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  • 1
    Electronic Resource
    Electronic Resource
    Stripping dynamics in the reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene—production of propargyl and methylpropargyl radicals (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1318-1324 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene were studied at three collision energies between 48 and 104 kJmol−1 employing the crossed molecular beam technique. Forward-convolution fitting of our data combined with electronic structure calculations suggests that the reactions proceed via stripping dynamics. Extremely short-lived allene and 1,2-butadiene intermediates decompose via atomic hydrogen emission to yield propargyl and methylpropargyl radicals, respectively. These production routes are of potential importance to form benzene, toluene, and o-/p-xylenes in circumstellar envelopes of carbon stars and combustion flames. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1428754
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  • 2
    Electronic Resource
    Electronic Resource
    A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. I. The 1,3-butadiene molecule, H2CCHCHCH2(X1A′) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9622-9636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction between ground state carbon atoms, C(3Pj), and 1,3-butadiene, H2CCHCHCH2, was studied at three averaged collision energies between 19.3 and 38.8 kJmol−1 using the crossed molecular beam technique. Our experimental data combined with electronic structure calculations show that the carbon atom adds barrierlessly to the π-orbital of the butadiene molecule via a loose, reactantlike transition state located at the centrifugal barrier. This process forms vinylcyclopropylidene which rotates in a plane almost perpendicular to the total angular momentum vector J around its C-axis. The initial collision complex undergoes ring opening to a long-lived vinyl-substituted triplet allene molecule. This complex shows three reaction pathways. Two distinct H atom loss channels form 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″) and H2CCCC2H3(X2A″), through tight exit transition states located about 20 kJmol−1 above the products; the branching ratio of 1- versus 3-vinylpropargyl radical is about 8:1. A minor channel of less than 10% is the formation of a vinyl, C2H3(X2A′), and propargyl radical C3H3(X2B2). The unambiguous identification of two C5H5 chain isomers under single collision has important implications to combustion processes and interstellar chemistry. Here, in denser media such as fuel flames and in circumstellar shells of carbon stars, the linear structures can undergo a collision-induced ring closure followed by a hydrogen migration to cyclic C5H5 isomers such as the cyclopentadienyl radical—a postulated intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs). © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290285
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  • 3
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV. Chemical dynamics of cyanoacetylene (HCCCN; X 1Σ+) formation from reaction of CN(X 2Σ+) with acetylene, C2H2(X 1Σg+) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8656-8666 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemical reaction dynamics to form cyanoacetylene, HCCCN (X 1Σ+), via the radical–neutral reaction of cyano radicals, CN(X 2Σ+;ν=0), with acetylene, C2H2(X 1Σg+), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol−1. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e=51 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the π electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the 2A′ surface via indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond rupture through a tight transition state located 22 kJ mol−1 above the cyanoacetylene, HCCCN (X 1Σ+) and H(2S1/2) products (microchannel 1). To a minor amount (15%) trans HCCHCN shows a 1,2-H shift via a 177 kJ mol−1 barrier to form a doublet H2CCCN radical, which is 46 kJ mol−1 more stable than the initial reaction intermediate (microchannel 2). The H2CCCN complex decomposes via a rather loose exit transition state situated only 7 kJ mol−1 above the reaction products HCCCN (X 1Σ+) and H(2S1/2). In both cases the geometry of the exit transition states is reflected in the observed center-of-mass angular distributions showing a mild forward/sideways peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a solid background for the title reaction to be included in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289530
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  • 4
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X 1A′) formation from reaction of C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4994-5001 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X 1A′), via reactive collisions of the phenyl radical C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X 1A1), was used to identify the site of the carbon–hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol−1. Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the π electron density of the C1 carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C6H5)HCCCH3 intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the 2A′ surface. The phenylallene isomer channel was not observed experimentally. The dynamics of the title reaction and the identification of the phenyl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photospheres of carbon stars and oxygen poor combustion systems. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481054
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  • 5
    Electronic Resource
    Electronic Resource
