| Clusters have been widely investigated by material-chemist, theory-chemist and physical scientist, especially the Boron nitrides (BN)n clusters.As a new material, the boron nitride is similar to the graphite; it is also called white graphite. The most differences of the boron nitride are white, nonconductor, heat resistance in air and many other properties. It is like carbon, every crystal states have special properties.Since people have already studied on carbon clusters by means of theory and experiment, they found the interesting structure of fullerenes, nanotube and et al. (BN)n clusters have the same electrons as corresponding C2n molecules and they might be possible to create new material like fullerenes and carbon nanotubes. Additionally, a series of boron nitride clusters and nanobubes have been detected by high-resolution electron microscope image. No matter however their structure will be changed, they remain have the excellent properties such as heat resistance in air, insulation, wide bandgap. So they may have a better prospect than fullerene carbon in electron-apparatus, semi-conductor, and high quality insulated lubricant et al.In this paper, (BN)n (n=1～20) isomers were designed by using the molecular figure software G-view. The full geometry optimization were performed at sto-3g level of HF method by the Gaussian03 program, after eliminating the repeat configurations, the full geometry optimization and harmonic vibration frequency analysis were performed at 6-31G (d) level using density functional theory B3LYP method, then the energy, geometry structure, bonding character, NBO analyse frontier molecular orbital (FMO) and mulliken population have been analyzed.We studied the structural characteristics and stability of (BN)n isomers and obtained the following conclusions: (1) The single-ring structures were the lowest energy isomers for (BN)n (n=1～10) ,the single-ring structures with Dnh symmetry is obvious lower than any other structures in each group. The N atom holds sp2 to bond while the B holds sp. There is a 2nc-2ne delocatedπbond in each single-ring structure (except (BN)2). Triple bonds between boron and nitrogen were formed, mainly because the inter reaction between the unoccupied orbital of B and the lone pair of N atom to form feedbackπbonding. (2) (BN)2 is the special configuration, we guess there is a single electron B-B bond in this molecule, boron atom does not bond in HOMO. The HOMO-LUMO gap is 0.844eV, which is lower than any other single-ring structure; the bond length is the longest in these complexes. Since it has a special bonding, the aromaticity is very remarkably. (3) The B-B weak bonds are formed in some four-number ring structures; it is easy to destroy because the bond lengths are relatively longer than other bonds. (4) From the (BN)6, the triple-ring with C2h or C2v symmetry is easy to formπbonding. (5) There are single and triple states in linear BN)n structures, but the energy of triple state is lower than the single one and it cost lower in computation. (6) Generally speaking, the one is more regular with good symmetry; the energy is close to the single-ring structure. (7) In a handle-like structure, it has no imaginary frequency when the node is B atom, otherwise, when the node is N atom, it maybe have.We analyzed the average bond-length, bond-angle and binding energies differences of the single-ring (BN)n (n=1～20) clusters. When the cluster size increases, the bond-length decreases, the values of angle∠NBN approach to 180°and the∠BNB approach to the internal angles of n-side polygons. According to the binding energy, it increases as the size increases; when the size is small, the Eb is rapidly increased, when n >6, Eb increased tiny; when n>8, it changes slightly. It indicates, increasing the BN units play little role to the physical and chemical properties of (BN)n clusters.When we evaluate to the aromaticity, the (BN)n clusters with odd n are aromaticity, even n are antiaromaticity; when n>12, the value of NICS is close to zero; thus, (BN)n clusters trend to nonaromaticity when radiuses of rings are big. The results of HOMO-LUMO bandgap shows, the (BN)n clusters with odd n are more stable than (BN)n with even n.When n>6, all the lowest vibration frequencies are accord to the twisty vibration of ring in the planar face, the differences of vis close to the bond angle, the smaller the bond angle twisted, the smaller vibration frequency value is gained, vice versa. |
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