The dissociation dynamics and thermochemistry of the acrolein ion studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation dynamics of the acrolein ion (CH2CHCHO.+). The two lowest energy dissociation channels to C2H4.+ + CO and C3H3O+ + H-. are observed at photon energies between 10.0 and 12.0 eV. The C2H4.+ ion time-of-flight distributions exhibit characteristics of a two-component reaction rate. A three-well-two-channel model is proposed to explain the multi-component dissociation rate. The simulation that fits both the time-of-flight distributions and the breakdown diagram shows that the slow component of the reaction rate for C2H4.+ production is dominantly caused by tunneling through the isomerization barrier connecting the acrolein ion (A) and the distonic ion,(CH2CH2CO+)-C-. (B). After being produced, only small amounts of B isomerize to the lowest energy conformer, the methylketene ion (C). The energy barrier heights of the isomerization from A to B and the C3H3O+ ion production are 0.87 +/- 0.02 and 0.92 +/- 0.02 eV, respectively. The 0 K appearance energy of the C3H3O+ ion is determined to be 11.03 +/- 0.02 eV. Using the acrolein heat of formation of -69 +/- 100/mol, the 298 K heat of formation of the CH2CHCO+ ion is determined to be 783 +/- 10 kJ/mol. (C) 2002 Elsevier Science B.V. All rights reserved.