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Letter

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

Nicholas S. Shuman, Abneesh Srivastava,^† Carl A. Picconatto,^‡ David S. Danese, Paresh C. Ray,^§ and James J. Valentini*

Department of Chemistry, Columbia University, New York, New York 10027

J. Phys. Chem. A, 2003, 107 (41), pp 8380–8382

DOI: 10.1021/jp035431t

Publication Date (Web): September 11, 2003

†

Present address: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550.

‡

Present address: The MITRE Corporation, MS N230, 7515 Colshire Drive, McLean, Virginia 22102.

Present address: Black Light Power Inc., 493 Old Trenton Road, Cranbury, New Jersey 08512.

Corresponding author. E-mail: jjv1@chem.columbia.edu.

Abstract

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.