Photoelectron spectroscopy and ab initio study of B3- and B4- anions and their neutrals

The two smallest boron clusters (B-3 and B-4) in their neutral and anionic forms were studied by photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were observed for B-3(-) at three photon energies (355, 266, and 193 nm), and the electron affinity of B-3 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 B-3(-) at 4.55 eV and its origin was theoretically characterized. We confirmed that both B-3(-) and B-3 are pi and sigma aromatic systems with D-3h symmetry. The photoelectron spectra of B-4(-) were also obtained at the three photon energies, but much broader spectra were observed. The B-4(-) anion was found to have the lowest electron detachment energy (similar to 1.6 eV) among all boron clusters with three or more atoms, consistent with its extremely weak mass signals. The neutral B-4 Cluster was found to have a D-2h rhombus structure, which is only slightly distorted from a perfect square. For B-4(-), we identified computationally two low-lying isomers (B-2(lu) and (2) A(g)) both with D-2h symmetry, with the B-2(lu) 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 pi and a 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.