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Title
| - Reaction Dynamics on the Formation of Styrene: A CrossedMolecular Beam Study of the Reaction of Phenyl Radicals withEthylene
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Abstract
| - The reaction dynamics of phenyl radicals (C6H5) with ethylene (C2H4) and D4-ethylene (C2D4) wereinvestigated at two collision energies of 83.6 and 105.3 kJ mol-1 utilizing a crossed molecular beamsetup. The experiments suggested that the reaction followed indirect scattering dynamics via complexformation and was initiated by an addition of the phenyl radical to the carbon−carbon double bond ofthe ethylene molecule forming a C6H5CH2CH2 radical intermediate. Under single collision conditions,this short-lived transient species was found to undergo unimolecular decomposition via atomic hydrogenloss through a tight exit transitions state to synthesize the styrene molecule (C6H5C2H3). Experimentswith D4-ethylene verified that in the corresponding reaction with ethylene the hydrogen atom was trulyemitted from the ethylene unit but not from the phenyl moiety. The overall reaction to form styrene plusatomic hydrogen from the reactants was found to be exoergic by 25 ± 12 kJ mol-1. This study providessolid evidence that in combustion flames the styrene molecule, a crucial precursor to form polycyclicaromatic hydrocarbons (PAHs), can be formed within a single neutral−neutral collision, a long-standingtheoretical prediction which has remained to be confirmed by laboratory experiments under well-definedsingle collision conditions for the last 50 years.
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