A Reexamination of the Dissociative Photoionization of Isoprene

We explore the ionization of isoprene and the unimolecular dissociation chemistry of its radical cation employing imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The origin band of the threshold photoelectron spectrum (TPES) indicates an ionization energy of 8.87 ± 0.01 eV in agreement with the calculated CBS-QB3 values. Franck-Condon (FC) simulations rationalize the vibrational structure of the ground-state band in the TPES, and Outer Valence Green's Function calculations for the excited electronic states are compared with the excited-state bands in the TPES. Four major fragment ions were observed at photon energies above 10.5 eV: C₅H₇⁺ (m/z 67, loss of H), C₄H₅⁺ (m/z 53, loss of CH₃), C₃H₆⁺ (m/z 42, loss of C₂H₂), and C₃H₄⁺ (m/z 40, loss of C₂H₄). Dissociative ionization paths were calculated at the CBS-QB3//B3LYP/6-311+G(d,p) level of theory, The literature mechanisms for CH₃ and C₂H₄ losses cannot be reconciled with the experimental results and the reaction paths and energetics are revisited. Methyl radical loss is found to form the methylcyclopropenyl cation, while ethylene loss results in the allene radical cation rather than ionized propyne.