Quantum Chemical Studies On The Structures,Chemical Bonding,and Dynamic Fluxionality Of Main Group Metal Doped Boron-Based Alloy Clusters

Boron is a typical electron-deficient element in the periodic table,which features a valence electron configuration of 2s²2p¹.The electron deficiency of boron gives rise to unusual geometric structures and exotic chemical bonding modes for boron-based nanoclusters.Boron tends to form multi-centered bonds with its surrounding atoms,thus leading to planar or quasi-planar structures for elemental boron clusters over a wide range of sizes.Also owing to the nature of electron deficiency,boron-based clusters have recently emerged as an intriguing field for dynamic structural fluxionality.For instance,a circular B19^-cluster is shown to be dynamical fluxional,askin to a nanoscale Wankel motor.Mixing or alloying a metal element with boron may lead to boron-based alloy clusters,whose structures can be delicately tailored and electronic properties tuned.For this purpose,main group metals are among the favorable choices in the rational design of novel boron-based clusters.In this thesis,the geometric structures,chemical bonding,and dynamic structural fluxionality of a series of boron-based binary or ternary alloy clusters are studied using the state-of-the-art quantum chemical methods.Specifically,the systems include an Al-doped Al2B8 binary cluster,a ternary Mg Al B8⁺cluster,as well as a ternary Be B7Al2^-cluster.All three clusters are shown to possess sandwich-type geometries and feature doubleπ/σaromaticity.The systems are also dynamically fluxional,which is governed by their unique chemical bonding patterns and vividly demonstrated through molecular dynamics simulations.The main research contents of the thesis are as follows.1.Al-Doped binary Al₂B₈ clusterThe exploration of the potential energy surface of the Al2B8 system is conducted through the coalescence kick（CK）global searches.This is followed by structural optimization and energy calculations using the density-functional theory（DFT）at the PBE0level,in combination with the single-point energy calculations at the CCSD（T）level.The above effort collectively allows the establishment of the global-minimum（GM）structure for Al2B8 cluster.The GM cluster shows a unique sandwich structure,which consists of an in-plane hypercoordinate B8 molecular wheel and two isolated Al atoms,the latter units being capped symmetrical above and below the molecular plane of B8 wheel.The Al atoms are slightly off-center relative to the molecular wheel.Charge analysis shows that one electron is transferred formally from each Al atom to the B8 molecular wheel,so that the cluster can be formulated as an[Al]⁺[B8]^2-[Al]⁺complex.Chemical bonding analysis indicates that there is no interaction between two Al atoms and the[B8]^2-molecular wheel has dual 6π/6σaromaticity.The Born-Oppenheimer molecular dynamics（BOMD）simulations show that the Al atoms can move virtually free of barrier along a circular“rail”on the B8 wheel,in which the rail is defined in between an inner B atom and an outer B8ring.Even at a temperature as low as 100 K,the system maintains its dynamic fluxionality.The dynamic behavor of the system is sort of similar to that of a magnetic levitation system.2.Boron-based nanocompass Mg Al B₈⁺clusterBased on a prior report from our lab regarding a nanocompass Mg2B8 cluster,we may conclude that the main reasons for the dynamic fluxionality of an alloy cluster are as follows.Firstly,there is a charge transfer between the fragments in the alloy cluster.Second,the dualπ/σaromaticity of B8 molecular wheel provides a homogeneous delocalized electron cloud that facilitates dynamic fluxionality.With this understanding,we design herein a ternary Mg Al B8⁺cluster via isoelectronic substitution.The GM structure of Mg Al B8⁺cluster is established using electronic structure calcalations at the PBE0 and single-point CCSD（T）levels.The GM Mg Al B8⁺cluster differs from that of Mg2B8 cluster in that the Mg-Al unit is oriented asymmetrically with respect to the B8 wheel.The Al site interacts more strongly with the B8 wheel,as compared to the Mg site.Thus,the ternary cluster closely mimics a nanocompass or nanoclock.The BOMD simulations indicate that Mg Al B8⁺cluster has dynamic fluxionality.Charge analysis shows strong charge transfer within the system.Chemical bonding analysis shows that Mg Al B8⁺cluster has three delocalizedσcanonical molecular orbitals（CMOs）and three delocalizedπCMOs,rendering it dualπ/σaromaticity.The continuous,homogeneous,and delocalizedπ/σelectron cloud is a key factor that underlies the dynamic fluxionality.3.Boron-based nanoclock Be B₇Al₂^-clusterThe third system in this thesis deals with a boron-based ternary Be B7Al2^-cluster,in which a boron cluster is doped with Be and Al atoms.The GM structure of Be B7Al2^-cluster is determined by potential energy surface searches,followed by electronic structure calculations at the DFT and CCSD（T）levels.The GM Be B7Al2^-cluster turns out to assume a sandwich-type geometry,differing markedly from that of Mg Al B8⁺cluster as described above.Physically,the GM Be B7Al2^-cluster consists of three distinct layers,that is,an Al2unit,a monocyclic B7 ring,and a single Be atom.The overall shape of the cluster may be described as a nanoclock.The Al2 unit is suspended above the B7 ring and can rotate freely.The CMO analysis indicates that the Al2 and Be units each transfer two electrons to the B7ring.The B7 ring in Be B7Al2^-cluster formally has a total of five negative charges and can be formulated as[B7]^5-.Accordingly,the sandwich cluster is described as a charge-transfer[Be]²⁺[B7]^5-[Al2]²⁺complex.Chemical bonding analysis shows that the cluster has 6π/6σdelocalized electrons,that is,it has double 6π/6σaromaticity.This bonding pattern underlies the dynamic structural fluxionality of the cluster.