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The two smallest boron clusters (B3 and B4) in their neutral and anionic forms were studied by photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were observed for B3- at three photon energies (355, 266, and 193 nm), and the electron affinity of B3 was measured to be 2.82 ± 0.02 eV. An unusually intense peak due to two-electron transitions was observed in the 193-nm spectrum of B3- at 4.55 eV and its origin was theoretically characterized. We confirmed that both B3- and B3 are π and σ aromatic systems with D3h symmetry. The photoelectron spectra of B4- were also obtained at the three photon energies, but much broader spectra were observed. The B4- anion was found to have the lowest electron detachment energy (∼1.6 eV) among all boron clusters with three or more atoms, consistent with its extremely weak mass signals. The neutral B4 cluster was found to have a D2h rhombus structure, which is only slightly distorted from a perfect square. For B4-, we identified computationally two low-lying isomers (2B1u and 2Ag) both with D2h symmetry, with the 2B1u state slightly more stable, which is confirmed through comparison of the calculated spectra with the experimental spectra. The chemical bonding of the two small boron clusters is discussed in terms of aromaticity and antiaromaticity both in the π and σ frameworks. We demonstrated that the aromaticity and antiaromaticity concepts provide us a clear explanation of the chemical structure and bonding in these two boron clusters.
Zhai et al. (Fri,) studied this question.