Theoretical Study On Structures And Properties Of Atomic Clusters Including Boron

With the development of chemistry, a new term -- atomic cluster are recognized since 1950s. Especially for the observing of C60 fullence, it has been employed in many fields, such as iatrology, biology and material and so on. This opens a new era for atomic cluster. Those clusters including boron atom have attracted much attention because many boron containing compounds are found to be as good candidates in technological application such as high temperature semiconductor devices, superconductor and nano-scale science. But till latest years, some small boron clusters are gradually synthesized and characterized. It's well known that atomic cluster is a huge system and it has various structures and characters. So it is significant and important for each atomic cluster to investigate their synthesis, structures and properties in detail. In this paper, the structures and characters of those clusters including boron, carton and silicon et al main group element are studies in theory. Some stable compounds are obtained and their electronic structures are analyzed. The calculated results enriched the functional characters of atomic clusters. The article is composed of four aspects as following:(1) Various isomers of B32 cluster and metal-poly-boron MB32 complexes are examined at the B3LYP/6-31G level to obtain the optimized geometries and vibrational frequencies. The expected B32 (Ih) isomer, proposed as conjugated polyhedron of C₆₀, can not be found as local energy minimum, owning to its large curvature strain and antiaromaticity. The tubular and quasi-planar B32 clusters are more stable than other three dimension (3D) structures. The geometry, NICS values and MOs analysis give the probable factor influencing the stability of B32 clusters: (1) curvature strain; (2) aromaticity; (3) delocalized bonding. What's more, some minima of MB32 (B32= cage (7), quasi-planar (4), bowl (6) and tubular(1); M=Li) are found, indicating bare B32 cluster can absorb a metal atom and form metal-poly-boron MB32 complexes. The interaction between metal atom and some B atom of B32 can be viewed as an ionic bond according to geometric, vibrational frequencies analysis, NBO calculation and the topological property ? 2ρ(r)calculation.(2) B₁₀, B₁₀-, B₁₀+ cluster are examined at the B3LYP/6-311+G (d) level to obtain the optimized geometries and vibrational frequencies. All calculations show quasi-planar B₁₀, B₁₀+ and B₁₀- clusters are, respectively, most stable structure of all obtained neutral, cation and anion B₁₀ clusters. And B₁₀ cluster is a closed-shell, stable and compact molecule, which suggests it may possess some special chemical properties. Then the investigations of MB₁₀ and B₁₀-M-B₁₀ (M = Li, Na, K) indicate quasi-planar B₁₀ can sandwich a metal atom to form half or full sandwich-type metal complexes. The NBO, MOs and vibrational frequencies analyses reveal stable quasi-planar B₁₀ may be as a new ligand. These sandwich complexes can also be viewed as quasi- diatomic (or triatomic) systems.(3) We first investigate the structure and electronic properties of poly-coordinate planar boron compounds that boron atom is the center and other main group elements are as the surrounding. Poly-coordinate planar B compounds BXn (X= B, Al, C, N and Si; n=3-8) are optimized at B3LYP/6-311++G (3df, p) theoretical level. For X=B, center B atom can coordinate 3-8 atoms, while for X= Al, C, Si and N, it can only coordinate 3-5 atoms. BCn clusters prefer planar ring type structure to planar coordinate one. It is difficult to gain the stable planar coordinate BNn. This is relative with their atomic radii, bond lengths and electro- negatives. The NBO analysis shows the center B atom does not violate the octet rule, though the numbers of coordinated atom even reach 8, which thanks to partial bond or multi-center bond between center boron atom and other boron atoms. The MO analysis and NICS values both indicate these poly-coordinated planar compounds BXn (X= B, Al, C, Si and N; n=3-8) are aromatic, either 2-fold (σandÏ€) or single (σorÏ€). The bonding nature possessing highly delocalizedσandÏ€bonding orbitals are responsible for their stability and arranging all atoms in one plane.(4) B₁₂ cluster and its metal-poly-boron MB₁₂ (M=Li, Na, K, Be, Mg, Ca) compounds are examined at the B3LYP/6-311+G (d) level. The analysis for the geometry of B₁₂ cluster shows it is a rigid and compact molecule and can be as a whole while interacting with the metal atom. The NBO and interaction energy calculations reveal that the bonding between metal atom and B₁₂ unit is an ionic bond. These results inspire us that B₁₂ cluster probably possesses some special characters. According to the calculated EA, wiberg bond indices (WBIs), natural atomic populations (NPAs) and vibrational frequencies, B₁₂ cluster seems to be as a super-atom. Moreover, [MB₁₂ ]-1 (M=Li, Na, K) compounds also present some optical properties. The calculated first hyperpolarizabilityβ0 of [MB₁₂]-1, especially for M=K, is large, 51342 a.u., which indicates [KB₁₂] -1 compound has large nonlinear optics (NLO) response and may be as a good candidate of NLO materials. In addition, we find the sequence ofβ₀ value for [MB₍120]^-1 is:β₀ (M=Li) <β₀ (M=Na) <β₀ (M=K), which reveals that the first hyperpolarizability has a dependence on the atomic number of the alkali metal. It is significant to investigate the optical property of MBn species.