title: Electronic structure trends in N-heterocyclic carbenes (NHCs) with varying number of nitrogen atoms and NHC-transition-metal bond properties.
abstract: Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.
authors: Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 (Singapore); Laboratoire des Mécanismes Réactionels, Department of Chemistry, Ecole Polytechnique and CNRS, 91128 Palaiseau Cedex (France).
authors: Jan C
authors: JC
authors: Bernhammer
authors: Gilles
authors: G
authors: Frison
authors: Han Vinh
authors: HV
authors: Huynh
keywords: bond analysis; carbenes; density functional calculations; donor-acceptor systems; heterocycles