title: Unimolecular reaction mechanism of an imidazolin-2-ylidene: an iPEPICO study on the complex dissociation of an Arduengo-type carbene.
abstract: The photoionization and dissociative photoionization of Im(iPr)2, 1,3-diisopropylimidazolin-2-ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone-pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IEad =7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate-determining step as hydrogen-atom migration from the isopropyl group to the carbene carbon center forming a resonance-stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene-loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance-stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate-sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.
authors.affiliation: Molecular Dynamics Group, Paul Scherrer Institut, 5234 Villigen PSI, Switzerland. patrick.hemberger@psi.ch
authors.forename: Patrick
authors.initials: P
authors.lastname: Hemberger
authors.forename: Andras
authors.initials: A
authors.lastname: Bodi
authors.forename: Thomas
authors.initials: T
authors.lastname: Gerber
authors.forename: Max
authors.initials: M
authors.lastname: Würtemberger
authors.forename: Udo
authors.initials: U
authors.lastname: Radius