Crossed-beam reaction of carbon atoms with hydrocarbon molecules. III: Chemical dynamics of propynylidyne (l-C{sub 3}H; X{sup 2}{Pi}{sub j}) and cyclopropynylidyne (c-C{sub 3}H; X{sup 2}B{sub 2}) formation from reaction of C({sup 3}P{sub j}) with acetylene, C{sub 2}H{sub 2}(X{sup 1}{Sigma}{sub g}{sup +})
- Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Division, Berkeley National Laboratory, Berkeley, California 94720 (United States)
The reaction between ground state carbon atoms, C({sup 3}P{sub j}), and acetylene, C{sub 2}H{sub 2}(X{sup 1}{Sigma}{sub g}{sup +}), is studied at three collision energies between 8.8 and 45.0 kJmol{sup {minus}1} using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C{sub 3}H at m/e=37 are recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions decreasingly forward scattered with respect to the carbon beam as the collision energy rises from 8.8 to 28.0 kJmol{sup {minus}1}, and isotropic at 45.0 kJmol{sup {minus}1}. Reaction dynamics inferred from the experimental data and {ital ab initio} calculations on the triplet C{sub 3}H{sub 2} and doublet C{sub 3}H potential energy surface suggest two microchannels initiated by addition of C({sup 3}P{sub j}) either to one acetylenic carbon to form s-{ital trans} propenediylidene or to two carbon atoms to yield triplet cyclopropenylidene via loose transition states located at their centrifugal barriers. Propenediylidene rotates around its B/C axis and undergoes [2,3]-H-migration to propargylene, followed by C{endash}H bond cleavage via a symmetric exit transition state to l-C{sub 3}H(X{sup 2}{Pi}{sub j}) and H. Direct stripping dynamics contribute to the forward-scattered second microchannel to form c-C{sub 3}H(X{sup 2}B{sub 2}) and H. This contribution is quenched with rising collision energy. The explicit identification of l-C{sub 3}H(X{sup 2}{Pi}{sub j}) and c-C{sub 3}H(X{sup 2}B{sub 2}) under single collision conditions represents a one-encounter mechanism to build up hydrocarbon radicals in the interstellar medium and resembles a more realistic synthetic route to interstellar C{sub 3}H isomers than hitherto postulated ion{endash}molecule reactions. (Abstract Truncated)
- Research Organization:
- Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 474830
- Journal Information:
- Journal of Chemical Physics, Vol. 106, Issue 5; Other Information: PBD: Feb 1997
- Country of Publication:
- United States
- Language:
- English
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