| At present, it has been one main investigation direction by means of quantum chemical theoretical calculation to predict and research the structures and properties of novel nanoclusters for chemists. Based on density functional theory and wave function theory, a systemic theoretical investigation has been performed on their structures and properties of boron double-chain nanoribbon clusters, polycyclic aromatic hydroboron clusters, borospherene (or called as all-boron fullerene) and endohedral and exohedral metalloborospherenes in this thesis. And photoelectron spectra of the corresponding anions have been predicted to facilitate their spectroscopic characterizations. Furthermore, an initial investigation were performed to explore the hydrogen storage capacity of borospherene. The results of this thesis will help people further know and understand borospherene chemistry and provide some theoretical basis for experimental researchs and applications in the future. The main contents are as follows:1. Boron Double-Chain Nanoribbons BnH22- and Li2BnH2We report an extensive density-functional theory and coupled-cluster CCSD(T) study on boron dihydride dianion clusters BnH22-(n=6-22) and their dilithiumated Li2BnH20/-salt complexes. Double-chain (DC) nanoribbon structures are confirmed as the global minima for the BnH22-(n=6-22) clusters. Charging proves to be an effective mechanism to stabilize and extend the DC nanostructures, capable of producing elongated boron nanoribbons with variable lengths between 4.3-17.0 A. For the dilithiumated salts, the DC nanoribbon structures are lowest in energy up to Li2B14H2 and represent true minima for all Li2BnH20/-(n=6 - 22) species. These boron nanostructures may be viewed as molecular zippers, in which two atomically-thin molecular wires are zipped together via delocalized bonds. Bonding analysis reveals the nature of σ plus π double conjugation in the lithiumated DC nanoribbon Li2BnH20/-(n up to 22) model clusters, which exhibit a 4n pattern in adiabatic detachment energies, ionization potentials, and second-order differences in total energies. Band structure analysis of the infinite DC boron nanoribbon structure also reveals that both σ and π electrons participate in electric conduction, much different from the monolayer boron a-sheet in which only π electrons act as carriers. A concept of "ribbon aromaticity" is proposed for this quasi-one-dimensional system, where regular a versus π alternation of the delocalized electron clouds along the nanoribbons results in enhanced stability for a series of "magic" nanoribbon clusters. The total number of delocalized σ and π electrons for ribbon aromaticity collectively conforms to the (4n+2) Huckel rule. Ribbon aromaticity appears to be a general concept in other nanoribbon systems as well.2. Polycyclic Aromatic Hydroboron (PAHB) Clusters B3nHmCalculations performed at ab initio level using the planar concentric π-aromatic B18H62+as building block suggest the possible existence of a new class of B3nHm polycyclic aromatic hydroboron (PAHB) clusters made of interwoven boron double-chain ribbons, which appear to be inorganic analogs of the corresponding CnHm polycyclic aromatic hydrocarbon (PAHC) molecules, in a universal atomic ratio of B:C=3:1. Detailed canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), and electron localization function (ELF) analyses indicate that, as hydrogenated fragments of boron snub sheet, these PAHB clusters are aromatic in nature, and exhibit the formation of islands of both σ- and π-aromaticity. The predicted ionization potentials of PAHB neutrals and electron detachment energies of small PAHB monoanions should permit them to the characterized experimentally in the future. The results expand the domain of planar boron-based clusters to a region well beyond B20, and experimental syntheses of these snub B3nHm clusters through partial hydrogenation of the corresponding bare B3n may open up a new area of boron chemistry parallel to that of PAHCs in carbon chemistry.3. Thermodynamic and Dynamic Stability of Borospherene B40By joint photoelectron spectroscopy (PES) and first-principle investigations, we and coworkers successfully characterized the first cage-like all-boron fullerene B40-, for which we propose the name "borospherene". Theoretical calculations show that a quasi-planar isomer of B40- with two adjacent hexagonal holes is slightly more stable than the fulierene structure. In contrast, for neutral B40 the fullerene-like cage is calculated to be the most stable structure. Both B40- and B40 have D2d symmetry, with 16 tetracoordinated and 24 