Planar Networks of Boron Triangles: Analogies to Benzene and Other Planar Aromatic Hydrocarbons

Abstract

The lowest energy structures for B n species (n = 6 to 23 except for 20) observed experimentally in the gas phase with a mass spectrometer are planar networks of boron triangles. Such networks are considered to consist of trigonal planar sp 2 -hybridized boron atoms having perpendicular p orbitals similar to the carbon atoms in benzene and other planar aromatic hydrocarbons. Electron bookkeeping for reasonable chemical bonding topologies of wheel-like structures such as B@B n−1 (n = 6−9) leads to two π-electrons for B 6 and B 7 + similar to the cyclopropenyl cation and six π-electrons for B 8 2− and B 9 − similar to benzene. Related chemical bonding topology analyses for low-energy oval B 10 and B 11 − structures as well as for larger planar boron triangle networks with 12, 13, and 14 boron atoms suggest six π-electrons in such structures. Planar networks of boron triangles having 16−19 boron atoms are shown to be systems with 10 π-electrons similar to naphthalene. Similarly, low-energy planar B 22 and B 23 − structures are shown to be 4 π-electron systems 1analogous to linear anthracene and angular phenanthrene, respectively. Intermediate B 15 − and B 21 − systems are shown to be systems with 4k rather than 4k + 2 π-electrons with 8 and 12 π-electrons, respectively. Structures based on planar networks of boron triangles are strongly energetically disfavored for B 20 relative to a nonplanar decagonal antiprism structure with ideal D 10d symmetry.

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

“…Although the geometries of the B 15 cluster in the cationic, neutral and anionic states have extensively been determined in previous studies, [12][13][14][15][16][19][20][21][22] we would take this opportunity to explore them again using different quantum chemical methods. The lower-lying isomers are separately searched in each charge state.…”

Self-Termination of Borophene Edges

“…Zhang and co-workers have found that, on the silver substrate, some specific types of borophene edges are very stable and might be self-passivated as the interaction between the borophene edge and the substrate is very weak . Some theoretical studies have proposed similar chemical bonding models with peripheral two-center two-electron (2c–2e) and central three-center two-electron (3c–2e) bonds for planar boron nanoclusters, showing different bonding configurations at the edge and bulk of 2D boron materials. , Unfortunately, an insightful understanding of the superior stability of the borophene edge or the mechanism of dangling orbital self-termination at the edge is still missing.…”

“… 17 Some theoretical studies have proposed similar chemical bonding models with peripheral two-center two-electron (2c–2e) and central three-center two-electron (3c–2e) bonds for planar boron nanoclusters, showing different bonding configurations at the edge and bulk of 2D boron materials. 18 , 19 Unfortunately, an insightful understanding of the superior stability of the borophene edge or the mechanism of dangling orbital self-termination at the edge is still missing.…”