Crossed-Beam Imaging of the H Abstraction Channel in the Reaction of CN with 1-Pentene

نویسندگان

  • Armando D. Estillore
  • Laura M. Visger
  • Ralf I. Kaiser
  • Arthur G. Suits
چکیده

The reaction of the cyano radical with 1-pentene has been studied in crossed molecular beams experiments at a range of collision energies from 5 to 9 kcal/mol. In these studies, the pentenyl radical product is probed using singlephoton ionization at 157 nm, and the product flux contourmap is obtained directly using DC slice imaging. The results clearly demonstrate the presence of the hydrogen abstraction channel in this reaction, which has largely been neglected in studies of analogous systems. The reaction likely gives rise to the resonantly stabilized C5H9 radical. The yield of this channel is estimated by comparison to the reaction of the cyano radical with n-pentane. The results have implications for hydrocarbon growth and nitrile incorporation in formation of haze particles on Saturn's moon, Titan. SECTION Dynamics, Clusters, Excited States T he cyano radical (CN(X 2Σþ)) is an extremely reactive species that can abstract hydrogen from saturated hydrocarbons without a barrier and add to π bonds of unsaturated hydrocarbons to form strongly bound intermediates also without a barrier. Thesemay then be stabilized by collision or decay through a variety of pathways, predominantly via atomic hydrogen or alkyl radical elimination, forming nitriles. Owing to its ability to react without barriers and its relative abundance in the atmosphere of Titan, the cyano radical plays a key role in the low-temperature chemistry on Titan, including hydrocarbon growth processes and nitrogen incorporation into the haze particles that are a dominant feature of its atmosphere. Cyano radical reactions with unsaturated hydrocarbons have been examined in a variety of kinetics studies, including someusing supersonic flows at temperatures down to 20K. The reaction rates are found to be extremely fast and show a negative temperature dependence, with rates of 2.5( 0.2 10 cmmolecule s reported for the reaction of cyano with ethylene at 298 K. Two recent kinetics studies are particularly noteworthy in that products were detected for reactionwith propene. The Seakins group at Leeds detected H atom elimination following CN addition and determined absolute yields for this channel; then, they compared these to a master equation calculation using a model potential surface. Trevitt et al. also studied the CNþ propene reaction in a flow reactor using tunable synchrotron radiation to obtain isomer-specific detection of products. They detected H and CH3 elimination and identified distinct product isomers but saw no evidence for formation of the allyl radical when the ionization energy was tuned above the threshold for detection of allyl but below that for ionization of other products. In addition to these kinetic studies, a number of crossed-beam studies for cyano radical plus unsaturated hydrocarbons have been reported, notably from the Kaiser laboratory. Also, Huang et al. used ab initio theory at the CBS-CCSD(T) level to examine the possible product channels for crossed beam results at a detailed level for this reaction. They found slight barriers to H abstraction but acknowledged that these may disappear at higher levels of theory. Gannon et al. used an IRC search at the UMP2/6311þG(d,p) level and concluded there was no barrier. In any case, the traditional crossed-beam approach has little sensitivity to the abstraction reaction, owing to unfavorable kinematics and background at the product masses; therefore, this pathway has often been omitted in the analysis. We have been using crossed-beam slice imaging with single-photon ionization at 157 nm as a sensitive and direct probe of crossed-beam reactive scattering for a variety of reactions. We earlier reported cyano radical reaction with n-alkanes and have recently embarked on systematic studies of chlorine atoms with alkanes and alkenes. In the present study, we investigate the reaction between the cyano radical and 1-pentene at four collision energies from 5 to 9 kcal/mol. The sliced images of the C5H9 radical, corrected for thedensity-to-fluxeffects, are shown inFigure 1 with the most probable Newton diagrams superimposed. As discussed in the Experimental Section, the photochemistry background interferes in the vicinity of the pentene beam, so that portion of the image is suppressed. From these images, we extract the center-of-mass translational energy and angular distributions shown in Figure 2. The translational energy Received Date: June 24, 2010 Accepted Date: July 22, 2010

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تاریخ انتشار 2010