Theoretical Studies Of Boron Carbide Clusters

Clusters have the size between those of atoms and macroscopical systems, and anomalous physical and chemical properties that have provided a way for producing and developing new material. So many experimental and theoretical researches have been done about them. By undertaking calculations of the special structures and electrical characters on clusters, it can help the experimentalists in identifying novel structures for investigation.Boron-carbon compounds have the special structure, electrical character and have the tremendous appliance values and tempting appliance foreground. Therefore, compounds of boron and carbon are found to be good candidates for various technological applications, such as control rods in nuclear fission reactors, target tiles in nuclear fusion reactions, protective coating of carbon fiber materials, against atomic materials oxygen in spacecraft and extra hardness materials. So the studies about the preparation and academic study of sorts of size the boron-carbon clusters has been attracted increased interest by scientists.This paper includes four parts : in the first part, stable isomers of B4C clusters were searched at the B3LYP/6-31G(d) levels and analyzed in detail, furthermore, the conversions among various isomers were reseached. In the second part, stable isomers of B5C and C5B clusters was searched at the B3LYP/6-311+G(d) levers and analyzed in detail, moreover, the conversions among various isomers were reseached. In the third part, the small clusters B_nC and B_nC₂ (n=3-10) were studied in detail at the B3LYP/6-311G** levels. Bonding Energy, Vertical Electron Detachment Energies (VDE), Adiabatic Electron Detachment Energies (ADE), Vertical Electron Affinities (VEA) and Adiabatic Electron Affinities (AEA) of the most stable linear and cyclic of B_nC and B_nC₂ clusters were analyzed. In the fourth part, stable isomers of P₆, P₆⁺, P₆^-, P_n (n=2-10) clusters were reseached and analyzed detailedly.In the first part, geometry optimizations and vibration frequencies of B₄C clusters were performed with the density functional theory Becke-3LYP method using 6-31G(d) basis set. Fourteen stable isomers were found, and the most stable structure among them is the five-member ring containing two three-member boron rings. We also analyzed these stable isomers in detail, and the results show the structures containing three-member boron ring are predominant in energy for B₄C clusters. In terms of MO and NBO analysis, the three-centered bond and theÏ€-electron delocalization play important roles in stabilizing the planar five-member rings of these B₄C clusters. The calculated nucleus-independent chemical shifts (NICS) of the structures of isomer 11, 12, 13 and 14 are all negative values, which indicate their aromatic characters. Our calculations suggest that isomer4 can be converted into isomer7 with only an energy barrier of 0.31kJ.mol^-1 at the B3LYP/6-311+g(3df) levels, therefore, isomer4 would not be created successfully. The planar structures of the five-member rings (isomer 12, 13 and 14) can be converted with each other, the conversions of isomer 14 to isomer 13 and isomer 13 to isomer 12 have high energy barriers of 70.99 and 68.51kJ.mol^-1 at the B3LYP/6-31g(d) level, respectively.In the second part, geometry optimizations and vibration frequencies of B₅C and C₅B clusters were performed at B3LYP/6-311+G(d) levels. Stable configurations of B₅C and C₅B clusters were obtained respectively and analyzed in detail. The most stable structure of B₅C is the planar six-member ring. However, for C₅B clusters, the most stable structure is the linear structure with boron atom in the position3. Various configurations of B₅C clusters which contain three-member boron ring have predominance in energy aspect. While configurations of C₅B clusters which contain three-member carbon ring have inferior position. In B₅C clusters, if isomer 2 has an excess energy≥43.83 kJ.mor^-1, it would be converted into isomer 1. Similarly, in C₅B clusters, if the isomer 5 was given excess energy≥19.66kJ.mor^-1, it would be converted into isomer 2. And if the isomer 7 has the excess energy >20.57kJ.mol"1, it would get across the energy barrier and be converted into isomer 2.In the third part, the small clusters of B_nC and B_nC₂(n=3-10)were studied in detail at the B3LYP/6-311G** levels, and stable configurations of them were obtained. Bonding energy, Vertical electron detachment energies (VDE), Adiabatic electron detachment energies(ADE), Vertical electron affinities (VEA) and Adiabatic electron affinities (AEA) of the most stable linear and cyclic of B_nC and B_nC₂ clusters have been analyzed at the same level. The most stable structures of B_nC and B_nC₂ clusters are n+1 and n+2 member ring respectively. When n is small, the assumption that the structures containing three-member boron ring are predominance in energy is reasonable. However, when n is large, the assumption is not reasonable. The most stable cyclic structures of B_nC and B_nC₂ clusters mostly haveÏ€-orbitals and the three-centered bond; these may be the important causes for the stabilities of them.In the fourth part, through the studies of isomers of P₆,P₆⁺,P₆^- clusters, stable isomers were obtained, and analyzed in detail. The most stable structures of P₆, P₆⁺, P₆^- clusters is derived from tetrahedral P₄ by adding two two-fold phosphorous atoms. At the same time, we have studied the P_n(n=2-10) clusters. The bonding energies of P₁₀ containing two tetrahedron, tetrahedral P₄ and cueane P₈ are three higher ones, and they are three more stable structures of P_n(n=2-10) clusters. Therefore, we support that tetrahedral P₄ and cueane P₈ have predominance in energy, and they are important components for construction of larger clusters and could be important candidates in designing phosphorous clusters models.