- Mechanism of thermal decomposition of allyltrichlorosilane with formation of three labile intermediates: dichlorosilylene, allyl radical, and atomic chlorine
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It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis (~10–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical, dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is a typical reaction of homolytic decomposition to two radicals (C3H5 and SiCl3) at the weakest Si—C bond. Due to weakness of the Si—Cl bond in the SiCl3 radical, the energy of which is even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl3, this radical undergoes further dissociation to SiCl2 and Cl, thus resulting in three intermediates of different classes of highly reactive species formed from AllSiCl3.
- Boganov,Promyslov,Krylova,Zaitseva,Egorov
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p. 1216 - 1224
(2017/02/05)
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- Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
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Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
- Jackson, Leon V.,Walton, John C.
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p. 1758 - 1764
(2007/10/03)
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- Polar effects on iodine atom abstraction by charged phenyl radicals
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The ability of differently substituted charged phenyl radicals (a class of distonic radical cations) to abstract an iodine atom from allyl iodide was systematically examined in the gas phase by using Fourier transform ion cyclotron resonance mass spectrometry. The reaction products and second-order reaction rate constants were determined for several radicals that differ by the type and/or number of substituents located in the ortho- and/or meta- position with respect to the radical site. All the radicals also carry a para-pyridinium group needed for mass spectrometric manipulation. These electron-deficient phenyl radicals react with allyl iodide by predominant iodine atom abstraction. The reaction is facilitated by the presence of neutral electron-withdrawing substituents, such as F, CF3, Cl, or CN. The extent of rate increase depends on the type and number of the substituents, as well as their location relative to the radical site. Based on molecular orbital calculations (PM3 and Becke3LYP/6-31G(d)+ZPVE), the indicated variations in the transition state energy are not related to enthalpic factors. Instead, the results are rationalized by polar effects arising from a variable contribution of a stabilizing charge transfer resonance structure to the transition state. A semiquantitative measure for the barrier-lowering effect of each substituent is provided by its influence on the electron affinity of the radical (the electron affinities were calculated by Becke3LYP/6-31+G(d) and AM1, which were found to produce similar values). Methyl substitution does not significantly affect the electron affinity, and accordingly, does not have a detectable effect on reactivity. Methyl groups located at ortho-positions are an exception, however. o-Methyl-substituted phenyl radicals undergo exothermic rearrangement to a benzyl radical in competition with iodine abstraction from allyl iodide.
- Heidbrink, Jenny L.,Thoen, Kami K.,Kenttaemaa, Hilkka I.
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p. 645 - 651
(2007/10/03)
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- Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
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Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro- and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF3, and CCl3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester. At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO radicals added to the C=N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.
- McCarroll, Andrew J.,Walton, John C.
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p. 2399 - 2409
(2007/10/03)
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- Experimental study of the reaction between vinyl and methyl radicals in the gas phase. Temperature and pressure dependence of overall rate constants and product yields
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The vinyl-methyl radicals are critical intermediates in hydrocarbon combustion systems with elementary reactions of CH3 and C2H3 influencing the rate and products of the overall combustion process. The vinyl-methyl cross-radical reaction was studied using laser photolysis/photoionization MS. C2H2 yields did not experience any pressure dependence at 310, 500, and 900 K. The C3H5 decreased with an increasing pressure, which was well resolved at 310 and 500 K. The overall C2H3 + CH3 rate constants and quantitative product yields were obtained in direct real-time experiments at 300-900 K and bath gas (He) density (3-12) x 1016 molecules/cc. The primary products of the C2H3 + CH3 reaction were propylene, acetylene, and allyl radicals. A mechanism consisting of two major routes was proposed, i.e., via direct abstraction of a hydrogen atom from the vinyl radical by the methyl radical resulting in the formation of acetylene and methane and via the formation of chemically activated propylene that can undergo collisional stabilization or further decomposition into allyl radical and hydrogen atom.
- Stoliarov, Stanislav I.,Knyazev, Vadim D.,Slagle, Irene R.
