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 D2d B20 (), to the disk-like C2v B20H2 (), and then to the cage-like C2 B20H4 (). Both the singly charged C2v B20(-) () and C2 B20H2(-) () favor 2D disk-like planar structures with a filled hexagon (B7) at the center, while C2 B20H4(-) () follows its neutral counterpart with a 3D cage-like geometry. All the doubly charged C2v B20(2-) (), C2 B20H2(2-) (), and C1 B20H4(2-) () turn out to prefer planar or quasi-planar 2D structures over 3D geometries, with the most stable B20H4(2-) () possessing a unique hexagon hole (B6) at the center. Detailed CMO and AdNDP analyses reveal that both the perfect planar B20(2-) () and B20H2 () 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 (D6h C24H12). The quasi-planar C2 B20H2(2-) () and C1 B20H4(2-) () also appear to be π aromatic with one π system following the 4n + 2 rule. The B20H4(2-) () structure with a hexagon hole may serve as the embryo for monolayer boron α sheet. Both the cage-like C2 B20H4 () and C2 B20H4(-) () appear to be 3D aromatic with the large negative NICS values of -51.5 and -55.5 ppm, respectively. The structural changes from to 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(-) () and B20H4(-) () are predicted to facilitate their future experimental characterizations and production.