An Ab Initio Investigation of Reactions of Carbon Atoms, C(³P_j), with C₂H₄and C₃H₆in the Interstellar Medium

The reactions of ground state atomic carbon C(P-3(j)) with the two simplest olefines ethylene (C2H4) and propylene (C3H6)are investigated computationally to explore the formation of carbon-bearing radicals in extraterrestrial environments. The calculations revealed that the reactions proceed on the triplet surface and are initiated by a barrier-less addition of the carbon atom to the pi -bond forming three-membered ring adducts cyclopropylidene and methylcyclopropylidene. Both complexes reside in deep potential energy wells and ring-open to allene and cis/trans 1,2-butadiene, respectively. The decomposition of these intermediates is dominated by an H atom loss to form the propargyl radical (C(P-3(j))/C2H4) as well as 1-, 3-methylpropargyl, and 1,3-butadienyl-2 radicals (C(P-3(j))/C3H6). Additionally, triplet 1,2- butadiene can lose a CH3 group to give a propargyl radical. To a minor amount, triplet allene was found to undergo a hydrogen migration followed by fragmentation of the H2CCHCH complex to acetylene (C2H2) and triplet carbene (CH2). All reaction channels have no entrance barrier, show exit barriers well below the energy of the reactant molecules, and are strongly exothermic. Therefore, binary collisions of carbon atoms with ethylene and propylene present compelling candidates to form (substituted) propargyl radicals even in cold molecular clouds. Since these radicals are suggested to play an important role in the formation of polycyclic aromatic hydrocarbons, all isomers should be incorporated in future chemical models of interstellar clouds and circumstellar envelopes around carbon stars.