Insights on the aromaticity of imidazolylidene carbenes by means of DFT calculations

The effect of sixteen substituents on the structure of imidazolylidene carbenes was studied using five aromaticity criteria: Singlet-Triplet energy gap (STG), a modified version of the Harmonic Oscillator Model of Aromaticity (HOMA) geometric descriptor, the Aromatic Stabilization Energy (ASE) based on the energy of hydrogenation of the carbenes, the Nuclear Independent Chemical Shift (NICS) and the Aromatic Fluctuation Index (FLU). These calculations were conducted at the B2PLYP/aug-cc-pVDZ//B3LYP/6-311+G^∗ level of theory. Even if these five descriptors are intended to quantify the “aromaticity” of the substituted carbenes, the correlation between them is far from being statistically significant. After excluding some “outliers”, a detailed analysis reveals three important correlations: (i) STG and ASE, (ii) HOMA and ASE and (iii) NICS(1)_zz and FLU. The first one supports the well-known connection between the STG and the energy of hydrogenation of carbenes. The second is strongly influenced by the interplay between energetic and “steric” factors associated with each substituent group. The last one is clearly due to the extent of the electronic delocalization in these heterocyclic compounds.