Boron Silicon B₂Si₃^qand B₃Si₂^pClusters: The Smallest Aromatic Ribbons

Abstract

The small binary boron silicon clusters B2Si3 q with q going from −2 to +2 and B3Si2 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 2n ], [...π 2n+1 σ 2n ], [...π 2n σ 2n ], and [...π 2n+1 σ 2n–1 ], respectively. Geometry optimizations of the B2Si3 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 B2Si3 is indeed stable in a planar form (C s ). A new global equilibrium structure for the anion B2Si3 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.

A reinvestigation of the boron cluster B15+/0/−: a benchmark of density functionals and consideration of aromaticity models

“…We have recently validated the TPSSh functional for its suitability in exploring the structural characteristics of complex boron clusters involving silicon or lithium, 40,41 while the PBE functional has demonstrated its theoretical adequacy in cases involving a competition between 2D and 3D geometrical configurations. 42…”

“…We have recently validated the TPSSh functional for its suitability in exploring the structural characteristics of complex boron clusters involving silicon or lithium, 40,41 while the PBE functional has demonstrated its theoretical adequacy in cases involving a competition between 2D and 3D geometrical configurations. 42 One intriguing observation is that despite the substantial differences in relative energies obtained from DFT calculations, particularly between the 3D and 2D forms, 4n compared to 1n, and 4c compared to 1c, the relative energies derived from single-point CCSD(T) energy calculations using DFT optimized geometries show negligible deviation.…”

Small chromium-doped silicon clusters CrSin: structures, IR spectra, charge effect, magnetism and chirality

“…For the location of the cationic CrSi n + and neutral CrSi n isomers, initial guess structures are generated by considering stable geometries of cationic, anionic, neutral metal and non-metal doped Si clusters reported in the literature. 3,9,31,32,34,39,40,59,62–69 Additionally, geometries of the pure Si clusters in different charge states taken from previous studies 31,34,37,42,44,70–73 are used as starting structures on which the Cr atom is added at various positions or an atom of the Si frame is replaced by a Cr atom. These structures are subsequently optimized in different multiplicities, from the singlet to the septet state for systems with an even number of electrons, and from the doublet to the octet state for systems with an odd number of electrons.…”