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Issue 46, 2011
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Detection of pentatetraene by reaction of the ethynyl radical (C2H) with allene (CH2[double bond, length as m-dash]C[double bond, length as m-dash]CH2) at room temperature

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Abstract

The reaction of ethynyl radical (C2H) with allene (C3H4) at room temperature is investigated using an improved synchrotron multiplexed photoionization mass spectrometer (MPIMS) coupled to tunable vacuum ultraviolet (VUV) synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory (LBNL). The orthogonal-accelerated time-of-flight mass spectrometer (OA-TOF) compared to the magnetic sector mass spectrometer used in a previous investigation of the title reaction (Phys. Chem. Chem. Phys., 2007, 9, 4291) enables more sensitive and selective detection of low-yield isomeric products. The C5H4 isomer with the lowest ionization energy, pentatetraene, is now identified as a product of the reaction. Pentatetraene is predicted to be formed based on recent ab initio/RRKM calculations (Phys. Chem. Chem. Phys., 2010, 12, 2606) on the C5H5 potential energy surface. However, the computed branching fraction for pentatetraene is predicted to be five times higher than that for methyldiacetylene, whereas experimentally the branching fraction of pentatetraene is observed to be small compared to that of methyldiacetylene. Although H-atom assisted isomerization of the products can affect isomer distribution measurements, isomerization has a negligible effect in this case. The kinetic behavior of the several C5H4 isomers is identical, as obtained by time-dependent photoionization spectra. Even for high allene concentrations (and hence higher H-atom concentrations) no decay of the pentatetraene fraction is observed, indicating that H-assisted isomerization of pentatetraene to methyldiacetylene does not account for the difference between the experimental data and the theoretical branching ratios.

Graphical abstract: Detection of pentatetraene by reaction of the ethynyl radical (C2H) with allene (CH2 [[double bond, length as m-dash]] C [[double bond, length as m-dash]] CH2) at room temperature

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Publication details

The article was received on 14 Aug 2011, accepted on 22 Sep 2011 and first published on 14 Oct 2011


Article type: Paper
DOI: 10.1039/C1CP22609F
Citation: Phys. Chem. Chem. Phys., 2011,13, 20820-20827
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    Detection of pentatetraene by reaction of the ethynyl radical (C2H) with allene (CH2[double bond, length as m-dash]C[double bond, length as m-dash]CH2) at room temperature

    F. Goulay, S. Soorkia, G. Meloni, D. L. Osborn, C. A. Taatjes and S. R. Leone, Phys. Chem. Chem. Phys., 2011, 13, 20820
    DOI: 10.1039/C1CP22609F

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