title: Intramolecular C-N bond activation and ring-expansion reactions of N-heterocyclic carbenes.
abstract: Intramolecular ring-expansion reactions (RER) of the N-heterocyclic carbene 1,3-dimethylimidazolin-2-ylidene were observed upon vacuum ultraviolet (VUV) photoexcitation. Similarly to RERs reported in the solvent phase, for the reaction of NHCs with main-group-element hydrides, hydrogen transfer to the NHC carbon atom is the crucial initial step. In an ionization-mediated protonation, 1,3-dimethylimidazolin-2-ylidene forms an imidazolium ion, which is the rate-limiting step on the pathway to two six-membered ring products, namely, methylpyrimidinium and -pyrazinium ions. To unravel the reaction path, we have used imaging photoelectron photoion coincidence spectroscopy with VUV synchrotron radiation, as well as high-level composite method calculations. Similarities and differences between the mechanism in the gas phase and in the condensed phase are discussed.
authors.affiliation: Molecular Dynamics Group, Paul Scherrer Institute, WSLA/115, 5232 Villigen (Switzerland) http://www.psi.ch/sls/vuv/. patrick.hemberger@psi.ch.
authors.forename: Patrick
authors.initials: P
authors.lastname: Hemberger
authors.forename: Andras
authors.initials: A
authors.lastname: Bodi
authors.forename: Johannes H J
authors.initials: JH
authors.lastname: Berthel
authors.forename: Udo
authors.initials: U
authors.lastname: Radius
keywords: carbenes; internal energy selection; reaction mechanisms; threshold photoionization; unimolecular ring expansion