Electronic parameter for N-heterocyclic carbenes based on core ionization energies (X-ray photoelectron spectroscopy)

The electronic structures of stable, singlet N-heterocyclic carbenes (NHC) have been studied via calculated K-shell (C1s and N1s) vertical, ionization energies. We propose an electronic parameter NEP (Novak electronic parameter) which can describe electronic structure properties of free NHC molecules. NEP is derived from K-shell core ionization energies for C and N atoms of carbene moiety which are deducible from X-ray photoelectron spectroscopy. NEP allows estimating σ-electron donating and π-electron accepting abilities NHC. The carbenes studied include classical and non-classical NHC. The calculations were performed using GW method implemented in NWChem software. We pay special attention to the effects of alkylation, aromatic ring substitution and ring enlargement on NHC electronic structure. • Electronic structures of N-heterocyclic carbenes (NHC). • Calculation of core-electron binding energies for K-shell electrons of C and N. • Estimation of electron donor and electron acceptor properties of NHC. • Study of influence of heterocyclic ring substitution on electron donor and electron acceptor properties. • Proposition of new electron parameter for NHC.