Electronic Structure and Properties of Silicon-Doped Boron Clusters BnSi withn= 15–24 and Their Anions

Abstract

We investigated the structures and properties of medium size silicon-doped boron clusters in both neutral and anionic states BnSi0/– with n = 15–24. While geometries were optimized using DFT with both TPSSh and PBE functionals, energies were determined using the coupled-cluster theory (U)­CCSD­(T) with the 6-311+G­(d) basis set, with a calibration using the 6-311+G­(3df) basis set. Average binding energies, second-order energy differences, dissociation energies, and electron detachment energies were predicted by using the (U)­CCSD­(T) + ZPE energies for the entire series. The growth of the BnSi series considered does not follow a regular pattern, but a few trends can be established as follows: (i) most of the lowest-energy isomers of BnSi0/– clusters arise from a replacement of a B atom of the Bn+1 species by an Si atom or an addition of one Si atom into the neutral, anionic, or dianionic Bn species; (ii) the predominance of the planar structures for BnSi0/– clusters can be interpreted as a result of an effect of either the pure boron clusters or the Si dopant or also an effect of the negative charge; and (iii) the Si dopant prefers to be placed at an outer place of the Bn framework in order to exchange for, or to connect to, the peripheral B atoms in forming a low coordination number. The B19Si– anion whose planar motif, MOs shapes, electron distribution, and magnetic ring current are similar to those of the pure anions, B182–, B19–, and B202–, can be considered as a disk aromatic species involving 12 valence π-electrons. The MOs and electron distribution of the pseudotubes B20Si, B22Si, and B24Si can be interpreted by the hollow cylinder model. The first stable pseudotubular shape was found for the size B24Si whose thermodynamic stability is enhanced by both σ and π aromatic characters.