On The Structures,Chemical Bonding,and Aromaticity Of Boron Based Nanoclusters

Due to the electron deficiency of boron,elemental boron clusters and relevant doped boron alloy clusters have novel structures and nonclassical chemical bonding.Over the past three decades,a series of combined experimental and computational studies using anion photoelectron spectroscopy（PES）and quantum chemical calculations have been carried out to systematically characterize and elucidate the geometric structures,electronic properties,and chemical bonding of size-selected boron clusters and doped-boron clusters.In the meantime,recent works have shown that the simplest diatomic BO cluster（or boronyl）is similar to CO or CN in terms of bonding,owing to the fact that BO can be regarded as aσ-radical with robust triple B≡O bonds.Boronyls and aromaticity/antiaromaticity are exactly rivaling components in boron oxide clusters,because the former attempts to destroy delocalizedp/sbonding frameworks and leads to new cluster structures.Furthermore,boron-based clusters are susceptible to metal doping,in which the metal elements can donate electrons and help compensate for boron’s intrinsic electron-deficiency.This effort may significantly expand the structural and bonding diversity in boron-based clusters.As we all know,alkaline-earth metals usually form bonds using their ns atomic orbitals.However,recent studies have found that there is no clear boundary between alkaline-earth metals and transition metals.When heavy element Ba is combined with boron clusters,one question arises whether the binary clusters can exhibit properties similar to those of transition metals.The purpose of this thesis is to study the geometric structures,chemical bonding,and aromaticity/antiaromaticity of boron-based nanoclusters using quantum chemical calculations and a diversity of modern computational tools for chemical bonding analyses.The main research contents of this thesis are as follows:1.Chemical bonding model for all-boron benzene B₁₂ cluster.Quasi-planar C_3v B₁₂cluster has long been known as an all-boron benzene system with 6paromaticity.However,an in-depth chemical bonding analysis for B₁₂ cluster is still lacking.In this work,chemical bonding of B₁₂ cluster is approached using an array of modern quantum chemistry tools,such as canonical molecular orbital（CMO）analysis,adaptive natural density partitioning（AdNDP）,and natural bond orbital（NBO）analysis.The concerted computational data reveal unique three-fold 6p/2s/10saromaticity,including spatially separated inner 2sand outer 10saromaticity.The 6paromaticity is clouded primary on three shorter edges,thus leading to a pseudo-Kekuléall-boron benzene cluster.The 10sframework appears be more delocalized than the 6pcounterpart.2.Structures and chemical bonding of boron-based B₁₂O and B₁₁Au clusters.A counterexample in boronyl chemistry.Boron oxide clusters have structural diversity and unique chemical bonding,and recent literature shows that boronyl complexes dominate boron-rich oxide clusters.A counterexample in boronyl chemistry is presented in this work.We shall report on the computational design of two boron-based quasi-planar or planar clusters:B₁₂O and B₁₁Au.Contrary to anticipation,the global-minimum（GM）structure of B₁₂O cluster,as established via computer coalescence kick（CK）searches,has a circular quasi-planar shape with a peripheral B-O-B bridge,which resembles bare B₁₂ cluster.It does not contain a boronyl ligand.The isomeric boronyl complex turns out to be 10.32 kcal mol^-1 higher in energy at the single-point CCSD（T）level.In contrast,the B₁₁Au cluster behaves normally with an elongated B₁₁ moiety and a terminal Au ligand.Chemical bonding analyses reveal three-foldp/saromaticity in circular B₁₂O cluster,including global 6paromaticity,as well as spatially isolated inner 2saromaticity and outer 10saromaticity.The three-fold 6p/2s/10saromaticity underlies the stability of B₁₂O cluster.This bonding picture is unknown in the literature for bare B₁₂ cluster or its derivatives,except for a very recent follow-up work from this lab.Elongated B₁₁Au cluster has conflictingp/saromaticity（with 6pversus 