Li Hai-Ru, Jian Tian, Li Wei-Li, Miao Chang-Qing, Wang Ying-Jin, Chen Qiang, Luo Xue-Mei, Wang Kang, Zhai Hua-Jin, Li Si-Dian, Wang Lai-Sheng
Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China.
Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.
Phys Chem Chem Phys. 2016 Oct 26;18(42):29147-29155. doi: 10.1039/c6cp05420j.
Size-selected boron clusters have been found to be predominantly planar or quasi-planar (2D) in the small size regime with the appearance of three-dimensional (3D) borospherene cages of larger sizes. A seashell-like B cluster was previously shown to be the smallest borospherene, which competes with a quasi-planar isomer for the global minimum. Here we report a study on the structures and bonding of the B and B clusters using photoelectron spectroscopy (PES) and first-principles calculations and demonstrate the continued competition between the 2D and borospherene structures. The PES spectrum of B displays a complex pattern with evidence of low-lying isomers. Global-minimum searches and extensive theoretical calculations revealed a complicated potential energy surface for B with five low-lying isomers, among which the lowest three were shown to contribute to the experimental spectrum. A 3D seashell-like C (2, A') isomer, featuring two heptagons on the waist and one octagon at the bottom, is the global minimum for B, followed by a 2D C (3, A) isomer with a hexagonal hole and a stingray-shaped 2D C (1, A') isomer with a pentagonal hole. However, by taking into account the entropic effects, the stingray-shaped isomer 1 was shown to be the lowest in energy at room temperature and was found to dominate the PES spectrum. Isomers 2 and 3, which have lower electron binding energies, were also found to be present in the experiment. Chemical bonding analyses showed that isomer 1 is an all-boron analogue of benzo[ghi]fluoranthene (CH), whereas the borospherene isomer 2 possesses 18π electrons, conforming to the 2(N + 1) electron counting rule for spherical aromaticity. For the B neutral cluster, the seashell-like borospherene isomer is the global minimum, significantly lower in energy than the stingray-shaped quasi-planar structure.
人们发现，尺寸选择的硼簇在小尺寸范围内主要是平面或准平面（二维）结构，而较大尺寸时会出现三维（三维）硼球烯笼状结构。先前已表明，一种贝壳状硼簇是最小的硼球烯，它与一种准平面异构体竞争全局最小值。在此，我们报告一项使用光电子能谱（PES）和第一性原理计算对B和B簇的结构与键合的研究，并证明二维结构和硼球烯结构之间持续存在的竞争关系。B的PES谱显示出一个复杂的图谱，有低能异构体的迹象。全局最小值搜索和广泛的理论计算揭示了B的一个复杂的势能面，有五个低能异构体，其中最低的三个异构体对实验谱有贡献。一种三维贝壳状C(2, A')异构体，腰部有两个七边形，底部有一个八边形，是B的全局最小值，其次是具有六边形孔的二维C(3, A)异构体和具有五边形孔的鲾鱼状二维C(1, A')异构体。然而，考虑到熵效应后，发现鲾鱼状异构体1在室温下能量最低，并在PES谱中占主导地位。实验中还发现了具有较低电子结合能的异构体2和3。化学键分析表明，异构体1是苯并[ghi]荧蒽（CH）的全硼类似物，而硼球烯异构体2拥有18个π电子，符合球形芳香性的2(N + 1)电子计数规则。对于B中性簇，贝壳状硼球烯异构体是全局最小值，能量比鲾鱼状准平面结构低得多。
