
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- A Comparative Analysis of the CO-Reducing Activities of MoFe Proteins Containing Mo- and V-Nitrogenase Cofactors
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The Mo and V nitrogenases are structurally homologous yet catalytically distinct in their abilities to reduce CO to hydrocarbons. Here we report a comparative analysis of the CO-reducing activities of the Mo- and V-nitrogenase cofactors (i.e., the M and V clusters) upon insertion of the respective cofactor into the same, cofactor-deficient MoFe protein scaffold. Our data reveal a combined contribution from the protein environment and cofactor properties to the reactivity of nitrogenase toward CO, thus laying a foundation for further mechanistic investigation of the enzymatic CO reduction, while suggesting the potential of targeting both the protein scaffold and the cofactor species for nitrogenase-based applications in the future.
- Lee, Chi Chung,Tanifuji, Kazuki,Newcomb, Megan,Liedtke, Jasper,Hu, Yilin,Ribbe, Markus W.
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p. 649 - 653
(2018/04/16)
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- Hydrogen/Deuterium-Exchange Reactions of Methane with Aromatics and Cyclohexane Catalyzed by a Nanoscopic Aluminum Chlorofluoride
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H/D-exchange reactions between methane and deuterated solvents such as [D6]benzene and [D12]cyclohexane were heterogeneously catalyzed by nanoscopic aluminum chlorofluoride (ACF=AlClxF3?x, x≈0.05–0.3) under very mild conditions. 13C NMR spectroscopy experiments at labeled methane revealed the formation of all isotopologues. AlCl3, AlBr3, HS-AlF3, γ-Al2O3, and γ-Al2O3 preheated at 700 °C did not show any H/D-exchange reaction of methane or [D6]benzene. Mechanistically, electrophilic activation of methane was suggested at the ACF surface.
- Calvo, Beatriz,Braun, Thomas,Kemnitz, Erhard
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p. 403 - 406
(2017/12/26)
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- Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes
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Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe2 complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe2 complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.
- Rudenko, Andrey E.,Clayman, Naomi E.,Walker, Katherine L.,Maclaren, Jana K.,Zimmerman, Paul M.,Waymouth, Robert M.
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supporting information
p. 11408 - 11415
(2018/09/12)
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- Bimetallic C-C Bond-Forming Reductive Elimination from Nickel
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Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C-C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp3-sp3 and sp2-sp2 couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.
- Xu, Hongwei,Diccianni, Justin B.,Katigbak, Joseph,Hu, Chunhua,Zhang, Yingkai,Diao, Tianning
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supporting information
p. 4779 - 4786
(2016/05/09)
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- Oxidative reactivity of (N2S2)PdRX complexes (R = Me, Cl; X = Me, Cl, Br): Involvement of palladium(III) and palladium(IV) intermediates
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A series of (N2S2)PdRX complexes (N2S2 = 2,11-dithia[3.3](2,6) pyridinophane; R = X = Me, 1; R = Me, X = Cl, 2; R = Me, X = Br, 3; R = X = Cl, 4) were synthesized, and their structural and electronic properties were investigated. X-ray crystal structures
- Luo, Jia,Rath, Nigam P.,Mirica, Liviu M.
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p. 3343 - 3353
(2013/07/19)
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- Alkyl groups as synthetic vehicles in gold-mediated oxidative coupling reactions
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The use of surface-bound alkyl and phenyl groups as synthetic vehicles in coupling reactions on oxygen-activated Au(111) is demonstrated by the formation of ethers via alkyl and phenyl iodides. Ethers are formed by successive additions of surface-bound alkyl groups to adsorbed atomic oxygen to form first the alkoxy and then the ether. The addition of the ethyl group to adsorbed oxygen on Au(111) is the rate-limiting step leading to diethyl ether formation. Alkyl groups also add to adsorbed alkoxy groups formed from alcohols. An unusual feature of the alkyl iodide reactions on Au is that oxygen is not required for the activation step; hence, opening new potential reactive pathways on metallic Au. This journal is
- Xu, Bingjun,Madix, Robert J.,Friend, Cynthia M.
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p. 3179 - 3185
(2013/05/08)
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- Structural and reactivity comparison of analogous organometallic Pd(iii) and Pd(iv) complexes
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The tetradentate ligands RN4 (RN4 = N,N′-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane, R = Me or iPr) were found to stabilize cationic (RN4)PdMe2 and ( RN4)PdMeCl complexes in both PdIII and PdIV oxidation states. This allows for the first time a direct structural and reactivity comparison of the two Pd oxidation states in an identical ligand environment. The PdIII complexes exhibit a distorted octahedral geometry, as expected for a d7 metal center, and display unselective C-C and C-Cl bond formation reactivity. By contrast, the PdIV complexes have a pseudo-octahedral geometry and undergo selective non-radical C-C or C-Cl bond formation that is controlled by the ability of the complex to access a five-coordinate intermediate. The Royal Society of Chemistry 2012.
