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The reactions of methanethiol on clean and modified Ni(110) have been studied under ultrahigh-vacuum conditions by temperature-programmed reactions (TPR), including deuterium incorporation studies.Surface bound molecular fragments were identified by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).The TPR data indicate that the major products of the reactions of methanethiol with clean Ni(110) surfaces are methane and hydrogen.Methane desorbs in a reaction-limited peak at 276 K, which does not shift with methanethiol exposure.Hydrogen desorption occurs in several peaks depending on the exposure.The coverage dependence of the methane yield indicates a competition between decomposition and reaction to form methane.At low coverages, decomposition is the major pathway while at higher coverages methane formation dominates.Vibrational spectroscopy (HREELS) indicates the presence of the methyl thiolate intermediate at temperatures less than 200 K.X-ray photoelectron spectroscopy and deuterium incorporation experiments confirm this assignment.A mechanism has been proposed based on hydrogenolysis of the methyl thiolate species and is consistent with all of the data.The appropriate rate equations associated with this mechanism have been solved numerically to predict the TPR data, and qualitative agreement was achieved .Methanethiol reacts with sulfur- and oxigen-modified Ni(110) surfaces to produce methane, hydrogen, and, in the case of the oxidized surfaces , water.The major effect of the modifier was to enhance the formation of methane relative to decomposition.These observations can be explained by either electronic or structural effects.

trans-Pt(CH2CMe2Et)Br(PPh3)2 induced C-H bond activation of CH4 under UV p irradiation leading to trans-PtMeBr(PPh3)2, while H-D exchange reaction of CH4 with D2 or D2SO4 took place in the photochemical reaction system containing Pt(PPh3)4 and neonexyl bromide.A possible reaction mechanism involving radical process is discussed on the basis of radical-trap experiment.

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.

The soluble iridium pentahydride (i-Pr3P)2IrH5 (activated by t-Bu-CH=CH2) catalyses H-D exchange between C6D6 and CH4 under mild conditions.

The reactivity of (1) (cp = η5-C5H5) has been studied.With (CH3)3CNC it gives >, with pyridine , with CH3CN and , with (CH3)3CNCO, which probably inserts into

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.

The interaction of gaseous D atoms with methyl bromide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied in order to elucidate the reaction mechanisms. The reaction kinetics at 85 K surface temperature were measured as a function of the methyl bromide precoverage by monitoring reaction products simultaneously with D atom exposure. On all substrates incoming atoms abstract the methyl group from adsorbed CH3Br via gaseous CH3D formation. In the monolayer regime of CH3Br/Pt(111) pure hot-atom phenomenology was observed in the rates. At multilayer targets the fluence dependence of the kinetics gets Eley-Rideal-like. With coadsorbed H present, the reaction of D with adsorbed methyl bromide revealed in addition to CH3D a CH4 product. This and simultaneous abstraction of adsorbed H via gaseous HD and H2 products clearly demonstrates that hot-atom reactions occur. At CH3Br adsorbed on a graphite monolayer on Pt(111) the abstraction kinetics of methyl was found to agree with the operation of an Eley-Rideal mechanism. These observations are in line with the expectation that hot-atoms do not exist on a CTPt(111) surface but on Pt(111) and H/Pt(111) surfaces. The methyl abstraction cross-sections in the monolayer regime of methyl bromide were determined as about 0.25 A2, irrespective of the nature of the substrate. This value is in accordance with direct, Eley-Rideal or hot-atom reactions.

Rate coefficients for the reactions CH3+H and CH3+D are presented over the temperature ranges 300* to generate CH2D+H is much faster than that to regenerate CH3+D under all conditions studied; in consequence the measured rate coefficient corresponds, in effect, to the high-pressure limit, kinfinite1(D).Fits to the CH3+H falloff data show that the high-pressure limit, kinfinite1(H), at 300 K exceeds that predicted from kinfinite1(D) by at least a factor of 2.This conclusion is confirmed by detailed master equation calculations that incorporate microcanonical dissociation rate coefficients calculated on the basis of a variational RRKM procedure.Parameters are provided that give a satisfactory representation of the CH3+H rate data, over the experimental pressure and tempereture ranges, with a temperature-independence value of kinfinite1(H) of 4.7E-10 cm3 molecule-1s-1 with uncertainties of Δlog kinfinite1(H) ca. +0.2 to -0.1 at 300 K rising to ca. +/-0.4 at 600 K.

