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

Extensive first-principles theoretical investigations indicate that neutral B-20 undergoes a dramatic structural transition upon partial hydrogenation, from the tubular D-2d B-20 (1), to the disk-like C-2v B20H2 (4), and then to the cage-like C-2 B20H4 (7). Both the singly charged C-2v B-20(-) (2) and C-2 B20H2 (5) favor 2D disk-like planar structures with a filled hexagon (B7) at the center, while C-2 B20H4- (8) follows its neutral counterpart with a 3D cage-like geometry. All the doubly charged C-2v B-20(2-) (3), C-2 B20H22- (6), and C-1 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 (B-6) at the center. Detailed CMO and AdNDP analyses reveal that both the perfect planar B-20(2-) (3) and B20H2 (4) possess concentric dual pi aromaticity, with two pi-electrons delocalized over the filled hexagon B-7 at the center and ten pi-electrons delocalized between the filled B-7 and the B-13 outer ring, each separately conforming to 4n + 2 Hucket's rule. They are therefore the boron analogues of coronene (D-6h, C24H12). The quasi-planar C-2 B20H22- (6) and C-1 B20H42- (9) also appear to be pi aromatic with one pi system following the 4n + 2 rule. The B20H42- (9) structure with a hexagon hole may serve as the embryo for monotayer boron alpha sheet. Both the cage-like C-2 B20H4 (7) and C-2 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.