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Competitive Bond Rupture in the Photodissociation of Bromoacetyl Chloride and 2- and 3-Bromopropionyl Chloride: Adiabatic versus Diabatic Dissociation

Authors

  • Ming-Yi Hsu,

    1. Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
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    • These authors contributed equally to this work.

  • Dr. Po-Yu Tsai,

    1. Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
    2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106 (Taiwan)
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    • These authors contributed equally to this work.

  • Zheng-Rong Wei,

    1. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071 (People's Republic of China)
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  • Meng-Hsuan Chao,

    1. Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
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  • Prof. Bing Zhang,

    1. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071 (People's Republic of China)
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  • Prof. Toshio Kasai,

    1. Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
    2. Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043 (Japan)
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  • Prof. King-Chuen Lin

    Corresponding author
    1. Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
    2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106 (Taiwan)
    • Department of Chemistry, National Taiwan University, Taipei 106 (Taiwan)
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Abstract

Competitive bond dissociation mechanisms for bromoacetyl chloride and 2- and 3-bromopropionyl chloride following the 1[n(O)→π*(C[DOUBLE BOND]O)] transition at 234–235 nm are investigated. Branching ratios for C[BOND]Br/C[BOND]Cl bond fission are found by using the (2+1) resonance-enhanced multiphoton ionization (REMPI) technique coupled with velocity ion imaging. The fragment branching ratios depend mainly on the dissociation pathways and the distances between the orbitals of Br and the C[DOUBLE BOND]O chromophore. C[BOND]Cl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(C[DOUBLE BOND]O) and np(Cl)σ*(C[BOND]Cl) bands. In contrast, C[BOND]Br bond fission is subject to much weaker coupling between n(O)π*(C[DOUBLE BOND]O) and np(Br)σ*(C[BOND]Br). Thus, a diabatic pathway is preferred for bromoacetyl chloride and 2-bromopropionyl chloride, which leads to excited-state products. For 3-bromopropionyl chloride, the available energy is not high enough to reach the excited-state products such that C[BOND]Br bond fission must proceed through an adiabatic pathway with severe suppression by nonadiabatic coupling. The fragment translational energies and anisotropy parameters for the three molecules are also analyzed and appropriately interpreted.

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