From the time dependence of the formation of monodeuterioalkanes in D2O, and related isotopic labelling experiments, the base-catalysed alkane-forming reactions of ethyl- and methyl-(aquo)cobaloximes have been found to occur via different mechanisms.

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.

Tetradentate pyridinophane ligands have been shown to stabilize uncommon high-valent palladium and nickel organometallic complexes. Described herein are the synthesis and detailed characterization of a series of NiII- and NiIII-dimethyl complexes supported by modified tetradentate pyridinophane ligands in which one or both of the N-methyl substituents were replaced with electron-withdrawing p-toluenesulfonyl groups, thus reducing the amine N atom donicity and favoring the formation of Ni complexes with lower coordination numbers. The corresponding NiII-dimethyl complexes exhibit accessible oxidation potentials, and their oxidation generates NiIII species that were characterized by EPR and X-ray crystallography. Moreover, the NiII-dimethyl complexes exhibit selective ethane formation upon oxidatively induced reductive elimination using various oxidants - including O2 and H2O2, without the generation of any C-heteroatom products. Overall, these results suggest that the (RN4)NiIIMe2 complexes with more weakly donating axial ligands are more reactive toward ethane formation, likely due to destabilization of the corresponding high-valent Ni intermediates and formation of 5- and 4-coordinate conformations for these Ni species.

An iridium methyl complex, [Cp?Ir(bpy)(CH3)]+, was prepared by electrophilic methylation of Cp?Ir(bpy) with CH3I and characterized electrochemically, photophysically, crystallographically, and computationally. Irradiation of the MLCT transition of [Cp?Ir(bpy)(CH3)]+ in the presence of CH3I in acetonitrile produces ethane, methane, propionitrile, and succinonitrile. A series of mechanistic studies indicates that C-C bond formation is mediated by free methyl radicals produced through monometallic photochemical homolysis of the Ir-CH3 bond.

A detailed mechanism of the Rh(i)-catalyzed carboxylation of simple aromatic compounds via C-H bond activation was investigated. Kinetic studies with model compounds of the postulated key intermediates revealed that 14-electron complexes, RhMe(dcype) and RhPh(dcype), participated in the C-H bond activation step and the carboxylation step, respectively. Interestingly, the undesired carboxylation of RhMe(dcype) to give acetic acid was found to be much faster than the desired C-H bond activation reaction under stoichiometric conditions, however, the C-H bond activation reaction could occur under catalytic conditions. Careful controlled experiments revealed that C-H bond activation using RhMe(dcype) became competitive with its direct carboxylation under the condition that the concentration of CO2 in the liquid phase was rather low. This factor could be controlled to some extent by mechanical factors such as the stirring rate and the shape of the reaction vessel. The resting state of the rhodium species under catalytic conditions was found to be [RhCl(dcype)]2, and the proposed intermediates such as RhMe(dcype) and Rh(OBz)(dcype) were readily converted to the most stable state, [RhCl(dcype)]2, via transmetallation with [Al]-Cl species, thus preventing the decomposition of the active catalytic species.

The niobium methylidene [{(Ar′O)2Nb}2(μ2-Cl)2(μ2-CH2)] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar′O)2Nb(CH3)2Cl] (1) (OAr′=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC8 results in formation of the first niobium methylidyne [K][{(Ar′O)2Nb}2(μ2-CH)(μ2-H)(μ2-Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh3 reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1. Redox switch: Reduction of the niobium methylidene 1 with KC8 results in formation of the first niobium methylidyne 2 through binuclear α-hydrogen elimination (see scheme). Oxidation of 2 with ClCPh3 reforms 1 by virtue of a hydride migration. When 1 is prepared from [(Ar′O)2Nb(CH3)2Cl], isotopic labeling studies suggest a radical mechanism, and a binuclear α-hydrogen abstraction being the most likely operative pathway.

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%.

