Studies On The Structural And Bonding Characteristics Of Boron-based Nanoclusters

With the rapid development of computer technologies and calculation methods,quantum chemical calculations are playing an increasingly important role in chemical research and materials science.Because of the diversities and singularities in structures and propeties,nanoclusters which lie between atoms,molecules and macroscopical systems become an important object in experimental and theoretical research.Based on density functional theory（DFT）and photoelectron spectroscopy（PES）,this thesis reveals the structures,bonding patterns,and properties of a series of boron oxide clusters,metal-doped boron oxide and boron clusters,and hydroborons.The main contents of this paper are as follows: 1.Small B_nO_n^0/-（n=3-5）clusters,metal-doped boron oxides and hydroboronsPhotoelectron spectroscopy and first-principles theory study are combined to observe the structural and electronic properties and chemical bonding of B₃O₃-/0,B₃O₃H-/0 and B₄O₄^0/-clusters.The global-minimum structures of B₃O₃-,B₃O₃H-/0,B₄O₄^0/-and their neutrals are identified through computational structural searches and electronic structure calculations at the B3 LYP and CCSD（T）levels.It is found that B₃O₃-adopts a V-shaped structure,and B₃O₃H-contains an asymmetric OB-B-OBO zig-zag chain,where the central B atom has two singly bonded terminals（OBO and H）as well as one terminal BO.Note that the anionic structures are very different from those of their neutral counterparts both of which contain a rhombic B₂O₂ ring.The rhombic B₄O₄ contains a B₂O₂ core with two terminal boronyl groups bonding to it.While B₄O₄-is a Cs Y-shaped structure which features a B atom bonded terminally to one OBO unit and two boronyl groups.Chemical bonding analyses reveal 3c-4e π hyperbonds in B₃O₃H-and 4c-4e o-bonds in B₃O₃,B₃O₃ H and B₄O₄^0/-.Different from B₃O₃^0/-and B₄O₄^0/-clusters,B₅O₅+ and B₅O₅ species share nearly the same architecture that consists of a boroxol B₃O₃ ring and two terminal BO units,which can be alternatively formulated as B₃O₃（BO）2+ and B₃O₃（BO）2,respectively.The B₅O₅-anion cluster possesses a three-dimensional global-minimum structure,which is characterized with a tetrahedral B-center attached by three BO groups and one OBO unit and can be formulated as B（BO）3（OBO）-.B₅O₅+/0 are boron oxide analogs of the organic phenyl cation and phenyl radical,C6H5+/0,while the B₅O₅-cluster may be considered as a boron oxide analog of chloromethane or methylchloride（CH3Cl）.Obviously,a single charge can change the global-minimum structures and electronic properties drastically.A series of lithium and gold alloyed boron oxide clusters including B₂O₃-,LiB₂O₃-,AuB₂O₃-,LiAuB₂O₃-,and their neutrals were investigated by a joint PES and DFT and molecular orbital（MO）theory study.The electronic and structural properties and chemical bonding of these clusters were analyzed.The electron affinities（EAs）of B₂O₃,Li B₂O₃,AuB₂O₃,and LiAuB₂O₃ are measured from the PES spectra to be 1.45 ± 0.08,4.25 ± 0.08,6.05 ± 0.08,and 2.40 ± 0.08 eV,respectively.Structural searches using the Coalescence Kick（CK）and Basin Hopping（BH）methods lead to establishment of the global-minimum cluster structures: B₂O₃-is bent,whereas the three LiB₂O₃-,AuB₂O₃-,and LiAuB₂O₃-alloy clusters are linear or quasi-linear with a metal center inserted between the BO and OBO subunits.Chemical bonding analyses reveal interesting features in these species,such as charge transfer complexes,covalent gold,hyperhalogen,and three-center four-electron（3c-4e）π hyperbonds.The concerted experimental and computational data hint the possibility to alter and fine-tune the properties of the boron oxide systems via alloying,which may result in novel,tailored electronic properties and chemical reactivities.Systematical DFT investigations on planar D_2h B₂₆H₈,D_2h B₂₆H₈2+,and C2 h B26H6 which are the smallest hydroboron clusters composed of a double chain（DC）framework with a twin-hexagonal hole at the center help to model the nucleation and growth processes of mono-layer boron sheets（MLBSs）.Such clusters are expected to serve as the building blocks of stable η2/14,η2/15,η3/24,η4/28,η4/33 MLBSs with twin-hexagonal holes（THHs）and further indicate the H/BO isolobal analogy.Detailed canonical molecular orbital（CMO）,nucleus independent chemical shift（NICS）,electron localization function（ELF）,and adaptive natural density partitioning（AdNDP）analyses indicate that they are overall aromatic in nature and analogous to D_2h C16H14,and D_2h C16H142+ in p bonding,respectively.2.From planar B_2nO_2n（n≤5）to tubular B_2nO_2n（n≥6）clustersBased on extensive theoretical calculations,we present herein the possibility of tubular B_2nO_2n（n≥6）clusters which can be rolled up from the one dimensional （BO）₂ chain and contain new bonding elements that are different from most reported traditional boron oxides.These tubular B2nO2 n clusters do not contain any terminal BoO bonds.The energy of planar B10O10 and tube one are nearly degenerate.While for B12O12,the double-ring tube is more stable than the planar one reported in literature.The calculated IR,Raman and UV-Vis spectra of these tubes are simulated which could facilitate their forthcoming experimental characterizations.3.Exohedral Saturn-like metalloborospherenes Li₄&B₃₆,Li₅&B₃₆+,and Li₆&B₃₆²⁺Based on extensive first-principles theory calculations,we present the possibility of the cage-like charge-transfer complexes Li₄&B₃₆,Li₅&B₃₆+,and Li6&B₃₆2+.Meanwhile,the structures of cage-like D_2h Li2&[Ca@B₃₆],C2 v Li3&[Ca@B₃₆]+,and D_2h Li₄&[Ca@B₃₆]2+ with an endohedral Ca²⁺ are also predicted.As typical exohedral charge-transfer complexes,D_2h Li₄&B₃₆,C2 v Li₅&B₃₆+,and Th Li6&B₃₆2+ possess 44 delocalized 3c-2e σ bonds and 12 delocalized 5c-2e π bonds evenly distributed on the cage surface,matching the σ+π double delocalization bonding pattern of the borospherene family perfectly.The vibrational frequencies and electron detachment energies of the concerned species are computationally predicted to facilitate their experimental characterizations.4.From Quasi-Planar B₅₆ to Penta-Ring Tubular Ca?B₅₆Based upon extensive first-principles theory calculations,we present herein the possibility of doping the quasi-planar C2 v B₅₆ with an alkaline-earth metal to produce the penta-ring tubular Ca?B₅₆ which is the most stable isomer of the system obtained and can be viewed as the embryo of metal-doped（4,0）boron α-nanotube Ca?BNT（4,0）.Ca?BNT（4,0）can be constructed by rolling up the most stable boron α-sheet.Detailed bonding analyses show that the highly stable planar C2 v B₅₆ is the boron analog of circumbiphenyl(C₃₈H₁₆)in π-bonding,while the C4 v Ca?B₅₆ possesses a perfect delocalized π system over the σ-skeleton on the tube surface.Metal dopants encapsulated in cage-like borospherenes to form metalloborospherenes,inserted in planar borophenes to form metalloborophenes,or wrapped up in boron nanotubes to form metal-doped boron nanotubes may effectively enhance the chemical stabilities and tune the transport properties of the boron nanostructures.