- Tang, Fengzhi,Qu, Fengrui,Khusnutdinova, Julia R.,Rath, Nigam P.,Mirica, Liviu M.
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p. 14046 - 14050
(2013/01/15)
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- Tracing the hydrogen source of hydrocarbons formed by vanadium nitrogenase
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Hydrocarbons from CO: The vanadium-nitrogenase-catalyzed reduction of carbon monoxide involves the adenosine triphosphate (ATP)-dependent protonation of CO and the subsequent formation of C - C bonds, leading to the production of small hydrocarbons, such as C2H4, C2H 6, C3H6, and C3H8 (see picture). Isotope-substitution studies monitored by GC-MS analysis show that protons are the source of hydrogen for the CO reduction. Copyright
- Lee, Chi Chung,Hu, Yilin,Ribbe, Markus W.
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experimental part
p. 5545 - 5547
(2011/07/08)
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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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- Deuterium Isotope Effect in Vinyl Radical Combination/Disproportionation Reactions
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The deuterium isotope effect for the vinyl radical combination and disproportionation reactions have been investigated.Protonated or deuterated vinyl and methyl radicals are produced from the 193 nm photolysis of protonated or perdeuterated methyl vinyl ketone.On the basis of product yield measurements, no isotope effect for the combination reactions of either vinyl-vinyl or vinyl-methyl has been observed.From the relative yields of ethylene, an isotope effect of kH/kD = 1.20 is determined for the vinyl-vinyl disproportionation reaction.
- Fahr, Askar,Laufer, Allan H.
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p. 262 - 264
(2007/10/02)
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- Magnitude and Origin of the Deuterium Isotope Effect during Methane Coupling and Related Reactions over Li/MgO Catalysts
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The deuterium kinetic isotope effect during methane coupling over a Li/MgO catalyst at 746 deg C has been determined as a function of methane concentration (10-80percent) at two different oxygen concentrations (5 and 10percent).In contrast to one earlier report, the overall rate ratio, CH4 versus CD4, appears constant (1.59 +/- 0.1) over this range of conditions.The isotope effect to ethane is slightly greater than the mean value.Those to ethylene and carbon monoxide are considerably greater still due to a second kinetic isotope effect in their production.The isotope effect tocarbon dioxide is correspondingly less.Similar experiments have been carried out for the oxidation of ethylene at 660 deg C.The overall isotope effect, C2H4 versus C2D4, is 1.45 +/- 0.12, and as with methane it is greater for carbon monoxide production than for carbon dioxide production.Methane oxidation has been modeled by coupling a simple literature model for surface steps involving Os(1-) to a model for the subsequent gas-phase reactions of surface generated methyl radicals.The calculations show that a parameter set suggested for the surface processes in earlier work does not provide a good fit to the observed kinetic orders in methane or oxygen.The set also underestimates the extent of 16O2/18O2 mixing during methane coupling.Alternative parameter sets which are more consistent with the kinetic and exchange data predict a small dependence of the expected kinetic isotope effect on pressure which falls withinb the bounds of the experimental measurements.It is concluded that over the range of conditions used here the rate of methane oxidation over Li/MgO catalysts is largely governed by the rate of bond breaking in methane.However that rate is sufficiently close to the rate at which surface oxidation sites are being created that the latter could be rate influencing under substantially different conditions as for example when using N2O as the oxidant.
- Cant, N. W.,Kennedy, E. M.,Nelson, P. F.
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p. 1445 - 1450
(2007/10/02)
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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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- Kinetics and Thermochemistry of the Reaction C2D6 + Cl C2D5 + DCl. The Heat of Formation of the C2D5 and C2H5 Radicals
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The bimolecular rate constant for the title reaction has been measured with the very low pressure reactor technique (VLPR) at 295 K.The rate constant at room temperature (295 K) was found to be k1=(8.30+/-0.7)x1E-12 cm3/(molecule*s).
- Parmar, Sucha S.,Benson, Sidney W.