A family of water-soluble dimethyl complexes of formula cis-[PtMe2(dmso)(NHC·Na)] (2), in which NHC is an anionic N-heterocyclic carbene bearing a sulfonatopropyl chain on one of the nitrogen atoms and a sulfonatopropyl (a), methyl (b), mesityl (c), or 2,6-diisopropylphenyl group (d) on the other, have been prepared. The hydrolytic stability of the Pt-C bonds in these complexes under different neutral, alkaline, and acidic aqueous conditions has also been studied. Complexes 2 were found to be quite stable at room temperature in water under neutral or alkaline conditions. Degradation occurred at higher temperatures but involved C sp3-H activation and C-C reductive elimination processes in addition to Pt-Me bond hydrolysis. Hydrolytic cleavage of the platinum-methyl bonds was favored by good nucleophiles. Thus, the addition of KCN to an aqueous solution of 2 resulted in formation of the monomethyl complexes K[PtMe(CN)2(NHC·Na)] (9), whereas the dimethyl complexes K[PtMe2(CNR)(NHC·Na)] (10) were formed with the isocyanide CNCH2COOK. The addition of stoichiometric amounts of protic acids to aqueous solutions of 2 resulted in the clean cleavage of one or both platinum(II)-methyl bonds. Thus, the reaction of 2 with HCl afforded the complexes [PtClMe(dmso)(NHC·Na)] (3) and [PtCl2(dmso)(NHC·Na)] (4), whereas [PtMe(OH2)(dmso)(NHC)] (5) and [Pt(OH2)2(dmso)(NHC)][BF4] (7) were obtained upon treatment with HBF4. The crystal structure of 9a is remarkable in light of the longitudinal channels around 6 ? in diameter internally decorated with Pt-Me bonds.

A tridentate, NNC-tb (where NNC-tb = 2-(pyridin-2-yl)benzo[h]quinoline) ligated IrIII complex (NNC-tb)Ir(Ph)(4-MePy)(TFA), 11 along with analogues are very active for CH activation as evidenced by rapid catalytic H/D exchange between benzene and trifluoroacetic acid-d1 (DTFA). The complexes were examined with a variety of oxidants for the catalytic conversion of benzene to phenyltrifluoroacetate. Herein, the synthesis and characterization of (NNC-tb)Ir complexes is described along with the reactivity of these complexes towards arenes and alkanes.

This article describes the high-yielding and selective oxidatively induced formation of ethane from mono-methyl palladium complexes. Mechanistic details of this reaction have been explored via both experiment and computation. On the basis of these studies, a mechanism involving methyl group transmetalation between PdII and PdIV interediates is proposed.

The deposition of K2[Ru4(CO)13], K 2[Os3(CO)11], K2[Fe 2(CO)8], and K[Re(CO)5] onto graphite-like carbon Sibunit followed by the thermal decomposition of the supported carbonylmetallate in a flow of dihydrogen or argon affords systems capable of activating C-H bonds of methane, ethylene, and acetylene and of introducing them into hydrogen-deuterium exchange reactions. In the case of ethylene and acetylene, the isotope exchange proceeds at room temperature, while in the case of methane reaction temperatures not lower than 150 C are needed.

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ( iPrCNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 C and 4 atm of H2 pressure, ( iPrCNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (iPrCNC)CoH, was accomplished by hydrogenation of (iPrCNC)CoCH3. Over the course of 3 h at 22 C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-iPrCNC)CoN2. Similar alkyl migration was observed upon treatment of (iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2- ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.

13C-Labeled ethylene polymerization (pre)catalysts [κ2-(anisyl)2P,O]Pd(13CH3)(L) (1-13CH3-L) (L = pyridine, dmso) based on di(2-anisyl)phosphine benzenesulfonate were used to ass

Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers). The model quantitatively reproduces the experiments and shows unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the crucial first step. When collisions at atmospheric pressure are included, the model quantitatively reproduces previously reported quantum yields for photodissociation at all pressures and wavelengths. The model also predicts that 21 ± 4% of the initially excited acetaldehyde forms stable vinyl alcohol, a known precursor to organic acid formation, which may help to account for the production of organic acids in the troposphere.

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.

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.

The key nickel enzyme for methanogenesis (MCR) catalyzes the formation of CH3D and CH2D2 in a deuterated medium. CH 2D2 is formed by an exchange of deuterium into the S-methyl group of the substrate. Deuterium is incorporated at both carbon atoms of the S-ethyl group of ethyl coenzyme M, and a 13C label is rapidly scrambled within the ethyl group (see scheme). Thus, at least one intermediate is formed and the isotope exchange pattern is consistent with formation of a σ-alkane-nickel complex.

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.

The heterogeneous acid-catalyzed activation step of alkanes leading to the reaction intermediates (carbocationic or alkoxy species) was up to now the matter of a longstanding controversy. Gas chromatography and online mass spectroscopy measurements show that H2 and methane are formed over H-zeolites, whereas HD and CH3D are formed over D-zeolites as the primary products in the reaction with isobutane. These results indicate that o-bond protolysis by strong acid sites is the first step for hydrocarbon activation on these catalysts at mild temperatures (473 K), in analogy to the activation path occurring in liquid superacid media.