pentacoordinated boron atoms. The all-boron fullerenes are elongated slightly along the two-fold main molecular axis, with two hexagonal holes at the top and bottom and four heptagonal holes on the waist. These structures are akin to a perforated chinese red lantern. Alternatively, they can be built from interwoven double-chain boron ribbons that consist of eight horizontal B9 ribbons and four vertical Bio ribbons, showing the important role of the boron double chain structural units in the formation of low-dimensional boron nanostructures. Molecular dynamics simulations of B40 show that it is highly robust at different temperatures. D2d B40 possesses a unique bonding pattern, with 120 valence electrons all being delocalized in multi-center σ and π bonds which cover the whole molecular surface almost evenly. To the best of our knowledge, such a bonding pattern only exists in these all-boron fullerences. The observation of all-boron fullerene represents the onset of all-boron fullerenes, a promising research area that possibly parallels that of the well-known carbon fullerenes.4. Metalloborospherenes M@B4o (M=Ca, Sr) and M&B40 (M=Be, Mg)The recent discovery of borospherenes D2d B4o -/0, paves the way for borospherene chemistry. We perform a density functional theory study on the viability of metalloborospherenes:endohedral M@B4o (M=Ca, Sr) and exohedral M&B40 (M=Be, Mg). Extensive global structural searches indicate that Ca@B40 (C2v, 1A1) and Sr@B4o (D2d,1A1) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B40 (Cs,1A’) and Mg&B4o (Cs,1A’) favor exohedral borospherene geometries with a η7-M atom face-capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of borospherene structural motif. The metalloborospherenes are characterized as charge-transfer complexes (M2+B402-), where an alkaline earth metal atom donates two electrons to the B40 cage. The high stability of endohedral Ca@B40 and Sr@B40 is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D2d B40 borospherene. And photoelectron spectra of C2 M@B40-(M=Ca, Sr) and Cs M&B40- (M=Be, Mg) have been predicted to facilitate future experimental characterizations. The results provide valuable information for potential applications of borospherene B40 as a molecular device.5. Endohedral Metalloborospherene Cations M@B40+(M=Sc, Y, La, Ac)Using the newly discovered borospherene D2d B40 as a molecular device, we perform a systematic first-principles theory investigation on four endohedral metalloborospherene cations C2v Sc@B40+, C2v Y@B40+, C2v La@B40+, and D2d Ac@B40+ which all turn out to be the most stable structures of the system. From Sc, Y, La, to Ac, the distances between the metal atom and B40 cage center decrease from 0.75,0.49,0.10, to 0.00 A and the nearest metal-boron distances increase from 2.52,2.68,2.99, to 3.10 A, indicating that Sc, Y, La are slightly off-centered, while Ac matches the B40 cage perfectly. With respect to M++B40=MB40+, the M@B40+ series have the huge formation energies of-163.8,-178.1,-157.4 and-129.5 kcal/mol for M=Sc, Y, La, and Ac at PBE0 level, respectively, suggesting the high possibility to produce these endohedral metalloborospherene cations in experiments. The infrared and raman spectra of the Czv La@B4o+are predicted to facilitate future spectroscopic characterizations of these endohedral metalloborospherene cations.6. Borospherene B40 as Hydrogen-Storage MaterialsAn initial study is performed on hydrogen adsorption and storage in borospherene B40 by means of density functional computations. Our results indicate that H atoms in B40Hn (n=2,4,6,8,16) species all can be bonded terminally with the tetracoordinate B sites of B40. And especially,16 terminal-H in D2d B40H16 exactly coordinate with 16 tetracoordinate B in borospherene B4o, however, its chemical hydrogen storage capacity is only 3.57 wt%. Then, we further study the hydrogen storage properties of Ca-and Li-coated B40 and find that B40 coated with six Ca or Li atoms capping onto the six holes in B40 all can store up to 30 H2 molecules with a binding energy of 0.27 and 0.31 eV/H2, respectively. Due to charge transfer between M and B40, Ca and Li atoms strongly bind to B40 with an average binding energy of 2.53 and 3.07 eV, separately. The charge redistribution induces strong electric field which polarizes H2 molecules and makes hydrogen adsorption feasible. The gravimetric densities of hydrogen of Ca6B4o(H2)3o and Li6B4o(H2)3o can rearch 8.2 and 11.2 wt%, respectively, so Ca-or Li-coated borospherene B40 may be a promising nanomaterial for hydrogen storage. |
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