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- A Study of the Mechanism of the Partial Oxidation of Methane over Rare Earth Oxide Catalysts Using Isotope Transient Techniques
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The mechanism of the partial oxidation reaction of methane over three catalysts (Sm2O3, Li/Sm2O3, and Pr6O11) having a range of activities and product selectivities has been studied by using isotope transient techniques.The most important conclusions from this study are (1) that large amounts of CH4 are adsorbed on all working catalysts, (2) that the reaction takes place on a small number of very active catalyst sites and does not involve the adsorbed CH4, (3) that gas-phase oxygen exchanges rapidly with the lattice oxygen atoms of the working catalysts, (4) that the rate of CH4 conversion is dependent on the rate of lattice oxygen exchange, and (5) that the carbon oxides are substantially formed by the secondary oxidation of the reaction products.A mechanism based on the formation and reactions of -> species is proposed for the C2+ products, but a different form of activated oxygen appears to be responsible for the formation of the carbon oxides.
- Ekstrom, Alfred,Lapszewicz, Jacek A.
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p. 5230 - 5237
(2007/10/02)
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- Kinetic Energy Release Distributions as a Probe of Transition-Metal-Mediated H-H, C-H, and C-C Bond Formation Processes: Reactions of Cobalt and Nickel Ions with Alkanes
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Product translational energy release distributions are used to investigate the potential energy surfaces for elimination of H2 and small hydrocarbons from ionic cobalt and nickel complexes with alkanes.The amount of energy appearing as product translation can be used to infer details of the potential energy surfaces in the region of the exit channel and has implications for the ease with which the reverse reactions may occur.The potential energy surfaces for hydrogen and alkane elimination reactions are discussed in view of the very different kinetic energy release distributions observed for these processes.For dehydrogenation reactions, both the shape of the distribution and the maximum kinetic energy release are correlated with the reaction mechanism.For example, the amount of energy apearing in product translation is quite distinctive between reactions known to involve metal-induced 1,2- and 1,4-hydrogen elimination.The selective dehydrogenations of 2-methyl-propane-2-d1 by Co+ and butane-1,1,1,4,4,4-d6 by Ni+ serve, respectively, as models for these processes.A comparison of these translational energy distributions with those observed for loss of H2, HD, and D2 from the dehydrogenation of butan-1,1,1,4,4,4-d6 by Co+ suggest that 1,4-elimination is dominant for the cobalt system and that the observation of different isotopic products results from scrambling processes.All the dehydrogenation processes examined were characterized by kinetic energy release distributions which could not be described by statistical theories.For these reactions, the maximum kinetic energy release approaches the estimated reaction exothermicity.In contrast, the more exothermic alkane eliminations have maximum kinetic energy releases which are less than half the reaction exothermicity, and the distributions can be fit with statistical models.For these processes the excess energy in the activated complex is approximately equal to the reaction exothermicity, suggesting a loose transition state for the disruption of a complex in which the intact alkane to be eliminated is interacting strongly with the metal center.Comparison of experiment with theory yields a Co+-propene bond strength of 48 +/- 3 kcal/mol, a Co+-CO bond strength of 34 +/- 3 kcal/mol, and a sum of the first and second metal bond strengths in Co(CD3)2+ of 110 +/- 3 kcal/mol at 298 K.The latter two values are derived from statistical kinetic energy release distributions observed for the loss of C2D6 and CO, respectively, in the reaction of Co+ with aceton-d6.
- Hanratty, Maureen A.,Beauchamp, J. L.,Illies, Andreas J.,Koppen, Petra van,Bowers, M. T.
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- COMPARATIVE STUDY OF cw CO2 LASER INDUCED AND SF6 SENSITIZED DECOMPOSITION OF METHYL IODIDES CH3I AND CD3I
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The decomposition of methyl iodide-d3 initiated by the irradiation of a cw CO2 laser in the presence of SF6 proceeds under comparable conditions faster, particularly in the shorter wavelength region of the CO2 laser spectrum, than the decomposition of methyl iodide.The differences can be explained by the different distribution of energy levels of both reactants.Observable differences are also in the composition of the reaction products.All these facts indicate the participation of at least two reaction mechanisms, a radical one and a molecular one, which canprobably proceed via the collision of two vibrationally excited molecules of CX3I.
