Theoretical Study On The Structures, Magnetic Properties And Superhalogen Behaviors Of Transition Metal-halogen Clusters TMX_n(TM= Fe, Co, Ni;X=F, Cl, Br; N=1-7)

Electron affinity (EA) is one of the most important parameters to characterize the chemical stability and the ability to attact additional electrons of atoms, molecules, and clusters. Atoms, molecules, and clusters with high electron affinities (EA) play important role in chemical reaction. Superhalogen clusters are composed of a central transition-metal or main-group atom surrounded by halogen atoms, in which the number of halogen atoms exceeds the normal valence of the metal atom, and thus the clusters have high electron affinities. In this thesis, based on the B3LYP hybrid density functional method, we have systematically studied the ground state equilibrium geometries, relative stabilities, magnetic properties and superhalogen behaviors of TMXn (TM=Fe, Co, Ni; X=F, Cl, Br; n=1-7) clusters. The results are summarized as follows.1. Our results show that, the number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atoms. In the case of neutral and anionic FeFn and CoFn clusters, a maximum of 6 F atoms can be bound atomically to the central metal atoms, the highest oxidation state of Fe and Co is+6. In the neutral and anionic NiFn clusters, only 4 atoms can be bound atomically to Ni atom, while remaining F atoms bind to other F atoms to form F2 molecules. In the neutral and anionic TMCln (TM=Fe, Co, Ni; n=1-7) clusters, a maximum of 4 Cl atoms can be bound atomically to the Fe or Ni atom, but in the neutral and anionic CoCln clusters,5 Cl atoms can be bound atomically to Co atom, in these clusters the highest oxidation state of these metal atoms is +4,+5,+4, respectively. In the case of neutral and anionic clusters TMBrn (TM=Fe, Co, Ni; n=1-7),+4 is the highest oxidation state of all metal atoms. Furthermore, we also predicted the most preferred fragmentation channel for all the ground state clusters.2. The computed adiabatic electron affinities (AEA) and vertical detachment energies (VDE) of TMXn clusters indicate that, when the number of halogen atoms n≥3, the EAs of all clusters are higher than the EAs of all halogen atoms, these clusters are thus classified as superhalogens.The maximum values of calculated AEA and VDE are 7.61eV and 8eV, respectively, far exceeding the EA of Cl that has the highest value (3.61eV) in the periodic table.3. Analysis on the magnetic properties of TMXn clusters shows that, these superhalogens are different from conventional superhalgens in that they carry large magnetic moments. For example, the magnetic moments of FeF, CoCl6 and NiBr6 clusters are 5μB,5μB,4μB, respectively. The magnetic moments of TMXn clusters contribute mainly from the 3d orbitals of transition-metal atoms, and depend strongly on the number of halogen atoms in a cluster as well as the charge state of the cluster.