Non-FCC Kernel Geometry and Multicenter Bonding in Au23(SR)17 Nanoclusters: Structural Isomerism and Cohesion Descriptors for the Au Skeleton

Abstract

Atomically precise structures provide molecular-level platforms for correlating metal-kernel geometry, ligand-shell topology, and electron delocalization in thiolate-protected gold nanoclusters. Here, Au23(SR)17 is used to examine structural isomerism and bonding descriptors associated with nonface-centered-cubic (non-FCC) Au-kernel geometry. Guided by the grand unified model and the ring model, five Au23(SR)17 isomers were constructed and compared with crystallographically determined Au23(SCTMS)17, denoted Au23–Iso1. All predicted structures are local minima with no imaginary harmonic frequencies. Among the predicted structures, Au23–Iso2 is lowest in energy, but remains 0.43 eV above Au23–Iso1 at the Perdew–Burke–Ernzerhof level. PBE0 and Tao–Perdew–Staroverov–Scuseria single-point calculations preserve the Au23–Iso1 < Au23–Iso2 < Au23–Iso3 order. The calculated ultraviolet–visible spectrum of Au23–Iso2 shows a broadly similar multiband profile to the reported spectra of unresolved Au23(SPh-tBu)17 and Au23(S-c-C6H11)17 clusters; however, the peak-deviation analysis does not favor Au23–Iso2 over the crystallographic Au23(SCTMS)17 spectral reference. Bonding and descriptor analyses identify a non-FCC kernel in Au23–Iso1, three clearly retained ADNDP-localized 4c–2e features and a threshold-sensitive fourth candidate, a distinct kernel–surface contact distribution, and the largest whole-skeleton cohesion descriptor in the selected isomer set.