Ring opening dissociation of cationic twisted conformer by conformer-specific cyclopentanone photoionization

We investigated the dissociation processes of a cationic conformer, induced by conformer-specific photoionization of cyclopentanone (CP) using a one-photon vacuum ultraviolet (VUV) laser pulse of energy in the range 9.24-9.92 eV for a few nanoseconds, generated by four-wave difference frequency mixing in a Kr cell. The adiabatic ionization energy of the CP was accurately determined to be 9.2697 ± 0.0009 eV, based on the VUV photoionization efficiency curve obtained using high-resolution VUV-photoionization time-of-flight (TOF) mass spectroscopy. The constructed potential energy contours, associated with the twisting and out-of-plane motions in the S₀ and D₀ states, revealed that the ionization energy value corresponded to a twisted conformer with C₂ symmetry at the global minimum. Subsequently, the low photon energy above the ionization onset of the twisted conformer in the CP led to C₂H₄ elimination, producing a C₃H₄O⁺ fragment directly prior to CO elimination for the C₄H₈⁺ fragment. The appearance energies for the C₃H₄O⁺ and C₄H₈⁺ were determined to be 9.7068 ± 0.0017 eV and 9.7483 ± 0.0017 eV, respectively, by measuring the fragmentation yield curves for two fragments analyzed in the TOF mass spectra. The formation enthalpy for each fragment ion at 0 K, evaluated using the measured and thermochemical data, enabled the realization of plausible structures for the produced fragment ions. Consequently, based on the results of the quantum chemical calculation on the dissociation processes of the twisted CP cation (t-CP⁺), we suggest that the fragmentation processes to C₃H₄O⁺ and C₄H₈⁺ correspond to the methylketene and (E)-2-butene cations, respectively.