- Thermal Decomposition Mechanism for Ethanethiol
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The thermal decomposition of ethanethiol was studied using a 1 mm x 2 cm pulsed silicon carbide microtubular reactor, CH3CH2SH + Δ → Products. Unlike previous studies these experiments were able to identify the initial ethanethiol decomposition products. Ethanethiol was entrained in either an Ar or a He carrier gas, passed through a heated (300-1700 K) SiC microtubular reactor (roughly ≤100 μs residence time) and exited into a vacuum chamber. Within one reactor diameter the gas cools to less than 50 K rotationally, and all reactions cease. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. Ethanethiol was found to undergo unimolecular decomposition by three pathways: CH3CH2SH → (1) CH3CH2 + SH, (2) CH3 + H2C=S, and (3) H2C=CH2 + H2S. The experimental findings are in good agreement with electronic structure calculations. (Chemical Equation Presented).
- Vasiliou, AnGayle K.,Anderson, Daniel E.,Cowell, Thomas W.,Kong, Jessica,Melhado, William F.,Phillips, Margaret D.,Whitman, Jared C.
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- Consecutive Infrared Multiphoton Dissociations in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer
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Consecutive infrared multiphoton dissociations (IRMPD) may be observed in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). This is the IRMPD equivalent of previous MSn experiments using CID. This work presents a versatile technique, using a bistable shutter to gate ON and OFF a continuous-wave (CW) CO2 laser for multiple irradiation periods of 0.1-1000 s duration. Consecutive photodissociations, up to MS4, are demonstrated for the proton-bound dimer of diethyl ether and the resulting fragment ions. The photoproducts are formed close to the center of the FTICR cell, resulting in high product ion recovery efficiency. This differs from CID products, which are formed throughout the FTICR cell causing reisolation/detection problems. The fragmentation resulting from the use of low-intensity, CW, infrared laser radiation is shown to be much more energy selective than CID. Photodissociation of C2H5OH2+ ion produces the lowest energy product ion exclusively, even though the two product channels differ only by ~5 kcal/mol. Low-energy CID, however, produces a mixture of C2H5+ and H3O+ products in the ratio of 1.3:1. Hence, the higher energy pathway (C2H5+) is substantially favored. The current results indicate that this IRMPD MSn technique may be successfully applied to large biomolecules prepared by electrospray or MALDI.
- Tonner, D. Scott,McMahon, Terrance B.
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- Association Reactions at Low Pressure. 2. The CH3+/CH3CN System
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The reaction between CH3+ and CH3CN has been examined at pressures between 8E-8 and 1E-3 Torr in an ion cyclotron resonance mass spectrometer.At low pressures the reaction is bimolecular, having a rate coefficient of 1.8E-9 cm3s-1.The major ion products are H2CN+ and C2H5+, but a bimolecular association channel also competes with these two main binary exotermic channels.At pressures above ca. 4E-5 Torr the termolecular associaton (CH3CNCH3+) reaction becomes the major reaction.The results are rationalized in terms of barriers on the potential energy surface of the binary exit channels.The rate coefficient observed for the termolecular associaton reaction CH3+ + CH3CN + M CH3CNCH3+ + M was found to be k=1.90E-22 cm6 s-1 when M=CH3CN, and when M=N2, Ne, and He, k=4.0E-23, 0.6E-23, and 1.0E-23 cm6 s-1 respectively.The lifetime of the collision complex was found to be >14 μs.
- McEwan, Murray J.,Denison, Arthur B.,Huntress, Wesley T. Jr.,Anicich, Vincent G.,Snodgrass, J.,Bowers, M. T.
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- Guided Ion Beam Studies of Scattering Dynamics and Energy Disposal: The CD3(1+)+C2H6 and the CD3(1+)+C3H8 Case
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The gas-phase ion-molecule reactions of CD3(1+)+C2H6 and of CD3(1+)+C3H8 were investigated in a guided ion beam apparatus.Several basic reaction mechanisms such as hydride, methylene, and ethylene transfer as well as eliminative and double-eliminative association were observed.Absolute integral cross sections were measured at collision energies ranging from 0.1 to 5 eV.The branching ratios are in good agreement with recent ion cyclotron resonance results.Velocity distributions of products (especially of very slow ones) were recorded by the guided ion beam time-of-flight (GIB-TOF) method.Thereby, direct type of reaction and reaction via a long-lived complex are distinguished.In contrast to previous crossed beam experiments, the hydride transfer has been identified to proceed via a direct mechanism, even at low collision energies.Partitioning of the total available energy into translational energy and internal excitation is estimated as a function of total energy.
