Observation of the Smallest Three‐Dimensional Neutral Boron Cluster
Cong‐Qiao Xu, Tiantong Wang, Chong Wang, Xinran Dong, Huijun Zheng, Ya Zhao, Li‐Li Pan, Jiayue Yang, Weiqing Zhang, Guorong Wu, Hua Xie, Gang Li, Jun Li, Ling Jiang, Xueming Yang, Lai‐Sheng Wang
Abstract
Abstract Despite major progress in the investigation of boron cluster anions, direct experimental study of neutral boron clusters remains a significant challenge because of the difficulty in size selection. Here we report a size‐specific study of the neutral B 9 cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B 9 is measured to be 8.45±0.02 eV and it is found to have a heptagonal bipyramid D 7h structure, quite different from the planar molecular wheel of the B 9 ‐ anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B 7 ring and between the B 7 ring and the capping atoms. Photoionization of B 9 breaks the single‐electron B‐B bond of the capping atoms, which undergo off‐axis distortion to enhance interactions with the B 7 ring in the singlet ground state of B 9 + . The single‐electron B‐B bond of the capping atoms appears to be crucial in stabilizing the D 7h structure of B 9 . This work opens avenues for direct size‐dependent experimental studies of a large variety of neutral boron clusters to explore the stepwise development of network structures.