Photoelectron spectroscopy (PES) in combination with computational chemistry has been used systematically over the past decade to elucidate the structures and chemical bonding of size-selected boron clusters. Small boron clusters have been found to be planar or quasi-planar, consisting of a monocyclic circumference with one or more interior atoms. The propensity for planarity has been found to be a result of both σ and π electron delocalisation over the molecular plane, giving rise to concepts of σ and π multiple aromaticity. In particular, the B36 cluster has been found to possess a highly stable planar structure with a central hexagonal vacancy. This finding provides the first indirect experimental evidence that single-atom layer boron-sheets with hexagonal vacancies, dubbed ‘borophene’, are potentially viable. Another exciting discovery has been the observation and characterisation of the first all-boron fullerenes. PES revealed that the cluster consisted of two isomers with very different electron binding energies. Global minimum searches led to two nearly degenerate isomers competing for the global minimum: a quasi-planar isomer with a double hexagonal vacancy and an unprecedented cage isomer. In the neutral, the B40 cage is overwhelmingly the global minimum, which is the first all-boron fullerene to be observed and is named ‘borospherene’. Rapid progresses in our understanding of the structures and bonding of size-selected boron clusters have been made during the past decade, which will be the focus of this review. The recent findings about borophenes and borospherenes have stimulated growing interests in boron clusters and will accelerate the pace of discovery in boron chemistry and nanostructures.

中文翻译：

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尺寸选择的硼簇的光电子能谱：从平面结构到硼烯和硼球烯

在过去的十年中，光电子能谱 (PES) 与计算化学的结合已被系统地用于阐明尺寸选择的硼簇的结构和化学键。已经发现小硼簇是平面或准平面的，由一个或多个内部原子的单环圆周组成。已发现平面性的倾向是分子平面上 σ 和 π 电子离域的结果，从而产生了 σ 和 π 多重芳香性的概念。特别是，已经发现 B36 簇具有高度稳定的平面结构，具有中心六边形空位。这一发现提供了第一个间接实验证据，即具有六边形空位的单原子层硼片，被称为“硼烯”，具有潜在的可行性。另一个令人兴奋的发现是对第一个全硼富勒烯的观察和表征。PES 显示该簇由两种具有非常不同电子结合能的异构体组成。全局最小值搜索导致两种几乎简并的异构体竞争全局最小值：具有双六边形空位的准平面异构体和前所未有的笼型异构体。在中性中，B40 笼绝对是全局最小值，这是第一个被观察到的全硼富勒烯，被命名为“硼球烯”。在过去十年中，我们对尺寸选择的硼簇的结构和键合的理解取得了快速进展，这将是本综述的重点。