Exploration of structures and stability of planar CnB3 (n = 1–8)

Jingling Shao , Chunyan He , Rongwei Shi , Cheng Wang , Xiaolei Zhu , Xiaohua Lu
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引用次数: 11

Abstract

The structures and relative stabilities of CnB3 (n = 1–8) clusters are explored and analyzed at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level. For low-lying isomers of CnB3 (n = 1–8), cyclic structures are more favorable in energy than branch as well as linear structures, and rings with exocyclic chains. For most isomers of CnB3 (n = 1–8), the structures with doublet states are lower in energy than the ones with quartet states. The lowest-energy isomers of CnB3 are all cyclic structures. The lowest-energy structures of CnB3 (n = 1–3) appear to be similar to those of pure boron clusters which have polycylic structures, while the lowest-energy structures of CnB3 (n = 4–8) tend to be analogous to the corresponding geometries of pure carbon, CnB, and CnB2 clusters which possess single-ring structures. The most stable structures of planar CnB3 (n = 3–8) clusters can be derived from combining a B–C–B–C–C–B building unit and a carbon chain. Some physical properties, such as the binding energy per atom, incremental binding energy, and second order energy difference suggest that for CnB3 clusters, odd-n clusters are more stable than even-n clusters. Results from NBO and molecular orbital analysis illustrate that the formation of the delocalized π MOs, the σ-radial and σ-tangential MOs are favorable to the stabilization of structures for lowest-energy isomers. It is interesting to find from the valence molecular orbital and NBO analyses that for the most stable isomers of CnB3 (n = 1–8), the numbers of π electrons for CnB3 (n = 1–8) is (n + 1) with an exception of C4B3 (6 π electrons), suggesting that CB3, C4B3 and C5B3 may have aromaticity, which is consistent with the results of the nucleus independent chemical shifts (NICS).

平面CnB3 (n = 1-8)的结构与稳定性探讨
在CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d)水平上对CnB3 (n = 1-8)簇的结构和相对稳定性进行了探索和分析。对于CnB3 (n = 1-8)的低处异构体,环状结构比支链、线性结构和带外环链的环状结构更有利。对于CnB3 (n = 1-8)的大多数同分异构体,具有重态结构的能量低于具有四重奏态结构的能量。CnB3的能量最低的同分异构体都是环状结构。CnB3的最低能量结构(n = 1-3)与具有多环结构的纯硼团簇相似,而CnB3的最低能量结构(n = 4-8)与具有单环结构的纯碳、CnB和CnB2团簇的相应几何结构相似。平面CnB3 (n = 3-8)簇最稳定的结构可以由一个B-C-B-C-C-B结构单元和一个碳链组合而成。单原子结合能、增量结合能和二阶能差等物理性质表明,对于CnB3簇而言,奇n簇比偶n簇更稳定。NBO和分子轨道分析结果表明,离域π型MOs、σ-径向型MOs和σ-切向型MOs的形成有利于低能异构体结构的稳定。从价电子轨道和NBO分析中发现,CnB3 (n = 1 - 8)最稳定的异构体(n = 1 - 8),除C4B3(6个π电子)外,CnB3 (n = 1 - 8)的π电子数均为(n + 1),这表明CB3、C4B3和C5B3可能具有芳香性,这与核无关化学位移(NICS)的结果一致。
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