N-Heterocyclic Carbenes: A Βenchmark Study on their Singlet-Triplet Energy Gap as a Critical Molecular Descriptor.

N-Heterocyclic Carbenes (NHC) are used extensively in modern chemistry and materials science. The in-depth understanding of their electronic structure and of their metal complexes remains an important topic of research and of experimental and theoretical interest. Herein, in contrast to the commonly used HOMO-LUMO(H-L) gap or Singlet-Triplet(S-T) vertical gap due to the simplicity of their calculations, which occasionally leads to controversial results, we establish the adiabatic singlet-triplet gap as a superior, quantifiable critical descriptor, sensitive to the nature and the structural diversity of the NHCs, for a successful rationalization of experimental observations and computationally extracted trends. The choice was supported by a benchmark study on the electronic structures of NHCs, using high-level ab initio methods, i.e., CASSCF, NEVPT2, MRCISD, DLNPO-CCSD(T) along with DFT methods such as BP86, M06, and M06-L, B3LYP, PBE0, TPSSh, CAM-B3LYP, and B2PLYP. It is found that the adiabatic S-T gap of the NHCs or their complexes is more sensitive to the nature and structural features of the NHCs and not to the type of the used methodology, or the used functional within DFT, and thus fulfils to an extent the requirements for a critical parameter.