Bifurcated dissociative photoionization mechanism of acetic acid anhydride revealed by imaging photoelectron photoion coincidence spectroscopy

Abstract

The fragmentation processes of internal energy selected acetic acid anhydride cations, Ac₂O⁺, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of CH₃C(O)OCO⁺ (m/z = 87) by a CH₃-loss. The 0 K appearance energy (E₀) was determined to be 10.289 ± 0.010 eV, in excellent agreement with the G4-calculated 10.28 eV transition state (TS) energy. Based on the thermochemical onset of CH₃C(O)OCO⁺, a reverse barrier of 40 kJ mol⁻¹ was found. The second dissociation channel leads to the formation of the acetyl cation, CH₃CO⁺ (m/z = 43). The appearance of trace amounts of acetone in the mass spectra, statistical modeling of the branching ratios, and quantum chemical calculations point to the existence of a post-transition-state bifurcation on the potential energy surface and a single TS leading to multiple products. That is, at higher excess energies, the CH₃-group may swerve back along an orbiting pathway to form the acetone cation by CO₂-loss instead of leaving directly. The acetone cation thus formed is then energetic enough to lose a methyl group and yield the acetyl cation at a phenomenological E₀ = 10.316 ± 0.015 eV. The acetyl cation, which dominates the breakdown diagram up to 16 eV photon energy, is also formed by sequential CO₂-loss from the CH₃C(O)OCO⁺ intermediate at E₀ = 10.53 ± 0.03 eV. The CH₃⁺ (m/z = 15) fragment ion appears above 13 eV photon energy. This species can be produced directly from the parent ion or via two sequential dissociation channels: by acetyl radical loss from the acetone cation or CO-loss from the acetyl cation.