Computational Studies On The BN Clusters (BN)_n

With the rapid advance in the research of computer and computing technology, applications of computer and computing technology were pervasive in almost every aspect of modern life and they become indispensible in the scientific research due to the power of fast computing. Exemplar application is computational chemistry, which studies the fundamental mechanism of chemistry with application of basic theory of quantum mechanics by solving the Schr?dinger equation heavily depends on the development of computer and computing technology. In the present thesis, the techniques of computing science have been applied in computational science to study the structures and properties of a novel type of nanomaterials: boron nitride cage-shaped clusters (BN fullerenes). The structure, stabilities, aromaticity, Infrared, Raman, and electronic spectra of BN clusters (BN)n (n=12, 16, 20, 24, 28, 36) and endohedral metal-doped BN clusters M@(BN)n (M=Ca, Zn) have been predicted within density functional theory based quantum chemical method B3LYP for its merits of computational efficiency along with Gaussian basis set 6-31G(d).The stability of BN cluster increases with the size of cluster. The squares in BN clusters have less polarized charges (i.e. less ionic characters with relatively small amount of charges compared to those atoms off squares), while the rest part of BN cluster have relatively larger amount of charges. The nature of the bonding of the HOMO is clearlyπbonding mainly from the p atomic orbitals of B and N. However, the bonding strength of HOMO is conspicuously weaker than that in carbon based fullerenes. Such weak conjugation of the HOMO (and other occupied molecular orbitals) along with the NA charges on B and N suggest partially ionic character of bonding in BN clusters. The major population of charges on the squares of BN clusters in the HOMO could render, according frontier molecular orbital theory, this region relatively reactive in chemical reactions.Because of space confinement of the inner side of BN clusters, the conjugation of the p orbitals (inducing ring current) is stronger in the inner side of those cages, which might be one of major causes for the aromaticity of BN clusters as indicated by the negative independent chemical shifts (NICS) at the cage center, ring center, and 1? above the ring center of the systems. According to those NICS values, theσaromaticity is stronger than theπaromaticity in all rings of BN clusters asπconjugation in BN clusters is not strong enough to induce large ring current ofπelectrons. Squares have the highest aromaticity due to the compact geometry. The aromaticity of the BN clusters does not change much with the size of clusters as indicated by the NICS value at the cage center. However, the aromaticity of BN clusters is weaker than that of carbon based fullerenes with the same size. The IR spectra of BN clusters have two major bands (peaks). One is around 800cm?1 and the other one is between 1400 and 1550 cm?1. The peaks around 800 cm?1 mainly involve the radial motion of the eight squares. The strongest IR peaks ranging from 1400 to 1550 cm?1 dictate the BN bonds stretching particularly the bonds involving squares. The Raman spectra of BN clusters have three major bands ca 200, 400, and 800 cm?1, respectively. The strongest peak corresponds to the breathing of eight squares which could serve as characteristic peak for BN clusters as indicated by the vibrational modes of such peaks. The Raman spectra of BN clusters have overall red-shift with the increase of the size of cluster. The electronic spectra of BN clusters do not have regular pattern with respect to the size of clusters. The major peaks arise from theππ* transition.The effect of doping of different metal in BN clusters on the properties of BN clusters have been clearly manifested by Ca and Zn. Essentially, the doped cage is stabilized with the increase of the cage size. The doping of Zn is more feasible if doping energy is used as criterion. The variation of doping energy (stability of doped cage) with size of cage has the similar change pattern of NA charge of the doped metal atom, i.e. the amount of charges on the metal atom increases with the size of BN cage and the doped cage get more stabilized with the increase of cage size. In the case of doping of Ca, except for Ca in B36N36, Ca donates electrons to BN clusters. For the case of Zn, promotion of electrons from 4s and 3d to 4p of Zn occurs in all three BN clusters with charge transfer from BN clusters to Zn. The doping of metal atom in BN clusters makes HOMO-LUMO gap smaller than that of bare BN clusters. Such smaller HOMO-LUMO gap might be that the confined space in cage for the orbital of metal atom destabilizes the highest occupied atomic orbital of metal atom and the interaction between cage and metal atom. The IR and Raman spectra of Ca doped BN clusters are quite different from those of pristine BN clusters, whereas the IR and Raman spectra of Zn doped BN clusters are very similar to those of bare BN clusters. This might result from the different interaction of Ca and Zn with BN clusters. With more diffusive outer shell atomic orbital in Ca, Ca has stronger interaction with BN cage, thus changes the electronic structure of BN cage much consequently inducing change in properties. More major absorptions occur in long wavelength region in the electronic spectra of Ca and Zn-doped BN clusters with respect to those of empty BN cages. The doping of Ca brings about stronger effect to the electronic structure of the doped BN clusters than the doping of Zn. The doping energies, IR, Raman, and electronic spectra of Ca-doped BN clusters have conspicuous difference from those of empty cages, while those of the Zn-doped BN clusters are similar to those of empty cages. Changing the doping metal could broaden the application of metal-doped BN clusters in chemistry and materials science, e.g. ensemble materials with tailored properties.