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

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.

The kinetics of thermolysis of acetyl propinyl peroxide in acetone-d 6 in the temperature range 323-373 K was studied using NMR spectroscopy and the effect of chemically induced nuclear polarization. The peroxide decomposes in acetone at rates comparable with the rates of thermolysis in alcohols, yielding numerous products. In the examined temperature range, the solvent molecules act as efficient donors of deuterium atoms, forming acetylmethyl-d5 radicals which recombine to a significant extent with the peroxide radicals. A scheme of the processes involved in decomposition of the peroxide was suggested. The parameters of the Arrhenius equation for the peroxide decomposition were determined. 2004 MAIK "Nauka/ Interperiodica".

Bimetallic palladium(II) methyl complexes, [Pd(dppmpH)(CH3)(μ-Cl)]2 (1) and [Pd(dippmpH)(CH3)(μ-Cl)]2 (2) were synthesized by the reaction of (COD)Pd(CH3)(Cl) (COD = 1,5-cyclooctadiene) with the new functional phosphine ligands 2-diphenylphosphino-4-methylphenol (dppmpH) and 2-diisopropylphosphino-4-methylphenol (dippmpH). The homolytic cleavage of the CH3-Pd bond was found to occur when complexes 1 and 2 were heated or photolyzed to form a methyl radical and the corresponding oxygen-bridged bimetallic palladium(II) complexes, [Pd(dppmp)(Cl)]2 (3) and [Pd(dippmp)(Cl)]2 (4), respectively. The molecular structures of complexes 1, 3 and 4 were determined by single-crystal X-ray diffraction. Reaction of small molecules, such as CO, SO2 and CH2=CH2, with complexes 1 and 2 was observed and characterized by IR, 1H, 13C and 31P{1H} NMR spectroscopy.

The relative reactivity and basicity of CH bonds in C3-C5 alkanes was studied using the strongest deuterated superacid, DF-SbF5, at various concentrations of SbF5. In all cases, at concentrations up to approxima

An EPR study was carried out to elucidate the hydrogen atom abstraction from methylsilane (CH3SiH3) by a methyl radical, CH3SiH3 + .CH3 → CH3SiH2. + CH4, in a solid solution of CH3SiH3 containing 1 mol% CH3I at the low temperatures of 3 K-115 K. The EPR spectra observed after UV-photolysis of the CH3I at 77 K were attributed to a mixture of the CH3SiH2. radical and the .CH3 radical. In the CH3SiH3 system, the CH3SiH2. radical was the major product immediately after the photolysis, while the .CH3 radical was the major one in the CH3SiD3 system. The .CH3 radicals decayed following first order kinetics in the dark in both systems. The decay rate constants for the reaction were experimentally determined to be k(Si-H) = 3.6 × 10-2 s-1 and k(Si-D) = 6.9 × 106 s1 (k(Si-H)/k(Si-D) = 5.2 × 103) at 77 K; the associated apparent activation energies were Ea(Si-H) = 0.85 kJmol s1 and Ea(Si-D) = 8.9 kJmol s1 (Ea(Si-H/Ea(Si-D) = 1/10) above 20 K. A non-linear Arrhenius plot was obtained for the rate constant, k(Si-H), and the rate became almost independent of the temperature below 20 K. These results suggest that the quantum mechanical tunneling effect contributes significantly to the H atom abstraction from the -SiH3 group.

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.

Kinetic measurements by 1H NMR spectroscopy and chemically induced dynamic nuclear polarization (CIDNP) effects were used to study thermolysis of solutions of acetyl propionyl peroxide in methanol-d4. It is found that the thermolysis may occur both by radical and by nonradical mechanisms. A scheme of the thermolysis is proposed. Parameters of the Arrhenius equation for the rate constants of the decomposition of the peroxide and the decarboxylation of acyloxyl radicals are obtained.

