Probing the barrier for CH 2CHCO→CH 2CH+CO by the velocity map imaging method

This work determines the dissociation barrier height for C H2 CHCO→C H2 CH+CO using two-dimensional product velocity map imaging. The C H2 CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent C H2 CHCO radicals that do not dissociate to C H2 CH+CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl (P 32 2) and Cl (P 12 2) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P (ET) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the C H2 CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce C H2 CH Ċ O or Ċ H2 CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl+CO. The experimentally determined barrier for C H2 CH Ċ O→C H2 CH+CO is 21±2 kcal mol-1, and the computed energy difference between the C H2 CH Ċ O and the Ċ H2 CHCO forms of the radical gives the corresponding barrier for Ċ H2 CHCO→C H2 CH+CO to be 23±2 kcal mol-1. This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the Ċ H2 CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable C H2 CH Ċ O radicals, most with an internal energy distribution ranging from ≈3 to ≈21 kcal mol-1. We discuss the results with respect to the prior work on the O (P3) +propargyl reaction and the analogous O (P3) +allyl system. © 2005 American Institute of Physics.