A new look at the structure of the neutral Au18 cluster: hollow versus filled golden cage

The geometry of the neutral Au18 gold cluster was probed by a combination of quantum chemical calculations and far-infrared multiple photon dissociation (FIR-MPD) spectroscopy of a Kr messenger complex. Two low-lying isomers are identified to potentially contribute to the experimental IR spectrum, both being derived from a star-like Au17 structure upon capping with one extra Au atom either inside (18_1) or outside (18_5) the star. In particular, the present detection of structure 18_1 by DFT computations where a golden cage encapsulates an endohedral Au atom, is intriguing as a stable core-shell isomer has, to our knowledge, never been found before for such small neutral gold clusters. DFT and local coupled-cluster (DLPNO and PNO-CCSD(T)) computations indicate that both Au18 isomers are close to each other, within ∼3 kcal mol-1, on the energy scale. Although the exact energy ordering is again method-dependent and remains, at present, inconclusive, the most striking spectral signatures of both isomers are related to vibrational modes localized at atoms capping the inner pentaprism sub-structure that result in prominent peaks centered at ∼80 cm-1, close to the most prominent experimental feature found at 78 cm-1. The calculated IR spectra of both core-shell and hollow isomers are very similar to each other and both agree comparably well with the experimental FIR-MPD spectra of the Au18Kr1,2 complexes.