Probing Cyclic π-Electron Delocalization in an Imidazol-2-ylidene and a Corresponding Imidazolium Salt

The extent of cyclic π-electron delocalization in the N-heterocyclic ring of an imidazol-2-ylidene (i.e., 1,3,4,5-tetramethylimidazol-2-ylidene) and its corresponding imidazolium salt (i.e., 1,3,4,5-tetramethylimidazolium chloride) has been investigated theoretically by using Bader's quantum theory of atoms in molecules (QTAIM) descriptors, delocalization indices, electron localizability indicators (ELI-Ds), and the source function tool. In addition, the experimental electron density distribution for the imidazolium salt has been obtained and analyzed from 100 K X-ray diffraction data. A significant drop is found in the ellipticity of the electron density along the C_carbene -N bond path in the imidazol-2-ylidene. This is shown to be a natural consequence of the σ lone pair of the C_carbene atom, which overwhelms the π-electron density, rather than a sign of a significantly diminished degree of π-electron delocalization in the imidazol-2-ylidene compared to its imidazolium salt. In fact, the source functions, the ELI-Ds, and the delocalization indices all probe a quite similar extent of cyclic π-electron delocalization in the N-heterocyclic rings of the two compounds.