Boron Silicon B 2 Si 3 q and B 3 Si 2 p Clusters: The Smallest Aromatic Ribbons.

The small binary boron silicon clusters B 2 Si 3 q with q going from -2 to +2 and B 3 Si 2 p with p varying from -3 to +1 were reinvestigated using quantum chemical methods. The thermodynamic stability of these smallest ribbon structures is governed by both Hückel and ribbon models for aromaticity. The more negative the cluster charge, the more ribbon character is shown. In contrast, the more positive the charge state, the more pronounced the Hückel character becomes. The ribbon aromaticity character can also be classified into ribbon aromatic, semiaromatic, antiaromatic, and triplet aromatic when the electron configuration of a ribbon structure is described as [...π 2( n +1) σ 2 n ] , [...π 2 n +1 σ 2 n ] , [...π 2 n σ 2 n ] , and [...π 2 n +1 σ 2 n -1 ] , respectively. Geometry optimizations of the B 2 Si 3 lowest-energy structure by some density functional theory (DFT) functionals result in a nonplanar shape because it possesses an antiaromatic ribbon character. However, its π aromaticity assigned by the Hückel rule is stronger in such a way that several other DFT and coupled-cluster theory CCSD(T) calculations show that B 2 Si 3 is indeed stable in a planar form ( C s ). A new global equilibrium structure for the anion B 2 Si 3 2- , which is a ribbon semiaromatic species, was identified. Some benchmark tests were also carried out to evaluate the performance of popular methods for the treatment of binary B-Si clusters. At odds with some previous studies, we found that with reference to the high accuracy CCSD(T)/CBS method, the hybrid TPSSh functional is reliable for a structure search, whereas the hybrid B3LYP functional is more suitable for simulations of some experimental spectroscopic results.