Treatment of CpMo(η4-diene)Cl2 (diene = 1,3-butadiene, C4H6, 1′; isoprene, C5H8, 1″; 2,3-dimethyl-1,3-butadiene, C6H10, 1″) in diethyl ether at low temperature with 2 equiv of alkylmagnesium RMgX reagents affords the corresponding dialkyl complexes CpMo(η4-1,3-diene)R2 (2, 2′, 2″, R = CH3, a; CH2Ph, b; CH2SiMe3, c). These species are isolable in moderate yields and have been fully characterized by EPR, elemental analyses, and cyclic voltammetry. They all show a reversible reduction process at relatively low potentials and an irreversible oxidation. The structure of 2″a was confirmed by single-crystal X-ray diffraction. The mixed complex CpMo(η4-C4H6)Cl(CH3), 3, has also been obtained by selective monomethylation of 1. A slow ligand redistribution process occurs between equivalent amounts of 1 and 2a to afford 3 quantitatively. Compound 3 slowly decomposes by elimination of a CH3 group, yielding compound [CpMo(η4-C4H6)Cl]2, 4, which has been structurally characterized by X-ray crystallography. Arylation with PhMgBr or (C6H2Me3-2,4,6)MgBr provides EPR evidence for formation of arylated Mo(III) products. However, the isolation and crystallographic characterization of [CpMo(η4-C4H4Me2-2,3)Br0.77Cl0.23]2, 4″, for the mesityl reaction indicates that halide exchange and electron transfer processes also take place competitively. Complex 2a does not react with Lewis bases such as phosphines or carbon monoxide nor with weak Bronsted acids (MeOH, H2O, CH3COOH, H3PO4, H3O+). Protonolysis of the Mo-R bond could only be observed by interaction with HCl, HBF4, and CF3COOD, with formation of CH4 or CH3D. Compounds CpMo(η4-C6H10)(CH3)(OCOCF3), 5, and CpMo(η4-C6H10)(OCOCF3)2, 6, have been isolated from the reaction between 2″a and 1 or 2 equiv of CF3COOD, respectively. Compound 6 has been structurally characterized by X-ray crystallography.

On the basis of the effects of chemically induced dynamic nuclear polarization of 1H and 13C and the yields of reaction products, the photolysis of methanol-d4 solutions of acetylpropionyl peroxide is investigated within the. temperature interval 193-333

The hydrogenation of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1) (Cp* = C5Me5) in the presence of PMe3 affords Cp*[C4H4B-N(i-Pr)2]Ta(H) 2(PMe3) (2) in essentially quantitative yield. Similarly, the hydrogenation of Cp*[C4H4B-Me]TaMe2 (3) in the presence of PMe3 affords Cp*[C4H4B-Me]Ta(H)2(PMe3) (4). Hydrogenation of 1 and 3 is accompanied by the reversible formation of side products. The most important of these complexes, Cp*[C4H4B-N(i-Pr)2]Ta(PMe 3)2 (5) and Cp*[C4H4B-Me]Ta(PMe3)2 (6), react slowly with dihydrogen forming 2 and 4, respectively. In the early stages of the hydrogenation of 1, the C-H activation product Cp*[C4H4B-N(i-Pr)2]Ta(H)(CH 2PMe2) (7) is also present. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr) 2]TaMe2 (8) in the presence of PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr) 2]Ta(PMe3)2 (9) as the exclusive product. The use of a bulkier phosphine, P(i-Pr)3, gives [C5H5B-Ph]-[C4H4B-N(i-Pr) 2]Ta(H)2[P(i-Pr)3] (10). Changing the phosphine to one of intermediate bulk, PEt3, leads to the formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr) 2]Ta(H)2(PEt3) (11t). The cis isomer (11c) is observable during early reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine loss followed by kinetically detectable rearrangement of the unsaturated intermediate prior to phosphine recoordination. Treatment of 11c with excess PMe3 results in the formation of 9 via a mixed-phosphine intermediate, [C5H5B-Ph][C4H4B-N(i-Pr) 2]Ta(PEt3)(PMe3) (12). The addition of [H(OEt2)2][B(C6H3(CF 3)2] to 11c results in the protonation of the nitrogen atom of the borollide ligand (H-11c+). H-11c+ is stable at room temperature for over a week. Treatment of 10 with excess PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr) 2]Ta(H)2-(PMe3) (13). Upon thermolysis in the presence of a large excess of PMe3, 13 is converted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.

