Structure Prediction And Electronic Structure Analysis Of Ligand Protected Gold Clusters

Ligand-protected gold clusters have attracted much attention in recent decades due to their particular properties and applications.The electronic configuration of Au is 5d106s1.Au element has full-filled d electrons and one single valence electron.The d10…d10 interactions in Au are aurophilic contacts,which are found to influence significantly a variety of structural and physical characteristics of gold.In this thesis,the perspectives on the energy landscape of Au-Cl binary system from the structural phase diagram of AuxCly（x+y=20）has been analyzed at first;secondly,we investigatethe electronic and geometric structures of AU30 clusters;thirdly,we report the evolution of 4e-superatom networks in Au4-（AuL）1-12 nanoclusters（L= Cl,SH,PH2,SCH3）;fourthly,we give a first principle study on the structural evolution and properties of（MCl）n（n= 1-12,M = Cu,Ag）clusters;Lastly,we study the electronic and geometric structure of Au70S20（PPh3）12 cluster.The main works are listed as follows:1.The structural phase diagram of AuxCly（x + y = 20）clustersTo give a direct and overall view of the energy landscape of Au-L binary systems,the AuxCly（x+y = 20）system is taken as a test case.By intensive global search of the potential energy surface at the level of density functional theory,a diverse set of global minimums and low-lying isomers are found at each composition,and the structural phase diagram is obtained.The unbiased global search is carried out using the method combining the genetic algorithm with TPSS functional.At x = 10 with the stoichiometric ratio of Au and Cl（1:1）,the cluster presents catenane structure.When x is in the range of 11-20,the clusters are Au-rich,and the Au-Cl system can be viewed as Cl-protected gold nanoclusters,where the gold cores consist of superatoms,superatom networks,or superatomic molecules in electronic structures.At x = 11-15,the gold cores consist of Au3,Au4 and Au5 2e-superatoms protected by staple motifs.At x = 16-20,the clusters are pyramidal superatomic molecules with one Au16 superatom core bonding with the four vertical atoms（Au or Cl）.When x is in the scope of 9-5,the clusters are Cl-rich,and the 5d electrons of Au participate in bonding,resulting in high multiplicities.The Au-Cl binary system shows great diversity and flexibility in electronic and geometric structures,and there are corresponding structures to most of the experimentally produced Au-L nanoclusters in our structural phase diagram.2.Evolution of 4e-superatom networks in ligand-protected gold（Au-L）nanoclustersThe superatom concept（SAC）and superatom-network（SAN）model are the two known tools,which are used to explain the electronic stability of Au-L nanoclusters.The size evolution of the superatom network in Au-L nanoclusters is still little known because lack of experimental data.To give a direct and overall view of size evolution of the superatom networks in Au-L clusters,the 4e Au4-（AuL）1-2（L = Cl,SH,PH2,SCH3）system is taken as a test case.The global minimum geometries have been explored using a first principle global minimization technique,namely,genetic algorithm from density functional theory geometry optimization（GA-DFT）.The SANs in these structures are composed by two of the Au3,Au4,Au5 or Au6 2e-superatoms protected by staple motifs.The SAN model is used to explain the chemical bonding patterns,which are verified by chemical bonding analysis based on the adaptive natural density partitioning（AdNDP）method.This work gives a clear size-evolution of the 4e-SANs for Au-L clusters with one to twelve ligands,which discovers the growth mechanism of Au-L clusters with different ligands.3.Electronic and geometric structures of thiolate-protected Au30 clustersDensity functional theory calculations have been performed to study the experimentally synthesized Au30S（SR）18 and two related Au30（SR）18 and Au30S2（SR）18 clusters.The patterns of thiolate ligands on the gold cores for the three thiolate-protected Au30 nanoclusters are on the basis of the“divide and protect”concept.A novel extended protecting motif with μ3-S,S（Au2（SR）2）2AuSR,is discovered,which is termed as tridentate protecting motif.The Au cores of Au30S（SR）18,Au30（SR）18 and Au30S2（SR）18 clusters are Au17,Au20 and Au14,respectively.The SAN and SAC models are used to explain the chemical bonding patterns,which are verified by chemical bonding analysis based on the AdNDP method and aromatic analysis on the basis of nucleus-independent chemical shifts（NICS）method.The Au17 core of Au30S（SR）18 cluster can be viewed as a SAN of one Au6 superatom and four Au4 superatoms.The shape of the Au6 core is identical to that revealed in the recently synthesized Au18（SR）14 cluster.The Au20 core of Au30（SR）18 cluster can be viewed as a SAN of two Au6 superatoms and four Au4 superatoms.The Au14 core of Au30S2（SR）18 can be regarded as a SAN of two pairs of two vertex-sharing Au4 superatoms.Meanwhile,the Au14 core is a 8e-superatom with 1S21P6 configuration.4.First principle study on the structural evolution and properties of（MCl）n（n =1-12,M = Cu,Ag）clustersThe structural evolution of（MCl）n（M = Cu,Ag,n= 1-12）clusters have been explored using GA-DFT technique.Benchmark calculations indicate that the method used in this work is reliable.The computational results indicate that the structures of（CuCl）n and（AgCl）n clusters share many similarities.The global minima of（MCl）_n clusters are planar and nonplanar monocyclic structures up to n= 5,and multi-ring or catenane structures when n≥ 6.The characters of the intramolecular interactions of（AgCl）3,（CuCl）3,（CuCl）4 and（CuCl）6 clusters have been studied by the noncovalent interactions（NCI）index.The electronic and bonding properties of（CuCl）3 and（AgCl）3 clusters are analyzed.At higher level,the refined energies of the isomers,the binding energies and second difference of binding energies are calculated,which are examined as a function of the cluster size.Several magic sizes with higher stability and symmetry are discovered.In particular,we find that the clusters adopt a new folding manner,tube conformation,at n = 6,9 and 12,where they are more stable than the adjacent clusters;moreover,they are aromatic.5.Study on the electronic and geometric structure of Au70S20（PPh3）12 clusterDensity functional theory calculations have been perfonned to study the experimentally synthesized Au70S20（PPh3）12 cluster.The patterns of thiolate ligands on the gold cores of the cluster is on the basis of the“divide and protect”concept.A novel extended protecting motif with ten μ₃-S and six S legs,[Au12（μ3-S）10]8-,is discovered,which is termed as six-tooth protecting motif.The Au core of the cluster is Au4616+.The AdNDP method and superatom network model are used to analyze the chemical bonding patterns of the cluster.The Au4616+ core can be viewed as a SAN of five edge-sharing Au6 superatoms,four vertex-sharing Au7 superatoms and two Au4 superatoms.The five Au6 superatoms comprise Au2212+ core.Meanwhile,the Au₂212+ core can also be regarded as a ten-electron superatomic molecule,super-N2 molecule.