Theoretical Studies On Aluminum-Doped Boron Clusters:Structural And Electronic Properties,Chemical Bonding,and Aromaticity

Boron clusters and relevant doped boron clusters are of current interest in cluster science,owing to their unique structural and bonding properties,as well as their potential as building blocks in novel nanomaterials.In recent years,gas-phase photoelectron spectroscopic experiments and high-level quantum chemistry calculations are used to probe the structures and bonding of size-selected boron clusters.Boron clusters feature unusual planar or quasi-planar structures,which are attributed to the intrinsic electron-deficiency of boron.The latter gives rise to delocalized π/σ bonds and helps develop bonding concepts such as multifold πaromaticity,antiaromaticity,and conflicting aromaticity.With the development in bare boron clusters,the modification of structural,electronic,and bonding properties by doping a second element has emerged in cluster science.Doping boron clusters with metals is anticipated to alter and enrich the structural and phyisco-chemical properties of boron clusters,because a metal element can be an electron donor.However,a boron-based binary cluster does not necessarily become richer in terms of valence electrons.In this thesis,computational studies are reported on boron-aluminum binary clusters,which are carried out basing on the density functional theory.Specifically,doping boron clusters with multiple aluminum atoms is chosen as the topic and focus of this thesis.The main contents,results,and conclusions are as follows:1.The Al-B substitution in boron-based alloy clusters is not completely isoelectronic:structures and bonding of B6Al2-/0/+clusters.Elemental boron and aluminum clusters show unique structures and chemical bonding.Consequently,binary B-Al clusters are normally studied following the idea of isoelectronic Al-B substitution.However,the concept is purely an assumption.We report on the structures,bonding,and aromaticity of a series of binary B6Al2-/0/+clusters in their different charge states,using computational global-minimum(GM)searches and electronic structure calculations.Relative energies of top low-lying structures are systematically assessed at the PBE0 and single-point CCSD(T)levels of theory,offering invaluable methodological information for this and relevant systems.The key cluster structures 1-4 have two distinct motifs:inverse sandwich versus heteroatomic molecular wheel.Neutral GM cluster 1 slightly favors the inverse sandwich,with molecular wheel structure 2 being a low-lying isomer.Anionic GM cluster 3 is also a molecular wheel,whereas cationic GM cluster 4 strongly favors the inverse sandwich.Upon sequential reduction from cation,neutral,to anion,the molecular wheel monotonically gains a relative advantage,and a structural transition from inverse sandwich to molecular wheel takes place in the anion.In terms of bonding,clusters 1/2 feature the 6π/4σ conflicting aromaticity,leading to elongated structural shapes.Cationic cluster 4 and anionic cluster 3 have the 3σ/5σ frameworks,which deviate from strict 4σ antiaromaticity.Thus,clusters 3/4 are relatively circular.The doping A1 sites fall into three distinct categories,being chemically valence three/two/one,respectively.Only one out of three categories conforms to the isoelectronic Al-B substitution.2.On the structures and chemical bonding of binary B11Al2-and B10Al2 clusters.We use the Coalescence Kick(CK)algorithm to search the GM structures of B11Al2--and B10Al2 clusters.The relative energies of candidate low-lying isomers are evaluated at the PBEO and single-point CCSD(T)levels.The computational data show that the GM structures of B11Al2-and B10Al2 clusters are similar to those of their bare boron counterparts(that is,B13-and B12).The B11Al2-and B10Al2 clusters assume elongated planar and quasi-planar structures,respectively.Their chemical bonding pattern is also the same as compared to that of bare boron counterparts,featuring double anti-aromaticity and double aromaticity,respectively.The substitution of Al atoms only leads to an enlarged peripheral ring in binary clusters.Canonical molecular orbitals(CMO)analysis indicates that all valence electrons in the binary clusters are involved in chemical bonding,and there are no lone pairs or non-bonding electrons.Therefore,all A1 atoms conform to the idea of isoelectronic Al-B substitution.The GM structure of B11Al2-cluster is exactly the combination of two B6Al clusters,suggesting that B6Al may potentially be developed as a structural unit for low-dimensional boron-based nanomaterials.There are three kinds of competitive isomers in B10Al2 clusters.The inner,B3 ring appears to be crucial in stablizing planar B-Al clusters.