Studies On Electronic Structure And Chemical Bonding Of Clusters By Density Functional Theory

B is an element in ⅢA,its electronic configuration is 1s22s2p1 and it is electron deficiency.In recent decades,boron and boron-based clusters have been studied extensively by many scientists because of their unique physical and chemical properties.Au is an element in IB,its electronic configuration is 5d106s1.Gold clusters have been a hot issue during the past few years due to their particular properties,such as the unique size-dependent catalytic activity,relativistic effects,aurophilic attraction,and hyperfine properties,and many experimental and theoretical investigations surged on this aspect,as well as potential applications in catalysis,biomedicine and nanoelectronics.Along with many applications of the computer technology,the rapid development of computational methods of quantum chemistry also have developed quickly.Now it is possible to study geometric/electronic structures and many other properties of clusters in computational chemistry.Density funct:ional theory method has become one of the widely used computational methods in theoretical chemistry.In this thesis,the binary clusters[BxAl13-x]-(x=0-13)are investigated by using a method combining the genetic algorithm with DFT.The results distinctly reveal a three dimensional(3D)to(quasi-)planar(2D)structural transition as a function of x upon increasing the number of boron atoms.Moreover,we designed XAuH2(X = F,Cl,Br,I)complexes to investigate its d10-σ interaction.The Au-H2 multicenter bond was studied,which is originated by charge-transfer.The main studies are as follows:1.Density Functional Theory Studies of the Binary Systems[BxAl13-x]-(x= 0-13)The global minimum search and structural optimization for the B-Al binary clusters[BxAl13-,]-(x= 0-13)are performed using the genetic algorithm(GA)method coupled with density functional theory(DFT).The effects of composition on the atomic structures,electronic properties including the energy gaps and vertical detachment energies of B-AI binary clusters are discussed.The results distinctly reveal a three dimensional(3D)to(quasi-)planar(2D)structural transition as a function of x upon increasing the number of boron atoms.When x varying from 0 to 7,the clusters are Al-rich and the B-Al binary systems maintain the 3D structure.Whereas,the binary system trends to be planar structure,and the critical B:Al ratios for the 2D-3D transition are between x = 7 and 8.To study the stability of the[BxAl13-x]-clusters,we defined the relative energy(Erel = E([BxAl13-x]-)-xE(B13-)/13-yE(Al13-)/13),where the cluster with a more negative Erel is more stable.At x =1,relative energy is the most negative,indicating the highest stability of clusters.To further understand the stability of clusters,the vertical detachment energies(VDE)and the HOMO-LUMO energy gaps(EH-L)of[BxAl13-x]-(x = 0-13)clusters are also calculated.The consequence reveal that the energy decreases with the increasing number of B atoms,indicating a lower stability.The largest EH-L of BAl12-cluster indicates that it is the most stable among all the series of this clusters.Molecular orbitals(MO)of BAl12-cluster are analyzed and the result shows that the electronic shells of Is2 and 1p6 are virtually unchanged when the central Al atom is replaced by the B atom.It also indicates that the electron shell closing model could be regarded as a simple but valid tool for explaining the structures and stabilities of metal clusters.Chemical bonding analysis by Adaptive Natural Density Partitioning(AdNDP)method for the B13-cluster reveals that it is a π-antiaromatic system with 8 delocalizedπ-electrons.2.Theoretical study of the d10-σ interaction in XAuH2(X = F,Cl,Br,I)The d10-d10 and d10-s2 closed-shell interactions have been frequently studied in theoretical chemistry.However,there is still little known about the d10-σ interaction.In this work,we constructed XAuH2(X = F,Cl,Br,I)complexes,and optimized them at the TPSSh/def2qzvp level of theory.To confirm the stability of complexes,we analyzed the frequencies and the results show that complexes are stable.Moreover,the XAuH2(X = F,Cl,Br,I)complexes are also confirmed to be stable from binding energy.It is supposed that there is charge transfer in XAuH2 complexes,which is certified by the analysis on bond length,NBO charge and bond orders.The results of AdNDP and MO analysis indicates that d10-σ interaction can be viewed as a 3c-2e multicenter bond.ELF and LOL verified the results of bonding analysis.Charger-transfer indirectly is the nature of chemical bonding of multicenter bonding:firstly,H2-σ is regard as ligand coming into sp(Au)hybrid empty orbital,after that Au atom shift electron to X in further by Au-X.The Chemical bonding,LOL and ELF analysis show that when X changes from F to I,the closed-shell interaction(or 3c-2e multicenter bonding)is becoming weaker,which is due to the strongest electronegativity of F.The analysis reveals that the 3c-2e multicenter bond is formed by the charge-transfer.Interestingly,these is a linear dependence between the binding energy,H-H bond length in XAuH2 complexes and the occupy number of Au in the 3c-2e bonding.The d10-σ interaction proposed in XAuH2 complexes is a new idea in the field of closed-shell interaction in theoretical chemistry.