Structural transition upon hydrogenation of B20 at different charge states: from tubular to disk-like, and to cage-like

Abstract

Extensive first-principles theoretical investigations indicate that neutral B₂₀ undergoes a dramatic structural transition upon partial hydrogenation, from the tubular D_2d B₂₀ (1), to the disk-like C_2v B₂₀H₂ (4), and then to the cage-like C₂ B₂₀H₄ (7). Both the singly charged C_2v B₂₀⁻ (2) and C₂ B₂₀H₂⁻ (5) favor 2D disk-like planar structures with a filled hexagon (B₇) at the center, while C₂ B₂₀H₄⁻ (8) follows its neutral counterpart with a 3D cage-like geometry. All the doubly charged C_2v B₂₀²⁻ (3), C₂ B₂₀H₂²⁻ (6), and C₁ B₂₀H₄²⁻ (9) turn out to prefer planar or quasi-planar 2D structures over 3D geometries, with the most stable B₂₀H₄²⁻ (9) possessing a unique hexagon hole (B₆) at the center. Detailed CMO and AdNDP analyses reveal that both the perfect planar B₂₀²⁻ (3) and B₂₀H₂ (4) possess concentric dual π aromaticity, with two π-electrons delocalized over the filled hexagon B₇ at the center and ten π-electrons delocalized between the filled B₇ and the B₁₃ outer ring, each separately conforming to 4n + 2 Hückel's rule. They are therefore the boron analogues of coronene (D_6h C₂₄H₁₂). The quasi-planar C₂ B₂₀H₂²⁻ (6) and C₁ B₂₀H₄²⁻ (9) also appear to be π aromatic with one π system following the 4n + 2 rule. The B₂₀H₄²⁻ (9) structure with a hexagon hole may serve as the embryo for monolayer boron α sheet. Both the cage-like C₂ B₂₀H₄ (7) and C₂ B₂₀H₄⁻ (8) appear to be 3D aromatic with the large negative NICS values of −51.5 and −55.5 ppm, respectively. The structural changes from 1 to 9 reflect a competition between 2D and 3D aromaticities in these clusters, depending on the extent of hydrogenation and electronic charge states. The PES spectra of B₂₀H₂⁻ (5) and B₂₀H₄⁻ (8) are predicted to facilitate their future experimental characterizations and production.