Observation of the Smallest Three-Dimensional Neutral Boron Cluster

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 B₉ cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B₉ 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 B₉^- anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B₇ ring and between the B₇ ring and the capping atoms. Photoionization of B₉ breaks the single-electron B-B bond of the capping atoms, which undergo off-axis distortion to enhance interactions with the B₇ ring in the singlet ground state of B₉⁺. The single-electron B-B bond of the capping atoms appears to be crucial in stabilizing the D_7h structure of B₉. 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.