Theoretical Study On Structure, Bonding Energies And Aromaticity Of Binary Coinage Transition-Metal Clusters

Aromaticity has attracted most attention in recent years. Alexander I. Boldyrev in Utah State University and Lai-Sheng Wang in Washington State University published an important paper on Science in 2001. They prepared a series of gas all-metal cluster MAl₄^- (M=Li, Na, Cu) and found that the most stable configuration of MAl₄^- is pyramidal structure (the configuration of AI₄^2- is square) by XPS experiment and quantum chemistry computing method. HOMO was found to be a total delocalizedÏ€MO and the others areσor isolated electron type MO from analyses of the valence MOs of the AI₄^2-. TwoÏ€electrons totally occupied the delocalized HOMO Mos. It proves to be aromatic system because it suit for the Hückel (4n + 2) rule (n=0). The scientific meaning of this paper consist that spread aromaticity which originally belonged to organic chemistry conception to transition metal rings system.Shape memory alloy (SMA) is a shape memory alloy series skies. Under certain conditions and a certain degree of deformation, the deformation of the material disappeared and returned to the shape before deformation when the outside conditions such as temperature changed. Meanwhile, the ordinary metal materials will leave a permanent deformation. NiTi shape memory alloy is the most widely used memory alloy material. Generally, the atomic number of Ni and Ti in the shape memory effect NiTi alloy is close and the atomic ratio is about 1:1. The parent of NiTi alloys is CsCl-type body-centered cubic B2 structure (a₀=0.301-0.302nm). The body-centered atom and the atoms around it are two different kinds of atoms. The body-centered atoms can be the Ti atom and also can be the Ni atom. The aromatic nature, the nature of bonding and the bond of suchmaterial were not able to study in detail. However, the study of aromatic nature will help to fully understand the special stability and the bonding energy of such material.The structure, bond energy and aromatic nature of NiTi compounds were studied systematically through DFT methods in this study, and NBO was used to optimize the geometry of equilibrium bond length, naturally charge, bond etc. Meanwhile, the interaction between the various elements debris in the geometry structure of the optimal balance can be theoretical calculated by the imaginary reaction and the electrostatic potential classic method, thereby the strength of the interaction between metal and ligand was determined. In addition, the frontier molecular orbital and the orbital relation map between molecular fragments were used to study the electronic structure of the ground state and the circulation of complexes. Finally, GIAO-NMR calculation method was used to calculate the central aromatic nature of the complexes and NBO program was used to analyze the contribution of different bond and atomic nucleus to NICS. Then basing the distribution characteristic we made the theoretical discrimination on the aromatic nature of complexes and the magnitude of aromatic nature.The main structures studied include: (a) Ni₄(D_4h), [Ni₄]²⁺(D_4h), [Ni₄]^2-(D_4h) (b) Ni₄Ti₂(D_4h), [Ni₄Ti₂]²⁺(D_4h), [Ni₄Ti₂]^2-(D_4h) and (c)Ni₄Ti4(D2h), [Ni₄Ti4]²⁺(D2h), [Ni₄Ti4]^2-(D2h). This divided into three groups design is based on Ni₄ ring, and increasing Ti atom number is used to study the effect of change caused by importing of Ti atom on the Ti ring system and the whole structure. From the structure of Ni₄Ti₂,[Ni₄Ti₂]²⁺ and [Ni₄Ti₂]^2- we can see that the importing of the two Ti atoms makes the Ni₄ ring well planar and the four Ni-Ni bonds length averaged. If importing two Ti atoms, it will form the structure of Ni₄Ti4, [Ni₄Ti4]²⁺ and [Ni₄Ti4]^2-. Then the averaged Ni₄ ring length will be broke and make it a rectangular planar structure.All the Ni atoms in Ni₄(D_4h),[Ni₄]²⁺(D_4h)å’Œ[Ni₄]^2-(D_4h),Ni₄Ti₂(D_4h),[Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h) bonded into a coplanar ring, and all the Ni-Ni bond were of the same length. Such structure properties ensured the aromaticity for the studied complexes. The incorporation of Ti atom enormously weakened the inner-ring Ni-Ni interaction.As assumed, by analyzing the bond energy in the corresponding reaction, we can predict that such clusters were easy to form crystals rather than mono-molecules. NICS calculations told us that these complexes possessed relatively strong magnetic responses. In detailed analysis, we discovered that the NICS for Ni₄Ti₂(D_4h) and Ni₄Ti4(D2h) were of positive numbers, while [Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h) were of negative numbers and [Ni₄Ti₂]^2-(D_4h) more negative.Analysis of the contributions to NICS by bonding energy and molecular orbital showed that the diamagnetism circumfluence in [Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h) caused by s and d orbitals was the main reason for their aromaticity. In detail, Ti atom provided s orbital and Ni provided d_z2 and d_x2—y2 orbitals, together to form a square ring for a spheric d orbital circumfluence. And in [Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h) there exist obviousÏ€orbital circumfluence.