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p. 9687 - 9697
(2007/10/03)
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- Detailed kinetics of cyclopentadiene decomposition studied in a shock tube
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Mixtures of cyclopentadiene diluted with argon were used to investigate its decomposition pattern in a single pulse shock tube. The temperatures ranged from 1080 to 1550 K and pressures behind the shock were between 1.7-9.6 atm. The cyclopentadiene concentrations ranged from 0.5 to 2%. Gas-chromatographic analysis was used to determine the product distribution The main products in order of abundance were acetylene, ethylene, methane, allene, propyne, butadiene, propylene, and benzene. The decomposition of cyclopentadiene was simulated with a kinetic scheme containing 44 species and 144 elementary reactions. This was later reduced to only 36 reactions The ring opening process of the cyclopentadienyl radical was found to be the crucial step in the mechanism. 1997 lohn Wiley and Sons, Inc.
- Burcat, Alexander,Dvinyaninov, Michael
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p. 505 - 514
(2007/10/03)
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- Investigating conformation dependence and nonadiabatic effects in the photodissociation of allyl chloride at 193 nm
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The experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C-Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C-Cl bond fission dominates, giving an absolute branching ratio of HCl:C-Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200°C. The branching ratio depends on the nozzle temperature; at 475°C, the absolute branching ratio measured is HCl:C-Cl=0.24±0.03. We analyze the experimental results along with Supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C-Cl fission reaction coordinate and its dependence on molecular conformation.
- Myers,Kitchen,Hu,Butler
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p. 5446 - 5456
(2007/10/03)
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- Ion-Molecule Reactions of CF3+ with Simple Unsaturated Aliphatic Hydrocarbons at Near-Thermal Energy
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Ion-molecule reactions of CF3+ with C2H2, C2H4, and C3H6 have been studied at near-thermal energy (0.05 eV) by using an ion beam apparatus.Initial product ion distributions and reaction rate constants were determined and compared with previous beam and selected ion flow tube (SIFT) data.The CF3+/C2H2 reaction produces exclusively the electrophilic adduct C3H2F3+ ion.For C2H4 and C3H6, hydride abstraction and electrophilic addition followed by HF elimination or fluoride transfer occur in parallel.The branching ratios of the former and latter reactions are 0.29 +/- 0.04:0.71 +/- 0.06 for the CF3+/C2H4 reaction and 0.07 +/- 0.02:0.93 +/- 0.07 for the CF3+/C3H6 reaction.On the basis of theoretical calculations of potential energies for the CF3+/C2H2 and CF3+/C2H4 systems, the lack of the HF elmination channel in the CF3+/C2H2 reaction, whereas the lack of the initial adduct ion in the CF3+/C2H4 reaction, is attributed to the different stability of the intermediate adduct ions for HF elimination.The reaction rate constants were 0.45 x 1E-9, 1.3 x 1E-9, and 1.6 x 1E-9 cm3 s-1 for C2H2, C2H4, and C3H6, respectively, which correspond to 46percent, 120percent, and 130percent of calculated rate constants from Langevin theory or a parametrized trajectory model.Although there are significant discrepancies in the product ion distributions between the present beam experiment and the previous beam data, the product ion distributions and the reaction rate constants obtained here are in reasonable agreement with the previous SIFT data.
- Tsuji, Masaharu,Aizawa, Masato,Nishimura, Yukio
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p. 3195 - 3200
(2007/10/02)
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- Radical-Stabilization-Energy - the MMEVBH Force Field
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Making use of the VB method of Malrieu et al. a force field has been developed, which allows to calculate heats of formation of hydrocarbons (conjugated and non-conjugated olefins, radicals and diradicals) with high accuracy.With this method radical stabilization energies (RSE) for a great number of delocalized radicals are calculated and compared with experimental values, derived from shock-tube measurements of dissociation energies or from rotational barriers of substituted olefins.A detailed analysis of the RSE with respect to structure, substituents, strain, and aromaticity is presented. - Key Words: Resonance energy / Heats of formation / Single pulse shock tube / Intrisic rotational barrier
- Roth, Wolfgang R.,Staemmler, Volker,Neumann, Martin,Schmuck, Carsten
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p. 1061 - 1118
(2007/10/02)
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- Reduction of 1-Substituted 2,4,6-Triphenylpyridinium Ions in Aprotic Media
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Some 1-substituted 2,4,6-triphenylpyridinium salts have been studied by cyclic voltammetry.The compounds exhibit two or three reduction peaks depending on the sweep rate and the rate of loss of the 1-substituent.The rate of cleavage of the one-electron reduction product (the radical) was measured by cyclic voltammetry and double potential step chronoamperometry, and the rate of cleavage of the two-electron reduction product (the anion) found by simulation of the voltammetric curves; the simulation suggested that the rate of cleavage of the radical was faster than the rate of cleavage of the anion.This unexpected finding is tentatively explained by different conformations of the phenyl groups in the 2 and 6 positions in the radical and in the anion.
