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.

Dissociative ionisation of adamantane: a combined theoretical and experimental study

“…This hints to a common rate limiting transition state for at least some of the channels, as was observed in the past for other systems as well. 38,39 A summary of the parallel dissociation channels and the accompanying co-fragments is shown in Table 1.…”

Electronic Structure and Excited‐State Dynamics of an Arduengo‐Type Carbene and its Imidazolone Oxidation Product

“…A mass spectrum of the isolated carbene ( m / z= 180) is depicted in Figure S1 (Supporting Information), showing some dissociative photoionization. Loss of isobutene (C 4 H 8 ) is the most important fragmentation pathway, indicating a behavior similar to i Pr 2 Im …”

“…examined in a synchrotron radiation (SR) study the dissociative photoionization of an iso ‐propyl substituted NHC and found two parallel dissociation pathways in the cation. Furthermore, they confirmed that ionization to the cationic ground state takes place from the in‐plane σ‐orbital at the carbene center . In addition, they investigated the ring‐expansion reaction of 1,3‐di‐methyl‐imidazoline‐2‐ylidene .…”

Dissociative Ionization and Thermal Decomposition of Cyclopentanone

“…This excess energy and the original thermal energy of the sample was available for fragmentation. When studying dissociative photoionization processes, the threshold ionization fractional parent and daughter ion abundances were plotted in the breakdown diagram, which often contains clues as to the fragmentation mechanism, and can be modeled quantitatively using statistical thermodynamics approaches to determine accurate dissociative photoionization thresholds . Furthermore, owing to the long acceleration region and low extraction field, it took several microseconds for the photoions to reach their terminal velocity in the mass spectrometer.…”