Time-Resolved Unimolecular Dissociation of Styrene Ion. Rates and Activation Parameters

Article abstract of DOI:10.1021/ja00197a010

The rate of unimolecular dissociation of styrene ion into benzene ion plus acetylene was measured by time-resolved photodissociation at 308 nm in the ICR ion trap.A rate constant of 1.10E5 s^-1 was obtained at a total ion internal energy of 4.20 eV.Using accurate heat of formation data for the reactant and products, a 0 K reaction enthalpy of 2.42 eV was assigned.RRKM rate-energy curves were calculated for comparison with the present measurement and with previous photoionization coincidence (PEPICO) data.The shape of the RRKM curve matches experiment, andquantitative rate agreement is obtained assuming an activation energy of 2.32 eV and a tight transition state.The activation parameters derived from the activated complex, ΔS(excit.)(1000 K) = -6.4 eu and A_infinite(1000 K) = 2.3E12 s^-1, are compared with values for other ion dissociations and with neutral-molecule rearrangements and rearrangement dissociations.