The study of the electronic structure of some N-heterocyclic carbenes (NHCs) by variable energy photoelectron spectroscopy

The photoionization of three N-heterocyclic carbenes (NHCs) has been studied in the valence and core regions using synchrotron radiation. We observed different variations in the relative band intensities with photon energy for the NHCs in the valence ionization region. This is due to the intra-ring interactions between the C=C bond, nitrogen and carbene lone pairs in the heterocyclic ring of NHCs. In the core ionization region we observed chemical shifts which are consistent with the relative electron affinities of atoms and intramolecular electron density shifts. The core electron binding energies calculated via the unrestricted DDFT (B3LYP and M06-2X) approach are in very good agreement with the experiment. The shake-up portion of the core photoionization spectra is adequately described by the time-dependent DFT calculations relying on the CAM-B3LYP functional.