    Chemical dynamics of cyclopropynylidyne (c-C3H; X2B2) formation from the reaction of C(1D) with acetylene, C2H2(X 1Σg+) (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 231-239 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction between electronically excited carbon atoms, C(1D), and acetylene was studied at two average collision energies of 45 kJ mol−1 and 109 kJ mol−1 employing the crossed molecular beam technique. The time-of-flight spectra recorded at mass to charge m/e=37(C3H+) and m/e=36(C3+) show identical patterns indicating the existence of a carbon versus atomic hydrogen exchange pathway to form C3H isomer(s); no H2 elimination to the thermodynamically favorable tricarbon channel was observed. Forward-convolution fitting of our data shows that the reaction proceeds via direct stripping dynamics on the 1A′ surface via an addition of the carbon atom to the π-orbital of acetylene to form a highly rovibrationally, short lived cyclopropenylidene intermediate which decomposes by atomic hydrogen emission to c-C3H(X 2B2). The dynamics of this reaction have important impact on modeling of chemical processes in atmospheres of comets approaching the perihelon as photolytically generated C(1D) atoms are present. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1330232
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  • 6
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of the cyano radical, CN(X 2Σ+), with methylacetylene, CH3CCH (X 1A1): Observation of cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2857-2860 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemical dynamics to form cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′), via the neutral–neutral reaction of the cyano radical, CN (X 2Σ+), with methylacetylene, CH3CCH (X 1A1), is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of the C4H3N products are recorded at m/e=65, 64, 63, and 62. The reaction of d3-methylacetylene, CD3CCH (X 1A1), with CN radicals yields reactive scattering signal at m/e=68 and m/e=67 demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the π electron density of the β carbon atom of the methylacetylene molecule to form a long lived CH3CCHCN collision complex. The latter decomposes via two channels, i.e., H atom loss from the CH3 group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479567
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  • 7
    Electronic Resource
    Electronic Resource
    Chemical dynamics of d1-methyldiacetylene (CH3CCCCD; X 1A1) and d1-ethynylallene (H2CCCH(C2D); X 1A′) formation from reaction of C2D(X 2Σ+) with methylacetylene, CH3CCH(X 1A1) (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 3488-3496 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The crossed beam reaction of the d1-ethynyl radical C2D(X 2Σ+), with methylacetylene, CH3CCH(X 1A1), was investigated at an average collision energy of 39.8 kJ mol−1. Our experimental results were combined with electronic structure calculations. The chemical reaction dynamics are indirect, involve three distinct channels, and are initiated via a barrierless addition of C2D to the acetylenic bond through long lived cis and trans CH3CCH(C2D), 1-ethynylpropen-2-yl, intermediates. The reduced cone of acceptance of the carbon atom holding the methyl group favors a carbon–carbon σ bond formation at the carbon atom adjacent to the acetylenic hydrogen atom. A crossed beam experiment of C2D with partially deuterated methylacetylene, CD3CCH, shows explicitly that the reactive intermediates decompose to form both methyldiacetylene, CD3CCCCD (channel 1, 70%–90%), and to a minor amount ethynylallene, D2CCCH(C2D) (channel 2; 10%–30%), isomers through exit transition states located 7–15 kJ mol−1 above the products. The computed reaction energies to form both isomers are −135 and −107 kJ mol−1, respectively, with respect to the separated reactants. A minor reaction pathway involves a H shift in CH3CCH(C2D) to an 1-ethynylpropen-1-yl radical which fragments to methyldiacetylene via a barrier of 8.8 kJ mol−1 (channel 3). Neither methyl group elimination nor the formation of the CC(CH3)(C2D) carbene was observed in our experiments. The experimentally observed "sideways scattering" and ab initio investigation verify our conclusions of a predominate formation of the methyldiacetylene isomer. These electronic structure calculations depict a hydrogen atom loss in the exit transition state to methyldiacetylene almost parallel to the total angular momentum vector J as found in our center-of-mass angular distribution. Since the title reaction and the corresponding reaction of the C2H radical with CH3CCH both have no entrance barriers, are exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the ethynyl versus H atom exchange suggests the formation of both isomers under single collision conditions in extraterrestrial environments such as cold, molecular clouds as well as the atmosphere of Saturn's moon Titan. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1330233
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  • 8
    Electronic Resource
    Electronic Resource
