Simulation of XANES spectroscopy and the calculation of total energies for N-heterocyclic carbenes on Au(111)

It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed {\it first-principles} total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene ($^B$NHC$^X$, with X=Me, Et, and iPr, respectively) and the bis-$^B$NHC$^X$ complexes with Au derived from these molecules. We present a comprehensive analysis of the energetic stability of both the $^B$NHC$^X$ and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the $^B$NHC$^X$/Au geometry and the N($1s$) $\rightarrow$ $\pi^\ast/\sigma^\ast$, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. Our simulations are compared with recent experimental results.