- Joergensen, Lars Viborg,Lund, Henning
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p. 759 - 764
(2007/10/02)
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- Oxidation Chemistry of Propene in the Autoignition Region: Arrhenius Parameters for the Allyl + O2 Reaction Pathways and Kinetic Data for Initiation Reactions
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The oxidation of propene has been studied at a total pressure of 60 Torr between 400 and 520 deg C, and a detailed product analysis made in the initial stages of reaction over a wide range of mixture composition.Mechanisms for the formation of the products are discussed.The initial rates of formation of hexa-1,5-diene (HDE) and carbon monoxide are used to obtain A8 = 109.66 +/- 0.35 dm3 mol-1 s-1 and E8 = 78.6 +/- 4.5 kJ mol-1, the former giving from the known value of k1 CH2=CHCH2* + CH2=CHCH2* -> CH2=CHCH2CH2CH=CH2 (1).CH2=CHCH2* + O2 -> CO + products.Arrhenius parameters are also given for alternative pathways of the allyl + O2 reaction.All involve high energy barriers.From measurements of the accelerating effect of small amounts of additives CH3CHO, HCHO, HDE and propene oxide, rate constants at 480 deg C are obtained (for the first three) for the initiation reaction (21) RH + O2 -> R + HO2.Very few independent data for this type of reaction are available.The accelerating effect of propene oxide is ascribed to an exothermic isomerisation product which is not thermally stabilised at 60 Torr and undergoes homolysis to radical fragments.HDE is shown to have a spectacular accelerating effect on propene oxidation and values of k21n/k10 = 1050 +/- 200 at 480 deg C is obtained.C3H6 + O2 -> CH2=CHCH2* + H2O.HDE + O2 -> *CH2CH=CHCH2CH=CH2 + H2O.
- Stothard, Nigel D.,Walker, Raymond W.
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p. 2621 - 2630
(2007/10/02)
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- Pyrolysis of 1,7-Octadiene and the Kinetic and Thermodynamic Stability of Allyl and 4-Pentenyl Radicals
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Dilute mixtures of 1,7-octadiene in argon have been decomposed in single-pulse shock tube experiments.The main reaction is the breaking of the carbon-carbon allylic bond leading to the formation of allyl and 4-pentenyl radicals.A minor pathway involving a retroene reaction yielded propene and 1,5-pentadiene.Most of the 4-pentenyl radical decomposes to form another allyl and ethylene.Contributions from a cyclization process resulting in the formation of cyclopentene have also been observed.Allyl radical decomposition rate constants have been measured.The following rate expressions have been determined: k(1,7-octadiene -> allyl + 4-pentenyl) = 1.2 +/- 0.8 * 1016 exp(-35700 +/- 400/T)s-1, k(1,7-octadiene -> propene + 1,4-pentadiene) = (3 +/- 1.5) * 1012 exp(-27900 +/- 250/T)s-1, k(4-pentyl -> cyclopentene + H)/k(4-pentenyl -> allyl + ethylene) = 6.9 +/- 4) * 10-3 exp(2118 +/- 500/T), and k(allyl -> allene + H) = 40 +/- 10 s-1 at 1080 K.When compared with lower temperature literature data on hydrogen addition to allene to form allyl, the present results are consistent with a resonance energy of 51 +/- 4 kJ/mol for the allyl radical, a step-size down for collisional deactivation in argon of 500 cm-1, and lead to the high-pressure rate expression k(allyl -> allene + H) = 1.5 * 1011T0.84 exp(-30053/T) s-1 and k(H + allene -> allyl) = 1.2 * 108T0.69 exp(-1513/T) L mol-1 s-1 over the temperature range 350-1200 K.The uncertainty for the decomposition reaction is estimated to be a factor of 1.3 at 1100 K, but may increase to 1 order of magnitude at the lower temperatures.For the addition reaction, the estimated uncertainties are a factor of 1.5 at 350 K increasing to a factor of 2.5 at 1100 K.The disproportionation to combination ratio for allyl radical under high-temperature conditions is considerably less than 0.005.