The reaction between [NiMe{Ph2PCH2CH2)2PPh}]+ and anhydrous HCl (in MeCN) involves initial protonation of nickel to form [Ni(H)Me{{Ph2PCH2CH2)2PPh}]2+

The interaction of gaseous D atoms with methyl iodide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied. Direct product rate measurements were employed to determine the reaction kinetics. On all substrates, incoming D atoms abstract the methyl group from adsorbed CH3I via gaseous CH3D formation. In the monolayer regime of CH3I/Pt(111) pure hot-atom phenomenology was observed in the rates. With multilayers as targets, the fluence dependence of the rates get Eley-Rideal-type. With a coadsorbed H monolayer present, the CH3D rates at a CH3I monolayer on Pt(111) are affected by the suppression of hot-atom sticking. Accordingly, the rate curves exhibit similar features as expected for Eley-Rideal phenomenology. However, CH4 as a product and simultaneous abstraction of adsorbed H via gaseous HD and H2 formation clearly demonstrate that hot-atom reactions occur. With CH3I adsorbed on a graphite monolayer on Pt(111), the abstraction kinetics of methyl was found to agree with the operation of an Eley-Rideal mechanism. This observation is in line with the expectation that hot atoms do not exist on a C/Pt(111) surface.

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.

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.

Selective decomposition of methanethiol (CH3SH) on carbon-covered W(001) to produce mathane is enhanced by 75percent compared to the clean surface.The maximum enhancement requires only 0.25 monolayers (ML) of preadsorbed C.On a surface percovered with 0.8 ML of C, the methane desorbs in peaks at 460 and 550 K compared to 360 K on the clean surface, suggesting a greater stability in the C-S and C-H bonds.Increased intramolecular bond stability is confirmed by the temperature dependence of the S 2p and C ls soft X-ray photoemission.Methyl thiolate, CH3S, forms upon adsorption at 100 K.Chemisorbed methanethiol, which is not stable on the clean surface, is also observed between 100 and 300 K.The chemisorbed thiol decomposes to form additional thiolate.The thiolate reacts along three competing pathways.It undergoes rehydrogenation and desorbs as methanethiol, it selectively decomposes to form desorbed methane and adsorbed S, or it totally decomposes to form S, C, and desorbed H2.

Matrix isolation and cryogenic thin film approaches have been employed for the synthesis, isolation, and characterization of 1:1 and 1:2 complexes of (CH3)2Zn with CH3OH and CH3SH.These complexes were characterized by a shifting of certain sensitive vibrational modes of the acid and base subunits in the complex.The ratio of the 1:1 and 1:2 complexes could be altered by changing the relative amounts of the two reagents employed in a given experiment or by warming the cryogenic thin film from 14 K to as high as 200 K.Merged jet mixtures of (CH3)2Zn with either CH3OH and its isotopomers or CH3SH were also pyrolyzed at temperatures as high as 370 deg C.For the (CH3)2Zn/CH3OH pair, significant production of CH4 and CH2O was observed with minor amounts of C2H4.In the (CH3)2Zn/CH3SH experiments a substantial yield of (CH3)2S and CH4 was obtained.These pyrolytic reactions and products have not been reported previously and may have implications for the chemical vapor deposition of ZnO and ZnS.

Previous studies have shown that alkyl iodides dissociate on metal substrates around 200 K to produce iodine atoms alkyl moieties on the surface; here we report a new low-temperature decomposition pathway for those compounds on Ni(100) that leads to the formation of a close to 1:1 alkane-alkene mixture below 150 K.This latter reactions is proposed to occur via a mechanism where alkyl iodide dissociation results in the direct formation of free radicals.A combination of thermal desorption experiments with isotope labeling and hydrogen coadsorption was used to establish the importance of the nickel surface in the overall process and to rule out either surface disproportionation or gas-phase reactions as the source of the low-temperature products.Evidence was also obtained for a possible rearrangement of the adsorbed alkyl iodide molecules from a fat geometry into an upright configuration at high coverages, a change that would explain the ease with which the radicals formed after C-I bond scission are released into the gas phase instead of being left on the surface as adsorbed alkyl surface moieties.A comparison with other systems is also presented.

The absence of a radical-scavenging effect by [2H4]germane in ArF laser-induced photolysis of acetone at relatively high radiation fluences is consistent with molecular expulsion of ethane from acetone.

