Rate Coefficients of C2H with C2H4, C2H6, and H2 from 150 to 359 K

Opansky, Brian J.; Leone, Stephen R.

doi:10.1021/jp9619604

Cited by 75 publications

(75 citation statements)

References 29 publications

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“…67 We used experimental activation barriers obtained from rate-vs-temperature data over a temperature range of about 150 K -350 K for which the simple Arrhenius form was suitable and for which reaction rates were available. 35,36,[68][69][70][71][72] It must be stressed that these experimentally deduced activation barriers depend on the temperature range used for the Arrhenius fit, 70 and that this complicates a direct comparison with reaction barriers computed quantum mechanically.…”

Section: Theoretical Methodologymentioning

confidence: 99%

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

et al. 2006

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Symmetric and non-symmetric hydrogen abstraction reactions are studied using stateof-the-art ab initio electronic structure methods. Second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles, doubles and perturbative triples [CCSD(T)] methods with large correlation consistent basis sets (cc-pVXZ, where X = D,T,Q) are used in determining the transition-state geometries, activation barriers, and thermodynamic properties of several representative hydrogen abstraction reactions. The importance of basis set, electron correlation, and choice of zeroth-order reference wavefunction in the accurate prediction of activation barriers and reaction enthalpies are also investigated. The ethynyl radical (·CCH), which has a very high affinity for hydrogen atoms, is studied as a prototype hydrogen abstraction agent. Our high-level quantum mechanical computations indicate that hydrogen abstraction using the ethynyl radical has an activation energy of less than 3 kcal mol −1 for * To whom correspondence should be addressed. E-mail addresses: sherrill@gatech.edu; merkle@cc.gatech.edu; rfreitas@rfreitas.com 1 hydrogens bonded to an sp 2 or sp 3 carbon. These low activation barriers further corroborate previous studies suggesting that ethynyl-type radicals would make good tooltips for abstracting hydrogens from diamondoid surfaces during mechanosynthesis. Modeling the diamond C(111) surface with isobutane and treating the ethynyl radical as a tooltip, hydrogen abstraction in this reaction is predicted to be barrierless.

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Section: Theoretical Methodologymentioning

confidence: 99%

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

et al. 2006

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“…We include neutral-neutral and ion-molecule bimolecular reactions, three body processes and thermal dissociations. Important reactions involved in the build up of small organic molecules at high temperatures are those of H 2 with radicals such as C 2 and C 2 H. These reactions have been studied in the laboratory over a relatively wide temperature range: C 2 + H 2 → C 2 H + H between 295 and 493 K (Pitts et al 1982) and C 2 H + H 2 → C 2 H 2 + H in the range 178-440 K (Peeters et al 1996;Opansky & Leone 1996). They have moderate activation barriers of about 1400 K, which make them very slow in the cold interstellar medium, although at high temperatures they become rapid enough to control the abundance of C-bearing species.…”

Section: Chemical Modelmentioning

confidence: 99%

Formation of simple organic molecules in inner T Tauri disks

Agúndez¹,

Cernicharo²,

Goicoechea³

2008

A&A

155

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Aims. We present time dependent chemical models for a dense and warm O-rich gas exposed to a strong, far ultraviolet (FUV) field aimed at exploring the formation of simple organic molecules in the inner regions of protoplanetary disks around T Tauri stars.Methods. An up-to-date chemical network is used to compute the evolution of molecular abundances. Reactions of H 2 with small organic radicals such as C 2 and C 2 H, which are not included in current astrochemical databases, overcome their moderate activation energies at warm temperatures and become very important for the gas phase synthesis of C-bearing molecules.Results. The photodissociation of CO and release of C triggers the formation of simple organic species such as C 2 H 2 , HCN, and CH 4 . In timescales between 1 and 10 4 years, depending on the density and FUV field, a steady state is reached in the model in which molecules are continuously photodissociated, but also formed, mainly through gas phase chemical reactions involving H 2 . Conclusions. The application of the model to the upper layers of inner protoplanetary disks predicts large gas phase abundances of C 2 H 2 and HCN. The implied vertical column densities are as large as several 10 16 cm −2 in the very inner disk (<1 AU), in good agreement with the recent infrared observations of warm C 2 H 2 and HCN in the inner regions of IRS 46 and GV Tau disks. We also compare our results with previous chemical models studying the photoprocessing in the outer disk regions, and find that the gas phase chemical composition in the upper layers of the inner terrestrial zone (a few AU) is predicted to be substantially different from that in the upper layers of the outer disk (>50 AU).

