Structural And Bonding Analyses Of Double-Chain Ribbon, Triple-Ring Tube, And Cage-Like Boron Clusters

Bare boron clusters and planar two-dimensional boron sheets have attracted extensive attention in recent years. The first borospherenes B400/-(all-boron fullerenes) were observed in 2014 in a joint theoretical and experimental investigation. This finding enriches boron stereochemistry. Recently, chiral borospherene B39- was confirmed combining PES experiment and theoretical calculation, and B41+ and B422+, as new members of boropherene family, have also been investigated. They are all composed of interwoven boron double-chain ribbons. Therefore, the double-chain boron ribbon should be taken as an elemental structural unit for constructing boron fullerenes and sheets. It is necessary to investigate the double-chain boron ribbon in suitable species. This forms the topic of the current research which includes two parts:On one hand, a series of double-chain BC22H2(n = 2-9)were studied using the density functional theory and the coupled cluster method, including their geometrical structure, electronic structure, and chemical bondings; On the other hand, we predicted and analyzed the large size boron cluster B420/2+. The main contents and conclusions are presented as the followings:1. Boron double chain (BDC) may serve as building blocks to form boron fullerenes and sheets. In this part, a series of B2HC2H2 clusters (n= 2-9) were extensively investigated using the density functional theory and the coupled cluster method. The most stable structures of B2nC2H2 are planar double-chain nanoribbon with lengths from 3.9 to 15.0 A. According to CMO analyses, the B2nC2H2 (n= 2-9) nanoribbon clusters are similar to polyenes in the π conjugation, in which a rhombic B4 or triangle CB2 units are equivalent to a C=C unit in the polyenes, respectively. The adaptive natural density partitioning (AdNDP) analyses show that there exist conjugated multi-center bonds along the nanoribbons. The two-dimensional contour pictures of the nucleus-independent chemical shifts (NICS) reveal that B2nC2H2 clusters have ribbon aromaticity that fluctuates along the ribbons. The photoelectron spectroscopy (PES) spectrum of the B2n2H2-monoanions are also simulated to facilitate their future experimental characterization. This finding will provide new insights on boron fullerenes and two-dimensional boron sheets.2. We perform a density functional theory study on B420/2+ clusters. From the theoretical calculations, the ground state of B42 cluster possesses triple-ring tubular structure, instead of borospherene, which composed of interwoven boron double-chain ribbons. However, the ground state of B422+ favors borospherene. HOMO-LUMO gap of triple-ring tube B42 equals to 2.13 eV, which indicate the high stability in thermodynamics. The shape of delocalized CMOs of B42 exactly parallel to the corresponding wave functions obtained by solving the Schrodinger equation for a particle in a hollow cylinder model, which was reported by Minh Tho Nguyen’s group.The number of electrons both radial and tangential conforms to 4N+2M rule. And we find that triple-ring tube B42 has triple aromaticity, which enhance its structure stability. Tubular B42 would be viewed as a smallest triple-ring nanotube, in which two double-ring tube B28 are connected by sharing a single ring and may be the first triple-ring tubular neutral boron cluster. AdNDP analysis reveals the bonding pattern of B422+ is similar to borospherene B40. We simulate the photoelectron spectroscopy (PES) spectrum of the corresponding B42- monoanion to facilitate their future experimental characterization.