- Tsang, Wing,Walker, James A.
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p. 8378 - 8384
(2007/10/02)
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- Decomposition of 4,4-Dimethylpent-1-ene in the Presence of Oxygen between 400 and 500 deg C: Oxidation Chemistry of Allyl Radicals
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The decomposition of 4,4-dimethylpent-1-ene (DMP) in the presence of O2 has been used as a source of allyl radicals over the temperature range 400-500 deg C.The reaction has been studied in both KCl-coated (peroxy species destroyed at the surface) and aged boric-acid-coated vessels (peroxy species preserved), and the basic mechanism shown to be: (1) (2).Previous studies have shown that 99percent of the t-butyl radicals undergo reaction (2), so that the system is an excellent source of allyl and HO2 radicals.A full product analysis has carried out over a wide range of mixture composition and associated mechanistic aspects discussed.It is shown that allyl radicals are unreactive towards both DMP and O2 and that they react mainly in radical-radical processes. (3) (8) (18).Values of k8/k18 = 0.37, 0.38 and 0.42 are obtained at 400, 440 and 480 deg C, respectively.From a reasonably reliable estimate of and , values of k8 = (3.3 +/- 0.9) * 1E9 and k18 = (7.4 +/- 2.4) * 1E9 dm3 mol-1 s-1 are obtained at 480 deg C.No previous estimates of these rate constants are available in the literature.Values of k4 have been determined and when combined with independent data at higher temperatures give log(A4/s-1) = 14.19 +/- 0.25 and E4 = 255.5 +/- 5.3 kJ mol-1 over the range 400-1025 deg C.DMP -> (CH3)2C=CH2 + CH3CH=CH2 (4).A value of K11 = (2.5 +/- 1) * 1E2 dm3 mol-1 s-1 has been obtained at 480 deg C and shown to be consistent with the reaction's high endothermicity. (11)
- Lodhi, Zulfiqar H.,Walker, Raymond W.
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p. 681 - 689
(2007/10/02)
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- The Heat of Formation of the Allyl and Methallyl Radical
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The decomposition of 1,5-hexadiene (1), 2-methyl-1,5-hexadiene (7), and 2,5-dimethyl-1,5-hexadiene into allyl- (2) and methallyl radicals (6) was studied by means of the shoke tube technique with and without oxygen as scavenger.From these data and from the temperature dependence of the equilibria 1 2 and 5 6, measured between 600 and 800 deg C, the heat of formation of the allyl (2) and methallyl radical (6) as well as the activation parameters for the recombination and disproportionation of these radicals have been deduced.
- Roth, Wolfgang R.,Bauer, Frank,Beitat, Arndt,Ebbrecht, Thomas,Wuestefeld, Michael
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p. 1453 - 1460
(2007/10/02)
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- Gas-phase measurements of the kinetics of BF2(+)-induced polymerization of olefinic monomers
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The initial steps in the BF2(+)-induced polymerization of the monomers of ethylene, propylene, cis-2-butene, isobutene, and styrene have been observed in the gas phase at room temperature using the Selected-Ion Flow Tube (SIFT) technique.Rate constants and product distributions have been determined for the initiation of the polymerization in each case.All five initiation reactions were found to be rapid (k >/= 5.0*10-10 cm3 molecule-1 s-1).The primary product ions that propagate polymerization have been identified and sequential addition reactions have been followed in all five systems.For ethylene the energetics of the initial steps have been followed using ab initio molecular orbital theory.Reaction of BF2+ with the vapours of water and benzene have also been characterized. Key words: ion-induced polymerization; alkenes; kinetics; gas phase ion chemistry
- Forte, Leonard,Lien, Min H.,Hopkinson, Alan C.,Bohme, Diethard K.
