Structural And Electronic Properties Of The Boron Atomic Clusters And Water Molecular Clusters

As the bridge between the microscopic atom/molecule and macroscopic condensed matters, the research of clusters is helpful to reveal the evolution from microscopic single atom/molecule to the condensed matters, and provide a scientific basis for the micro-scale materials design/modification, and also important for the study of life science, catalytic chemistry and metal-organic chemistry. In this thesis, density functional theory method was used to investigate the structural, electronic properties and infrared spectra of boron clusters and water clusters.Boron is the fifth element in the periodic table and the first one to possess a p electron. The unique characters of boron, such as short covalent radius, electron deficiency, and flexibility to adopt multi-center bonds, endow boron clusters probably the most intriguing chemical bonding, which continues to attract scientific interests. It is helpful to study boron clusters to further understand boron.The proposal of B80buckyball triggered a hot wave of examing the structural, electronic, magnetic properties of B80fullerenes, and designing B80-based solids, nano sheets, nanotubes and nanoribbons. Unbiased search on the potential energy surface of medium-sized boron clusters, with B32-56, and B68-80as representatives, was carried out using simulated annealing incorporated with first-principles molecular dynamics. For n=32-56, boron clusters are more stable than the previously proposed symmetric cages. The stability of these irregular cages can be understood by the three-center bonds as well as the polygonal holes on the cage surface. For B68, B74, and B8o, these boron clusters thermodynamically prefer the B12-centered core-shell structures, which resemble the fragment of bulk boron solids. Though these core-shell clusters lack a descriptive symmetry and may not be the true global minima, the core-shell B80is about25meV/atom lower in energy than the widely assumed highly stable "magic" B80fullerene. The electronic states and photoelectron spectra of these clusters are closely correlated to the structural motif, which may be helpful for detecting the cluster configurations in experiments.Prompted by the controversy of the most stable structure of B80, we carefully evaluated the performance of the desity functional methods in the energetics of B2o clusters, and found the BLYP and B3LYP density functionals are not reliable to describe the relative stabilities of boron clusters, the PBE, TPSS, TPSSh and PBEO perform better than the other examined functionals. The computations using the herein validated functionals for various B80isomers reassured that the core-shell (stuffed fullerene) B80cluster is the most energetically favorable. The preferred core-shell structural pattern for the medium-sized boron clusters can be extended to larger clusters like B101-B103, and the complete core-shell cluster can be obtained at B103. All the obtained low-lying boron clusters have strong aromaticity and considerable HOMO-LUMO gap energies.As the eternal theme, water has been attracting scientists to reveal its mysterious veil both experimentally and theoretically. Water has been recognized as the matrix of life, The geometric structures, stabilization energies, dipole moments, and vibrational frequencies of the neutral water clusters (H2O)n, with n=1-10, were investigated using density functional theory along with a variety of exchange-correlation functionals (LDA with SVWN5parameterization, GGA with BLYP, PW91, PBE, B3LYP, X3LYP, PBEO, PBE1W, M05-2X, M06-2X and M06-L parameterizations) as well as high-level ab initio MP2and CCSD(T) methods. Using the MP2and CCSD(T) results as benchmarks, the effects of exchange-correlation functionals and basis sets were carefully examined. Each functional has its advantage in certain aspects; for example, M05-2X and X3LYP yield better geometries, and the capability of these two functionals to distinguish the relative energies between isomers are more similar to MP2. The size of the splitvalence basis set (6-31G or larger), diffuse functions on the oxygen atom, and d(p) polarization on the oxygen (hydrogen) atom are crucial for an accurate description of intermolecular interaction in water clusters. The6-31+G(2d,p) basis set is thus recommended as a compromise between computational efficiency and accuracy for structural description. We further demonstrated that the numerical basis set, TNP, performs satisfactorily in describing structural parameters of water clusters.In the meantime, the growing-structural patterns of water clusters with30-48water molecules were further investigated by means of a combined Monte Carlo (MC) search algorithm and density functional theory (DFT) computations. The (H20)30-48clusters with amorphous structures are lower in energy than the previously reported fused cage and the tubular configurations. The significant differences of infrared spectra (IR) of three different structural patterns provide some clues to experimentally characterize the structural properties for medium-sized water clusters.