- Rejnek, Jaroslav,Engst, Pavel,Jakoubkova, Marie,Horak, Milan
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p. 215 - 222
(2007/10/02)
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- Hydrogenation of Ethylene over Platinum (111) Single-Crystal Surfaces
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The hydrogenation of ethylene with both hydrogen and deuterium was studied(111) platinum single-crystal surfaces under a total pressure of 110 torr and a temperature range of 300-370 K.An activation energy (Ea) of 10.8 +/- 0.1 kcal/mol and kinetic orders with respect to hydrogen and ethylene partial pressure of 1.31 +/- 0.05 and -0.60 +/- 0.05, respectively, were observed.The deuterium atom distribution in the product from the reaction with D2 peaks at 1-2 deuterium atoms per ethane molecule produced, similar to what has been reported for supported catalysts.The reaction takes place on a partially ordered carbon covered surface, where the carbonaceous deposits have a morphology similar to that of ethylidyne.However, this ethylidine does not directly participate in the hydrogenation of ethylene, since both its hydrogenation and its deuterium exchange are much slower than the ethane production.A mechanism is proposed to explain the experimental results.
- Zaera, F.,Somorjai, G. A.
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p. 2288 - 2293
(2007/10/02)
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- The Synthesis of Specifically and Selectively Deuteriated 4,4'-Bisalkoxyazoxybenzene Derivatives
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In connection with deuterium n.m.r. studies of molecular motion in liquid crystals we have developed a number of methods for the synthesis of selectively deuteriated 4,4'-bisalkoxyazoxybenzenes.This paper is concerned with (i) the labelling of specific methylene segments of the alkoxy-chains, (ii) the labelling of the aromatic nucleus, and (iii) the selective enrichment of the deuterium content of one alkoxy-chain relative to the other.Our studies of the n.m.r. spectra of these liquid crystals have shown that there is not (as has sometimes been supposed) a monotonic decrease in C-D quadrupole splitting in passing from the oxygen to the CD3 end of the alkoxy-chain.In -PAA (p-azoxyanisole) we have also shown that the smaller splitting is associated with the CD3O group nearest to the NO end of azoxy-group.Whereas the photorearrangement of azoxy-compounds is normally regiospecific, the photorearrangement of PAA selectively deuteriated in one methoxy-group is unusual in that it leads to isotopically scrambled products.
- Boden, Neville,Bushby, Richard J.,Clark, Leslie D.
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p. 543 - 551
(2007/10/02)
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- CW CO2 LASER-INDUCED AND SF6-SENSITIZED DECOMPOSITION OF METHYL IODIDE-d3
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The decomposition of CD3I initiated by the irradiation of a cw CO2 laser was studied in the presence of the SF6 sensitizer.The first reaction step of the decomposition procedures CD3 radical that yields either methane-d4 (the abstraction reaction of "hot" radicals), or ethane-d6 (the recombination reaction of "cold" radicals).The effect of the total pressure and the decomposition of the reaction mixture, that of the laser excitation line and output and that of the irradiation time upon the decomposition rate and the product distribution was examined.
- Rejnek, Jaroslav,Jakoubkova, Marie,Engst, Pavel,Horak, Milan
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p. 3261 - 3269
(2007/10/02)
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- Ionization of Normal Alkanes: Enthalpy, Entropy, Structural, and Isotope Effects
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Enthalpies and entropies of ionization (ΔHi0, ΔSi0) of C4 to C11 normal alkanes were determined from charge-transfer equilibrium measurement between 300 and 420 K by using photoionization high-pressure mass spectrometry.Large negative ΔSi0 values are observed in C7 and larger n-alkanes, from -4.7 cal mol-1 K-1 (-19.6 J mol-1 K-1) in heptane to -13.9 cal mol-1 K-1 (-58.1 J mol-1 K-1) in undecane; in contrast, ΔSi0 of C4-C7 n-alkanes is negligible. ΔHi0 values range from 10.35 eV (997.6 kJ mol-1) (butane) to 9.45 eV (910.9 kJ mol-1) (undecane); the incremental ΔHi0 values also suggest the occurence of an effect that stabilizes C7 and higher but not the lower molecular ions.Analogy with disubstituted alkanes suggests that the negative ΔSi0 values and excess stabilization in C7 and higher alkane ions are due to constrained cyclic conformations which result from noncovalent intramolecular bonding between the terminal -C2H5 groups in the large, flexible molecular ions.These effects are more pronounced in n-alkanes than in 2-methylalkanes.Isotope effects on ΔHi0 as measured by the equilibrium constant K290 for n-CmD2m+2+ + n-CmH2m+2 ->/+ + n-CmD2m+2 are significant for ethane (k291 = 4.5) but decrease with increasing m: in propane K290 = 3.2 and in hexane and octane K291 = 1.0.However, the isotope effects in cyclic alkanes are much larger than in corresponding normal alkanes: in cyclohexane, K321 = 3.3 compared with that in n-hexane, were K320 = 1.0.
- Meot-Ner (Mautner), M.,Sieck, L.W.,Ausloos, P.
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p. 5342 - 5348
(2007/10/02)
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