    A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. II. The dimethylacetylene molecule, H3CCCCH3(X1A1g) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9637-9648 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction of ground state carbon atoms, C(3Pj), with dimethylacetylene, H3CCCCH3, was studied at three collision energies between 21.2 and 36.9 kJmol−1 employing the crossed molecular beam approach. Our experiments were combined with ab initio and RRKM calculations. It is found that the reaction is barrierless via a loose, early transition state located at the centrifugal barrier following indirect scattering dynamics through a complex. C(3Pj) attacks the π system of the dimethylacetylene molecule to form a dimethylcyclopropenylidene intermediate either in one step via an addition to C1 and C2 of the acetylenic bond or through an addition to only one carbon atom to give a short-lived cis/trans dimethylpropenediylidene intermediates followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total angular momentum vector J. This complex fragments to atomic hydrogen and a linear 1-methylbutatrienyl radical, H2CCCCCH3(X2A″), via a tight exit transition state located about 18 kJmol−1 above the separated products. The experimentally determined exothermicity of 190±25 kJmol−1 is in strong agreement with our calculated data of 180±10 kJmol−1. The explicit verification of the carbon versus hydrogen exchange pathway together with the first identification of the H2CCCCCH3 radical represents a third pathway to form chain C5H5 radicals in the reactions of C(3Pj) with C4H6 isomers under single collision conditions. Previous experiments of atomic carbon with the 1,3-butadiene isomer verified the formation of 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″), and H2CCCC2H3(X2A″), respectively. In high-density environments such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring closure(s) and/or H atom migration(s) which can lead to the cyclopentadienyl radical. The latter is thought to be a crucial reactive intermediate in soot formation and possibly in the production of polycyclic aromatic hydrocarbon molecules in outflow of carbon stars. Likewise, a H atom catalyzed isomerization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290286
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  • 9
    Electronic Resource
    Electronic Resource
    A combined crossed-beam, ab initio, and Rice–Ramsperger–Kassel– Marcus investigation of the reaction of carbon atoms C(3Pj) with benzene, C6H6(X 1A1g) and d6-benzene, C6D6(X 1A1g) (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3248-3262 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of atomic carbon, C(3Pj), with benzene, C6H6(X 1A1g), and with d6-benzene, C6D6(X 1A1g) were investigated at twelve collision energies between 8.8 and 52.5 kJ mol−1 using the crossed molecular beams technique. Forward-convolution fitting of the data, high-level electronic structure calculations, and Rice–Ramsperger–Kassel–Marcus (RRKM) investigations on the singlet and triplet C7H6/C7D6 potential energy hyperface suggest that at low collision energies the chemical reaction dynamics are indirect and dominated by large impact parameters. As the collision energy increases, smaller impact parameters become more important, and the chemical dynamics is increasingly direct. At all collision energies, the reaction proceeds on the triplet surface via a barrierless addition of the carbon atom to form a bicyclic intermediate followed by ring opening of the initial collision complex to a seven-membered ring intermediate (cycloheptatrienylidene). The latter decomposes without exit barrier to the thermodynamically less stable 1,2-didehydrocycloheptatrienyl radical, C7H5(X 2B1)+H, and its deuterated C7D5(X 2B1)+D counterpart. The formation of a C7D6 adduct is observed as a second channel. The barrierless route for the destruction of benzene can help to model important pathways for the synthesis of higher polycyclic aromatic hydrocarbon derivatives in the interstellar medium, in outflows of dying carbon stars, in hydrocarbon-rich planetary atmospheres, as well as in oxygen-poor combustion flames. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1418744
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  • 10
    Electronic Resource
    Electronic Resource
    A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. III. 1,2-butadiene, H2CCCH(CH3) (X 1A′)—a non-Rice–Ramsperger–Kassel–Marcus system? (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5107-5116 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Crossed molecular beam experiments were conducted to investigate the reaction of ground state carbon atoms, C(3Pj), with 1,2-butadiene, H2CCCH(CH3) (X 1A′), at three collision energies of 20.4 kJ mol−1, 37.9 kJ mol−1, and 48.6 kJ mol−1. Ab initio calculations together with our experimental data reveal that the reaction is initiated by a barrier-less addition of the carbon atom to the π system of the 1,2-butadiene molecule. Dominated by large impact parameters, C(3Pj) attacks preferentially the C2–C3 double bond to form i1 (mechanism 1); to a minor extent, small impact parameters lead to an addition of atomic carbon to the C1–C2 bond yielding i2 (mechanism 2). Both cyclic intermediates i1 and i2 ring open to triplet methylbutatriene complexes i3′ (H2CC*CCH(CH3)) and i3″, (H2CCC*CH(CH3)); C* denotes the attacked carbon atom. i3′ is suggested to decay nonstatistically prior to a complete energy randomization via atomic hydrogen loss forming 1- and 4-methylbutatrienyl CH3CCCCH2 (X 2A″) and HCCCCH(CH3) (X 2A″), respectively. The energy randomization in i3″ is likely to be complete. This isomer decomposes via H atom loss to 3-vinylpropargyl, H2CCCC2H3(X 2A″), as well as 1- and 4-methylbutatrienyl radicals. In high-density environments such as the inner regions of circumstellar envelopes of carbon stars and combustion flames, these linear C5H5 isomers might undergo collision induced isomerization to cyclic structures like the cyclopentadienyl radical. This isomer is strongly believed to be a key intermediate involved in the production of polycyclic aromatic hydrocarbon molecules and soot formation. These characteristics make the reactions of atomic carbon with C4H6 isomers compelling candidates to form C5H5 isomers in the outflow of AGB stars and oxygen-deficient hydrocarbon flames. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1385794
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