Evidence of a Surprising Channeling of Ring-Opening Energy to the H2 Product in the H + c-C3H6 → H2 + C3H5 Abstraction Reaction

The product energy disposal in the abstraction reaction H + c-C3H6 → H2(v‘, j‘) + C3H5 at 1.6 eV collision energy has been characterized by quantum-state-selective, Doppler-resolved REMPI detection of the H2(v‘, j‘) product. The H2(v‘, j‘) product is observed with total energy, translational plus rotational plus vibrational, in excess of the total available energy if the C3H5 product is a cyclopropyl radical, but the total H2(v‘, j‘) energy matches the total available energy if the C3H5 product is allyl. This implies ring-opening of cyclopropyl to allyl concerted with abstraction and the deposition of a large fraction of the ring-opening energy into the H2(v‘, j‘) product. Such behavior is unprecedented and entirely unexpected. Calculations indicate that both direct H-by-H abstraction and H-addition/H2-elimination should occur too quickly to allow the isomerization to occur before product separation. Even if ring-opening did occur, the appearance of the total isomerization energy in the H2 product would seem dynamically forbidden. The reaction must follow an unanticipated and surprising path.