Furfural: The Unimolecular Dissociative Photoionization Mechanism of the Simplest Furanic Aldehyde

The unimolecular dissociation reactions of energy-selected furfural cations have been studied by imaging photoelectron photoion coincidence spectroscopy at the vacuum-ultraviolet (VUV) beamline of the Swiss Light Source. In the photon energy range of 10.9–14.5 eV, furfural ions decay by numerous fragmentation channels. Modeling the breakdown diagram yielded the 0 K appearance energies of 10.95 ± 0.10, 11.16, and 12.03 eV for the c-C4H3O–CO+ (m/z = 95), c-C4H4O+ (m/z = 68), and c-C3H3 + (m/z = 39) fragment ions, respectively, formed by parallel dissociation channels. An internal conversion from the A″ to the A′ electronic state via a conical intersection takes place along the reaction coordinate in the case of the H-loss channel (c-C4H3O–CO+ formation). Quantum chemical calculations and experimental results confirmed a fast conversion to the A′ state and that the rate-determining step is a tight transition state on the potential energy surface. Appearance energies were also derived for the sequential dissociation products from the furan cation, c-C4H4O+, for the formation of CH2CO+ (m/z = 42), C3H4 + (m/z = 40), and CHO+ (m/z = 29) at 12.81, 12.80, and 13.34 eV, respectively. Statistical rate theory modeling of the breakdown diagram can also be used to predict the fractional ion abundances and thermal shifts in mass spectrometric pyrolysis studies to help assigning the m/z channels either to ionization of the neutrals or to dissociative ionization processes, with potential use for combustion diagnostics. The cationic geometry optimizations yielded functional-dependent spurious DFT minima and a deviating planar MP2 optimized geometry, which are briefly discussed.