Hannover 2013 – scientific programme

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MO: Fachverband Molekülphysik

MO 26: Electronic Spectroscopy II

MO 26.3: Talk

Thursday, March 21, 2013, 14:30–14:45, F 107

Unimolecular Reaction Mechanism of an Imidazolin-2-ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo-type Carbene — •Patrick Hemberger — Paul Scherrer Institut, Villigen, Switzerland

The photoionization and dissociative photoionization of 1,3-di-iso-propylimidazolin-2-ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with VUV synchrotron radiation. Only 0.5 eV above the adiabatic ionization energy the carbene fragments, yielding propene or a methyl radical in a parallel dissociation. Both reaction channels appear at almost the same photon energy, suggesting that they share the transition state. This is confirmed by calculations, which reveal the rate determining step as hydrogen migration from the iso-propyl group to the carbene 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 for this reaction. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by H-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. iPEPICO is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.