- Mark, Stefan,Schellhammer, Christoph,Niedner-Schatteburg, Gereon,Gerlich, Dieter
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- Chemical ionization using CF3+: Efficient detection of small alkanes and fluorocarbons
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The trifluoromethyl ion CF3+ is evaluated as a chemical ionization (CI) precursor in a compact Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer. It reacts with alkanes by hydride abstraction allowing characterization and quantification of alkanes up to C4 and cyclic. With larger alkanes fragmentation occurs. Fluorocarbons react by fluoride abstraction. Rate coefficients have been measured for reaction with alkanes, fluoroalkanes, chlorofluoroalkanes as well as several common VOCs. Use of CF3+ for trace analysis in air has been tested on an air sample containing traces of acetone, toluene, benzene and cyclohexane. The results are consistent with those obtained with H3O+ precursor and allow additional cyclohexane quantification.
- Dehon, Christophe,Lemaire, Jo?l,Heninger, Michel,Chaput, Aurélie,Mestdagh, Hélène
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experimental part
p. 113 - 119
(2011/08/21)
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- Dissociative proton transfer reactions of H3+, N2H+, and H3O+ with acyclic, cyclic, and aromatic hydrocarbons and nitrogen compounds, and astrochemical implications
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A flowing afterglow-selected ion flow drift tube has been used to measure the rate coefficients and product ion distributions for reactions of H3O+, N2H+, and H3+ with a series of 16 alkanes, alkenes, alkynes, and aromatic hydrocarbons as well as acrylonitrile, pyrrole, and pyridine. Exothermic proton transfer generally occurs close to the collision rate. The reactions of H3O+ are mostly nondissociative and those of H3+ are mostly dissociative, but many reactions, especially those of N2H+, have both dissociative and nondissociative channels. The dissociative channels result mostly in H2 and/or CH4 loss in the small hydrocarbons and in toluene, loss of C2H2 from acrylonitrile, and loss of HCN from pyrrole. Only nondissociative proton transfer is observed with benzene, pyridine, and larger aromatics. Drift tube studies of N2H+ reactions with propene and propyne showed that increased energy in the reactant ion enhances fragmentation. Some D3+ reactions were also investigated and the results suggest that reactions of H3+ with unsaturated hydrocarbons B proceed through proton transfer that forms excited (BH+)* intermediates. Pressure effects suggest that a fraction of the (BH+)* intermediates decomposes too rapidly to allow collisional stabilization in the flow tube (t -8 s). The other low-energy (BH+)* intermediates are formed by the removal of up to 40% of the reaction exothermicity as translational energy, and these intermediates result in stable BH+ products. The results suggest that, in hydrogen-dominated planetary and interstellar environments, the reactions of H3+ can convert C2-C6 hydrocarbons to smaller and less saturated molecules, but polycyclic aromatics are stable against decomposition by this mechanism. The dissociative reactions of H3+ can therefore favor the accumulation of small unsaturated hydrocarbons and aromatics in astrochemical environments.
- Milligan, Daniel B.,Wilson, Paul F.,Freeman, Colin G.,Meot-Ner (Mautner), Michael,McEwan, Murray J.
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p. 9745 - 9755
(2007/10/03)
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- Ion - Molecule Reaction Studies of Hydroxyl Cation and Ionized Water with Ethylene
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Rate coefficients and product branching ratios for the ion - molecule reactions of the hydroxyl cation, ionized water, and their deuterated analogues with ethylene have been determined using a selected ion flow tube (SIFT) at room temperature and in 0.5 Torr of helium buffer gas. In all cases, reactions proceed at or near the collision rate. The major product is always charge transfer: 79% for L2O?+and 66% for LO+ and does not depend on the isotopic form of hydrogen present (L = H or D). For the L2O?+ reactions, the remaining 21% of products are from proton or deuteron transfer, with no evidence of an isotope effect on this step even in the HOD?+ reaction. The greater exothermicity of the initial charge transfer in the LO+ reaction is revealed by the observation of additional product channels, forming the vinyl cation and protonated carbon monoxide. Multistep mechanisms that proceed through rate-determining charge-transfer, followed by a product-determining step, are postulated to explain these observations.
