Unusually high electron density in an intermolecular non-bonding region: Role of metal substrate

We present a systematical density functional theory study on adsorbed bis(para-pyridyl)acetylene (BPPA) tetramer on Au(111) surface. Exceptionally large electron densities are found between two N atoms at an intermolecular “non-bonded” region, as observed a line-like feature in atomic force microscopy images. They originate from the N-Au-N three-center bonding which effectively compressed two lone electron pairs together forming non-bonding hybridization between two N atoms.