8selectron-counting）.The B₁₂O cluster is actually isoelectronic to bare B₁₂ cluster in terms of delocalizedp/sbonding,which inherits the structural and electronic robustness of the latter.3.Structures and bonding of boron-oxygen B₇O^+/0/-clusters:Competition between aromaticity/antiaromaticity and boronyls.The structural searches for B₇O^+/0/-clusters are carried out by using the CK algorithm,which are also aided with manual structural constructions.Chemical bonding and aromaticity of anionic B₇O^-cluster are elucidated using an array of quantum chemistry tools.The computational results show that the GM structures of B₇O^+/0/-clusters are quite similar.Specifically,anionic B₇O^-cluster has the singlet state with a circular quasi-planar shape.The O atom serves as a peripheral B-O-B bridge.Other than that,anionic B₇O^-cluster is similar to a bare B₇^-(C_2v,¹A₁)cluster.Chemical bonding analysis shows that GM B₇O^-cluster has 4p/6sconflicting aromaticity.Its competing isomer（in triplet state）is equivalent to the previously reported B₆Au^-cluster with a boronyl attached to an elongated B₆ unit.This isomer has 3p/3sdouble antiaromaticity.The 4p/6sconflicting aromaticity of the circular GM structure gains an advantage in competition with double 3p/3santiaromaticity,which provides a counterexample of boronyl chemistry.Compared to bare B₇^-cluster,the potential energy surface of anionic B₇O^-cluster changes significantly.Bare B₇^-cluster has three coexisting isomers.The lowest energy triplet state(C_6v,³A₁)structure of B₇^-appears to lose its energetic advantage upon addition of an oxygen atom in the system.4.Structure and chemical bonding of boron-oxygen B₉O₃⁺cluster.Based on quantum chemical calculations,we report on the geometric structure and chemical bonding of B₉O₃⁺cluster,which is a relatively boron-rich system.The B₉O₃⁺cluster turns out to consist of three structural units:a boronyl（BO）,a BO₂ radical unit,and an open B₇⁺molecular wheel.With this understanding,cluster B₉O₃⁺may be formulated formally as a[BO][OBO][B₇⁺]complex.It is of interest to describe the cluster as a quasi-planar“palm-leaf fan”with C_s symmetry.Chemical bonding analysis shows that the B₇⁺molecular wheel unit in cluster B₉O₃⁺maintains the same delocalized frameworks as bare B₇⁺cluster:2p/6sdouble aromaticity.This is the basis for the stability of B₉O₃⁺cluster.The BO₂ unit does not maintain its inherent linear structure.It acts as a radical and forms three B-B/B-O single bonds with the BO and B₇⁺units,which holds the whole B₉O₃⁺cluster together.This unique bridging approach is the key to the formation of B₉O₃⁺cluster and provides ideas for the design of novel boron oxygen clusters.5.Binary boron-barium inverse sandwich clusters:B₉Ba^-and B₉Ba₂^-.Alkaline-earth metals are usually bonded through their ns atomic orbitals.Recent works have found that they can,to some extent,behave like transition metals.In this work,the structure and chemical bonding of binary B₉Ba^-and B₉Ba₂^-clusters are studied using modern quantum chemistry.The B₉Ba^-cluster contains a molecular wheel B₉^-unit,which is deformed to an elongated“boat-shaped”structure,owing to electron transfer from Ba.The cluster has6p/8sconflicting aromaticity.The B₉Ba₂^-cluster assumes a triplet-state inverse sandwich structure.It has high symmetry with a nine-membered B ring.Such a monocyclic boron ring is not possible in bare boron clusters.The analyses of chemical bonding indicate that it hasp/sdouble aromaticity（with triplet state 8pand 6saromaticity）.Another competitive isomer of B₉Ba₂^-cluster has a similar structure relative to B₉Ba^-,which possesses conflicting aromaticity.This is also the reason why the inverse sandwich structure of B₉Ba₂^-cluster has an advantage energetically.According to zeroth-order approximation,the B₉ unit of B₉Ba₂^-cluster possesses five negative charges,which demonstrates that a disk-like B₉^-cluster can be converted into a monocyclic ring by acquiring a sufficient number of extra electrons.Chemical bonding analysis shows that there exists a small amount of d orbital components of Ba atoms in B₉Ba^-and B₉Ba₂^-clusters,especially for inverse sandwich B₉Ba₂^-cluster.