A Combined Crossed Beam and Ab Initio Investigation of the Gas Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with Propene (C3H6; X1A′): Identification of the Resonantly Stabilized Free Radicals 1- and 3-Vinylpropargyl

Abstract

The crossed molecular beam reactions of dicarbon, C2(X(1)Σg(+), a(3)Πu), with propene (C3H6; X(1)A') and with the partially deuterated D3 counterparts (CD3CHCH2, CH3CDCD2) were conducted at collision energies of about 21 kJ mol(-1) under single collision conditions. The experimental data were combined with ab initio and statistical (RRKM) calculations to reveal the underlying reaction mechanisms. Both on the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the addition of the dicarbon reactant to the carbon-carbon double bond of propene. These initial addition complexes rearrange via multiple isomerization steps leading ultimately via atomic hydrogen elimination from the former methyl and vinyl groups to the formation of 1-vinylpropargyl and 3-vinylpropargyl. Both triplet and singlet methylbutatriene species were identified as important reaction intermediates. On the singlet surface, the unimolecular decomposition of the reaction intermediates was found to be barrier-less, whereas on the triplet surface, tight exit transition states were involved. In combustion flames, both radicals can undergo a hydrogen-atom assisted isomerization leading ultimately to the thermodynamically most stable cyclopentadienyl isomer. Alternatively, in a third body process, a subsequent reaction of 1-vinylpropargyl or 3-vinylpropargyl radicals with the propargyl radical might yield to the formation of styrene (C6H5C2H3) in an entrance barrier-less reaction under combustion-like conditions. This presents a strong alternative to the formation of styrene via the reaction of phenyl radicals with ethylene, which is affiliated with an entrance barrier of about 10 kJ mol(-1).