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p. 1576 - 1583
(2007/10/02)
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- An E.S.R. study of radical cation cyclization in the radiolytic oxidation of but-3-en-1-ol solutions in freon matrices
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The radiolytic oxidation of but-3-en-1-ol in halogenoethane matrices produces e.s.r. signals from both the protonated tetrahydrofuran-3-yl radical and the allyl radical; the former species is readily attributable to the nucleophilic endo cyclization of radical cations generated from unassociated solute molecules whilst the allyl radical is thought to originate from the fragmentation of the alkoxyl radical produced from radical cations generated within solute clusters.
- Dai, Sheng,Wang, Jih Tzong,Williams, Ffrancon
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p. 1063 - 1064
(2007/10/02)
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- Gas-phase ligand substitution reactions with (OC)Fe(NO)2.-', (OC)2Co(NO).-, (η3-C3H5)Co(CO)2.-, (C3H5)Co(CO)3.-, and CpCo(CO)2.-
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Three 17-electron transition-metal complex negative ions LM(CO)x-1.- ((OC)Fe(NO)2.-, (OC)2Co(NO).-, and (η3-C3H5)Co(CO)2.-) and two parent molecular anion radicals LM(CO)x.- ((C3H5)Co(CO)3.- and CpCo(CO)2.-) were generated by electron impact on the corresponding LM(CO)x complexes in the gas phase. The ion-molecule reactions of these five metal complex negative ions were studied with the neutral molecules PF3, PMe3, NO, SO2, olefins with electron-donating and -withdrawing vinyl substituents, acetone and certain fluorinated derivatives, biacetyl, O2, CO, and CS2. In general, the LM(CO)x-1.- complexes and CpCo(CO)2.- reacted by ligand substitution involving the associative mechanism. In many cases, the product ions of adduct formation and ligand substitution were directly observed in the same reaction. These ligand substitution reactions appear to take advantage of the abbility of the NO, η3-C3H5, and Cp ligands to reduce their hapticities as the neutral ligand bonds to the metal. The reaction of (C3H5)Co(CO)3.- with PF3 occurred by fragmentation of the allyl radical. This latter result was considered to occur by radical β-fragmentation of the excited acyl complex [C3H5C(=O)Co(CO)2(PF3) .-]* formed by CO insertion into the Co-allyl bond. Both parent molecular anion radicals were observed to undergo electron transfer with several of the neutral substrates yielding EA((η3-C3H5)Co(CO)3) = EA(CpCo(CO)2) = 0.62 ± 0.1 eV; EA(η3-C3H5)Co(CO)2) = 0.9 ± 0.2 eV was also measured. The results of the reactions of the parent LM(CO)x.- species with the neutral ligands were consistent with their structures having a 17-electron configuration about the metal with η3-Cp and η1-C3H5 ligands.
- McDonald, Richard N.,Schell, Philip L.
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p. 1806 - 1820
(2008/10/08)
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- Selected-ion flow tube studies of reactions of the radical cation (HC3N)+. in the interstellar chemical synthesis of cyanoacetylene
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The radical cation (HC3N)+. was produced in a Selcted-Ion Flow Tube (SIFT) apparatus from cyanoacetylene by electron impact and reacted at room temperature in helium buffer gas with a selection of molecules including H2, CO, HCN, CH4, H2O, O2, HC3N, C2H2, OCS, C2H4, and C4H2.The observed reactions exhibited a wide range of reactivity and a variety of pathways including charge transfer, hydrogen atom transfer, proton transfer, and association.Association reactions were observed with CO, O2, HCH and HC3N.With the latter two molecules association was observed to proceed close to the collision limit, which is suggestive of covalent bond formation perhaps involving azine-like N-N bonds.For HC3N an equally rapid association has been observed by Buckley et al. with ICR (Ion Cyclotron Resonance) measurements at low pressures and this is suggestive of radiative association.The hydrogen atom transfer reaction of ionized cyanoacetylene with H2 is slow while similar reactions with CH4 and H2O are fast.The reaction with CO fails to transfer a proton.These results have implications for synthetic schemes for cyanoacetylene as proposed in recent models of the chemistry of interstellar gas clouds.Proton transfer was also observed to be curiously unfavourable with all other molecules having a proton affinity higher than (C3N)..Also, hydrogen-atom transfer was inefficient with the polar molecules HCN and HC3N.These results suggest that informations at close separations may lead to preferential alignment of the reacting ion and molecule which is not suited for proton transfer or hydrogen atom transfer.