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“…Our earlier work [Pedersen et al, 1993;Opansky and Leone, 1996] provided detailed measurements of (R1) down to 143 K using transient infrared laser absorption spectroscopy in a lowtemperature flow cell. Temperature-dependent rate constants were found to show a slight negative temperature dependence.…”

Section: Introductionmentioning

confidence: 99%

Direct measurements of rate coefficients for the reaction of ethynyl radical (C₂H) with C₂H₂ at 90 and 120 K using a pulsed Laval nozzle apparatus

Lee¹,

Samuels²,

Hoobler³

et al. 2000

J. Geophys. Res.

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Abstract. Rate coefficients for the reaction C2H + C2H2 -• C4H2 + H are measured at 90 and 120 K by using a new pulsed Laval nozzle apparatus equipped with laser ionization, time-of-flight mass spectrometric detection. The C2 H radicals are generated by 193 nm laser photolysis of C2H2, and the reaction product, C4H2, is directly detected by single-photon ionization at 118 nm (ls0.5 • eV). Rate coefficients of (2.7 _+ 0.5) x 10-•0 and Since there are relatively few measurements of rate coefficients at temperatures relevant for the outer planets, the models of the planetary atmospheres require extrapolations from higher-temperature rate data. The rate coefficient values presently used in the models of the planetary atmospheres are often in error by an order of magnitude, even before extrapolation to lower temperatures, and at the lowest temperatures of 70 K on Titan, the actual rate coefficients may differ even more.Currently, the Laval nozzle expansion type apparatus is the only method capable of measuring low-temperature rate constants without condensation of the reactants. The Laval nozzle produces a gas flow that is uniform in velocity, density, and temperature and endures for several tens of centimeters and some hundreds of microseconds downstream of the nozzle exit.In this paper we report the first rate coefficient data obtained with a new pulsed Laval nozzle apparatus designed to study neutral radical reaction kinetics. The apparatus incorporates vacuum ultraviolet laser ionization, time-of-flight mass spectrometer detection. The measurements presented here are the first direct kinetic measurements that monitor the reaction products. Therefore we can not only measure rate constants but also identify the reaction products of the low-temperature chemistry.In the present study, we report measurements of the rate coefficients for (R1) at 90 and 120 K using the pulsed Laval nozzle apparatus, and we probe neutral radical products directly using single-photon ionization at 118 nm (10.5 eV). The results show a marked increase in the reaction rate coefficient 15,085

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Rate Coefficients of C₂H with C₂H₄, C₂H₆, and H₂ from 150 to 359 K

Cited by 75 publications

References 29 publications

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

Formation of simple organic molecules in inner T Tauri disks

Direct measurements of rate coefficients for the reaction of ethynyl radical (C₂H) with C₂H₂ at 90 and 120 K using a pulsed Laval nozzle apparatus

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Rate Coefficients of C2H with C2H4, C2H6, and H2 from 150 to 359 K

Cited by 75 publications

References 29 publications

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

Formation of simple organic molecules in inner T Tauri disks

Direct measurements of rate coefficients for the reaction of ethynyl radical (C2H) with C2H2 at 90 and 120 K using a pulsed Laval nozzle apparatus

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Rate Coefficients of C₂H with C₂H₄, C₂H₆, and H₂ from 150 to 359 K

Direct measurements of rate coefficients for the reaction of ethynyl radical (C₂H) with C₂H₂ at 90 and 120 K using a pulsed Laval nozzle apparatus