A crossed molecular beams study on the formation and energetics of the resonantly stabilized free i-C4H3(X2A′) radical and its isotopomers

The chemical dynamics of the formation of the i-C4H3(X2A′) radical together with its partially deuterated isotopomers were investigated in eight crossed molecular beams experiments of dicarbon molecules in their X1Σg+ electronic ground and in first excited a3Πu state with (partially deuterated) ethylene at collision energies between 12.1 and 40.9 kJ mol−1. The center-of-mass angular distributions suggest that the reaction dynamics on the singlet and triplet surfaces are indirect and involve butatriene reaction intermediates. In case of the C2/C2H4 reaction, the 'symmetric' singlet butatriene intermediate would lead solely to a symmetric center-of-mass angular distribution; however, in combination with isotopically labeled reactants, we deduced that triplet butatriene intermediates excited to B/C like rotations likely account for the observed asymmetries in the center-of-mass angular distributions at higher collision energies. The translational energy distributions are also indicative of the involvement of both the triplet and singlet surfaces which lead both to the i-C4H3(X2A′) radicals through lose (singlet) and tight (triplet) exit transitions states. Also, our experiments helped to determine the enthalpy of formation of the i-C4H3(X2A′) radical to be about 504 ± 10 kJ mol−1 in good agreement with previous computational studies suggesting 498-499 kJ mol−1. The explicit identification of the resonance-stabilized i-C4H3(X2A′) radical proposes that the reaction of dicarbon with ethylene can lead to formation of i-C4H3(X2A′) in combustion flames; the n-C4H3(X2A′) isomer is not formed in this reaction. This conclusion correlates nicely with Hansen's et al. flame experiments at the advanced light source observing only the i-C4H3(X2A′) radical in hydrocarbon flames.