Theoretical Study of the Electronically Excited States of B₄, B₄ ⁺ and B₄ ⁻

Abstract

First-principle full valence complete active space calculations (CASSCF) are reported for the electronic states of the D2 h and C2 v planar isomers of B4, B4 + and B4 −. This approach is found to reproduce well the experimentally known transition energies for the two lowest excited triplet states of the most stable rhombic B4 isomer. The pattern of electronic transitions up to about 4.5 eV for B4 singlet and B4 + doublet states calculated in similar way should guide an UV spectroscopic search.

Comprehensive analysis of chemical bonding in boron clusters

“…1). Such a distortion from the perfect square comes from the second‐order (or “pseudo”) Jahn‐Teller effect, as it was discussed by Martin et al35 Because of the nature of the distortion, the barrier for “squareness” is rather small (0.7–0.8 kcal/mol)14a, 58 and even at moderate temperature, the B 4 cluster is effectively square.…”

“…They initially postulated the three‐dimensional structures for boron clusters. Consequent quantum chemical calculations32–62 have shown that boron clusters prefer planar or quasi‐planar structures. However, these computational predictions were not verified experimentally.…”

”Developing paradigms of chemical bonding: adaptive natural density partitioning

“…The rhombus shape of the global minimum structure 79 in this case is the result of the secondorder Jahn-Teller effect and even at moderate temperatures the cluster is effectively square. 80,81 Most general considerations of the structure of CMOs lead to the conclusion that the bonding can be described as the combination of four 2c-2e peripheral B-B bonds, a completely delocalized s-aromatic bond (originating from the completely bonding HOMO 2a g ), and a completely delocalized p-aromatic bond (originating from the completely bonding HOMO-1 1b 3u ). These predictions are confirmed by the results of AdNDP analysis (Fig.…”