From Exposed to Encapsulated: Structural Evolution and Enhanced Stability of Sn-Doped Gold Clusters, SnAu n (n = 2-20), Revealed by DFT.

A systematic density functional theory (DFT) investigation was conducted to explore the geometric evolution, electronic properties, and relative stability of tin-doped gold clusters SnAun (n = 2-20). The lowest-energy structures were identified using the CALYPSO structure prediction method and refined at the B3PW91/LANL2DZ level of theory. Our results reveal a distinct size-dependent structural transition: small clusters (n ≤ 8) adopt simple polyhedral geometries (some nearly planar, others three-dimensional) with Sn at peripheral sites, while larger clusters (n ≥ 9) evolve into three-dimensional structures with the Sn atom progressively encapsulated within the Au framework, achieving high coordination in tetrahedral or polyhedral cages for n ≥ 15. Analysis of the average binding energy, second-order difference energy, and HOMO-LUMO gaps identifies SnAu4, SnAu8, and SnAu16 as magic-number clusters with enhanced stability. Natural bond orbital (NBO) analysis suggests significant charge transfer from the electropositive Sn atom to the Au matrix. Theoretical infrared and Raman spectra were simulated, providing distinct vibrational fingerprints that evolve from sharp peaks in small clusters to broad, complex bands in larger systems. Quantum theory of atoms in molecules (QTAIM) analysis suggests that the bonding is predominantly closed-shell (ionic/metallic), with Sn-Au bonds showing enhanced polarization compared with Au-Au bonds. A possible weak localized character in the smallest clusters cannot be excluded, but the overall framework remains closed-shell dominated.