Crossed molecular beams studies of phenyl radical reactions with propene and trans-2-butene.

The reactions of phenyl radicals with propene have been studied at collision energies of 84 and 108 kJ/mol using the crossed molecular beams technique. The branching ratios between methyl radical elimination forming C8H8 and H-atom elimination forming C9H10 were found to be 10 ± 1:1 at 84 kJ/mol and 3 ± 1:1 at 108 kJ/mol. By using "soft" 9.9 eV vacuum ultraviolet photoionization for product detection, we were able to observe both product channels with negligible fragmentation of C9H10 to C8H8(+). Our finding that CH3 elimination is dominant is consistent with conclusions from a recent study employing a pyrolysis molecular beam reactor using photoionization detection. However, our C8H8/C9H10 branching ratios are significantly larger than inferred from previous CMB experiments and RRKM calculations. For comparison, we have also studied the reactions of phenyl radicals with trans-2-butene at Ecoll = 97 kJ/mol. In this case, the symmetry of trans-2-butene makes both alkene addition sites chemically equivalent. The intermediate formed in the reaction with trans-2-butene is similar to the 2-carbon addition intermediate in the reaction with propene. We observed only methyl elimination in the reaction with trans-2-butene, with no evidence for H-atom elimination, consistent with conclusions that C-C bond fission is the most favorable channel in these systems. Analogies between phenyl radical reactions with propene and trans-2-butene are used to provide insight into the mechanisms in the propene reaction.