- Fox, A.,Raksit, A. B.,Dheandhanoo, S.,Bohme, D. K.
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p. 399 - 403
(2007/10/02)
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- Abschaetzung kinetischer und thermodynamischer Daten fuer Wasserstoffabspaltungsreaktionen von niederen Olefinen
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Mit Hilfe der BEBO-Methode sind fuer 500 ... 1200 K die Aktivierungsparameter der H-Uebertragungsreaktionen .R1 + HR2 --> R1H + R2.(R1: H, CH3, C2H5, n-C3H7, i-C3H7, CH3CO, CH2COCH3; R2H: Ethen, Propen, But-1-en) berchnet worden.Unter Verwendung entsprechender Daten der Rueckreaktion wurden daraus ΔRGθ, ΔRHθ und ΔRSθ bestimmt und mit den aus thermochemischen Inkrementen zugaenglichen Werten verglichen.Die Beweiskraft der guten Uebereinstimmung beider Datensaetze fuer die Anwendbarkeit des BEBO-Verfahrens auf H-Abstraktionen von Olefinen wird diskutiert.Ausserdem wird gezeigt, dass bei der Pyrolyse von Olefinen die H-Abstraktion aus der Vinylgruppe bis ueber 1500 K gegenueber der aus den Alkylgruppen zu vernachlaessigen ist.
- Scherzer, K.,Lohse, K.,Loeser, U.
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p. 196 - 204
(2007/10/02)
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- Deuterium-Hydrogen Exchange and Scrambling Reactions in the Pyrolysis of Labeled Cyclopentene. A Radical Mechanism
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Inter- and intraradical mechanism that promote deuterium-hydrogen scrambling in labeled cyclopentene-d1 are investigated at 800 K and 1200 K.Rate constants are estimated for each step and for possible competing side reactions.The fast radical bimolecular exchange at 800 K and unimolecaular scrambling at 1200 K of labeled cyclopentene are shown to be faster than the Woodward-Hoffman allowed 1,4 concerted molecular elimination of hydrogen.The low-energy estimate of 8.0 kcal/mol by Lewis of the difference between the allowed 1,4 and disallowed 1,2 channels can thus be explained.No conclusions can be drawn concerning the 1,2 channel.The estimated rates of radical reactions are in agreement with experiments on the addition of D2 to cyclopentadiene at 300 deg C which shows only cis, 3-5 addition and with pyrolysis experiments at 500 +/- 20 deg C which show about 5percent contribution of a higher than first-order radical reaction.The mechanism of this radical decomposition is given and its steps are explicitly evaluated.
- Kosnik, Kenneth G.,Benson, Sidney W.
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p. 2790 - 2795
(2007/10/02)
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- Absolute Rate Constants for the Reaction of Triethylsilyl Radicals with Organic Halides
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The absolute rate constants for the reactions of Et3Si. radicals with a number of organic halides have been measured in solution by using laser flash photolysis techniques.The reactivities cover a wide range; e.g., the rate constants at ca. 300 K are 4.3 * 109, 1.1 * 108, 2.5 * 106, and 5 M-1 s-1 for CH3CH2I, C6H5Br, (CH3)3CCl, and C6H5CH2F, respectively.Arrhenius parameters were determined for a few representative substrates.Thus, for CCl4, Ea = (0.78 +/- 0.24) kcal/mol and log (A/M-1 s-1) = 10.2 +/- 0.2, data which can be combined with relative rates from the literature to yield absolute rate constants for some additional substrates.Comparison of the Arrhenius parameters for halogen abstraction from several substrates suggests the involvment of charge-transfer interactions in some of these reactions.Such rate-enhancing interactions can have a greater influence on the preexponential factor than on the activation energy.An explanation for this phenomenon is advanced.Combination of our rate constants with data from literature also allows evaluation of the rates of inversion at the silicon center in triorganosilyl radicals as being in the range (3-12) * 109 s-1 at temperatures from 0 to 80 deg C which implies that a considerable activation barrier is involved in such process.
- Chatgilialoglu, C.,Ingold, K. U.,Scaiano, J. C.
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p. 5123 - 5127
(2007/10/02)
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