Generation, characterization, and properties of iron-silylene (Fe=SiRR′) and iron-silene (Fe(CH2=SiRR′)) cations (R, R′ = H, CH3) are describe in the gas phase by using Fourier transform mass spectrometry (FTMS). Iron-(silylene/silene) cations were formed by reaction of Fe+ with appropriate silanes. Structures of these ions were probed by using both collision-activated dissociation (CAD) and ion/molecule reactions. CAD failed to yield structural information; however, reaction with isotopically labeled ethene provides compelling evidence for formation of iron-silene and iron-silylene species. There is no evidence for the interconversion of iron-silylene and iron-silene species, even upon slow collisional activation or by formation of ethene collision complexes (ca. 40 kcal/mol of excess energy). This indicates that there is a prohibitive barrier for iron mediated interconversion of silene and silylenes. Reactions of iron-silylene and iron-silene species with water and benzene are described. The nature of the bonding is presented and bond dissociation limits are obtained.

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.

Rate parameters are reported for hydrogen and deuterium abstraction of methyl radicals embedded in glassy mixtures of CH3OH and CD3OD.The mole fraction of CH3OH in these isotopomeric mixture is 0, 0.05, 0.075, 0.10, 0.15, or 1.The nonexponential time dependence of the radical concentration is analyzed in terms of distributions of first-order rate constants.For the isotopomerically pure matrices, lognormal distributions describe the decay satisfactorily.The large difference between characteristic H and D transfer rate constants indicates tunneling.In the mixtures, there is competition between H and D abstraction processes which depends on the local structure about a radical, so that the corresponding rate parameters contain information about this structure.On the basis of earlier work , the analysis begins with the assumption that the structure about a radical resembles one of the crystalline phases of methanol.The entire set of decay curves is described by a (disordered) β-phase structure in which the radical replaces a methanol molecule and is located near the position associated with a methyl group.However, this static picture is inadequate because the radical can diffuse through the glass on the time scale of the kinetic measurements.Diffusion allows the radical to encounter more CH3OH molecules than would be expected for the static structure on a statistical basis - the effective mole fraction of CH3OH in the mixtures is higher than the analytical concentration.For the xH = 0.05 mixture, we estimate that on the average the radical encounters approximately 26 methanol molecules before abstraction occurs.This corresponds to diffusion over roughly 1100pm through the lattice.

The mechanismus for the lowest energy barrier pathways for unimolecular dissociation of CH3SiH2(1+) and CH3Si(Cl)H(1+) were examined in the gas phase by using Fourier transform mass spectrome (FTMS).Collision activated dissociation (CAD) by using sustained off-resonance irradiation (SORI) was used to determine the lowest energy pathways for dissociation.The lowest energy pathway for decomposition of CH3SiH2(1+) is dehydrogenation.The mechanism for this dehydrogenation process was investigated by studying the decomposition of CH3SiD2(1+).Unfortunately, isotopic scrambling by reversible 1,2-hydrogen migrations precede dehydrogenation.Hence, no mechanistic information is obtained from this isotopic labeling experiment.SORI-CAD of CH3SiD2(1+) yields dehydrogenation as H2 (0.67) and HD (0.33) with no D2 loss.The lowest energy pathway for dissociation of CH3Si(Cl)H(1+) is elimination of HCl.In contrast to CH3SiD2(1+), CH3Si(Cl)D(1+) does not undergo isotopic scrambling upon CAD.SORI-CAD of CH3Si(Cl)D(1+) yields exclusive elimination of HCl (1,2-elimination) to yield CH2SiD(1+).Hence, the lowest energy pathway for dissociation of CH3Si(Cl)H(1+) is 1,2-elimination of HCl. 1,1-Elimination of DCl from CH3Si(Cl)D(1+) to yield CH3Si(1+) is 38 kcal/mol more favorable than the 1,2-elimination process.Consequently, there must be a prohibitive barrier for the energetically more favorable 1,1-elimination process.

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.

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.

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.

Rupture of the Ti-C bond in complexes with ligands containing nitrile groups (CH3CN, PAN, P-gr.PAN, P = polyethylene, polypropylene, polytetrafluoroethylene, PAN = polyacrylonitrile) takes place according to two mechanisms: coordinate and homolytic. CH3TiCl3·2CH3CN complexes in solution decompose according to the homolytic pathway, and complexes with polymer ligands decompose according to the coordinate pathway (with the participation of the CN or CH bonds in the polymer). The formation of kinetically homogeneous complexes in solution with homo-PAN and complexes of two types in the case of PE-gr.PAN and PPr-gr.PAN was detected. The rate constants of decomposition of these complexes were found. Binding of Ti-CH3 primarily by C≡N bonds in the case of PPr-gr.PAN and with CH bonds in PE-gr.PAN could be obtained by selection of the polymer supports.

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.