Hemberger P Bodi A Berthel JH Radius U 2015 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. https://sonar.ch/global/documents/278279 eng info:eu-repo/semantics/altIdentifier/doi/10.1002/chem.201406036 info:eu-repo/semantics/altIdentifier/pmid/25430962 Chemistry (Weinheim an der Bergstrasse, Germany). - 2015 carbenes internal energy selection reaction mechanisms threshold photoionization unimolecular ring expansion Intramolecular C-N bond activation and ring-expansion reactions of N-heterocyclic carbenes. http://purl.org/coar/resource_type/c_6501