- Fishman, Vyacheslav N.,Grabowski, Joseph J.
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p. 4879 - 4884
(2007/10/03)
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- Ion-Molecule Reaction of CO2+ with Butane and Isobutane at Thermal Energy
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Rate constants and product ion distributions have been determined for thermal energy reactions of CO+2 with n-C4H10 and i-C4H10 by using an ion-beam apparatus.The total rate constants are (9.8+/-2.0)*10-10 and (1.0+/-0.2)*10-9 cm3 s-1 for n-C4H10 and i-C4H10, respectively.These values amount to about 75percent of the collision rate constants estimated from the Langevin theory.C4H+9, C3H+n (n=5-7), and C2H+n (n=3-5) are produced from n-C4H10 with branching ratios of 6, 56, and 38percent, while C4H+9 and C3H+n (n=5-7) are formed from i-C4H10 with branching ratios of 7 and 93percent, respectively.The lack of C2H+n fragments from i-C4H10 is attributed to a low probability of significant rearrangement of chemical bonds for the formation of the C2H+n fragments.The product ion distribution in the CO+2/n-C4H10 reaction is in good agreement with that predicted from the fragmentation pattern of n-C4H+10 at 13.78 eV, indicating that the CO+2/n-C4H10 reaction proceeds through a near-resonant charge transfer without momentum transfer.
- Tsuji, Masaharu,Matsumura, Ken-ichi,Funatsu, Tsuyoshi,Nishimura, Yukio,Obase, Hiroshi,et al.
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p. 2864 - 2870
(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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- Stabilities of Halonium Ions from a Study of Gas-Phase Equilibria R(1+)+XR'=(RXR')(1+)
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The gas-phase ion equilibria R(1+)+B=RB(1+), where R(1+)=Et(1+), i-Pr(1+), c-Pe(1+), t-Bu(1+), 2-Me-2-Bu(1+), and 2-Nb(1+) and B=CH3Cl, CH2Cl2, CHCl2, CHCl3, SO2F2, CF3H, and CF4 were determined in a pulsed electron beam high pressure mass spectrometer. van't Hoff plots provide ΔG0300, ΔH0, and ΔS0.For the chloronium ions the following trends were observed.The bond energy D(R(1+)-ClR0), where R(1+) changes and R0 is constant, decreases with increasing electronic stabilization of R(1+), i.e., in the order Me(1+), Et(1+), i-Pr(1+), c-Pe(1+), t-Bu(1+), Nb(1+).The same order was observed earlier in this laboratory for D(R(1+)-Cl(1-)), i.e., the chloride affinity of R(1+).However, the changes of D(R(1+)-ClR0) for R(1+)=2-Me-2-Bu(1+), Nb(1+), and t-Bu(1+) are very small.This means that little differential, specific nucleophilic solvation of these ions in solution is to be expected when solvents of low nucleophilicity like CH2Cl2 and SO2ClF are used.The bond energies D(Me(1+)-ClR) increase in the order R=Me, Et, i-Pr, t-Bu.The bond energies D(t-Bu(1+)-B) decrease in the order B=C2H5Cl, CH2Cl2 ca.CH3Cl, CCl3H, SO2F2, CF3H, CF4.The significance of these trends is discussed.
- Sharma, Dilip K. Sen,Hoejer, Sarah Meza de,Kebarle, Paul
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p. 3757 - 3762
(2007/10/02)
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- The Study of Ion-Molecule Reactions in the Gas Phase using a Triple Quadrupole Mass Spectrometer. Part 1. The Reactions of CH3+, CD3+, and C2H5+ with Simple Olefins
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A triple quadrupole mass spectrometer has been used to study the reactions of simple carbocations with low molecular weight olefins in the gas phase at relatively high pressures (1E-3-1E-4 Torr).In each case a high energy 'addition complex' is formed which fragments spontaneously to give daughter ions, the extent of fragmentation depending to some extent on the pressure and on the translational energy of the primary ions.Also formed are 'second generation' ions due to reactions of the predominant daughter ions with the olefin.Ions with the same mass as the high energy 'addition complexes' have been fragmented by collision (C.I.D.) with inert molecules (N2) to yield similar daughter ions to those found from the 'addition complexes'.In the reactions, involving CD3+ ions, deuterium is widely, but not completely randomly, distributed among the daughter ions.
- Batey, Jonathan H.,Tedder, John M.
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p. 1263 - 1268
(2007/10/02)
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