Structural, Bonding, and Optical Properties of B18Ca2 Clusters: Double-Ring Forms
Abstract
The structural and electronic properties of the doubly calcium-doped boron cluster B18Ca2 have been systematically investigated using density functional theory calculations. Basin-hopping searches reveal that B18Ca2 adopts a double-ring geometry as its global minimum, consisting of two fused B9 rings symmetrically stabilized by calcium atoms located above and below the boron framework. Vibrational frequency calculations verify the dynamical stability of the low-lying structures, while infrared and UV-Vis spectra highlight strong Ca--B coupling and pronounced electronic delocalization within the boron scaffold. Atomic dipole-corrected Hirshfeld charge analysis indicates substantial charge transfer from Ca to the electron-deficient boron framework, with the donated electrons uniformly delocalized over the B18 skeleton. Real-space bonding analyses based on the electron localization function (ELF), Interaction Region Indicator (IRI), and the Laplacian of the electron density reveal an extended multicenter bonding network characterized by global σ-delocalization and Ca-induced polarization effects rather than localized two-center Ca--B bonds. Together, these results establish B18Ca2 as a prototypical boron toroidal cluster and provide fundamental insights into the role of alkaline-earth doping in stabilizing complex boron nanostructures.
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