Polyhedral boranes and elemental boron: direct structural relations and diverse electronic requirements

Details of the electronic and structural connections between macropolyhedral boranes and elemental boron are reported. The nature of electron deficiency in the β-rhombohedral polymorph of boron is analyzed by using a molecular fragments approach with boranes as model systems. The B57H36 molecule constructed from such an approach has three more electrons than mandated by the electron-counting rules (Balakrishnarajan, M. M.; Jemmis, E. D. J. Am. Chem. Soc. 2000, 122, 456. Jemmis, E. D.; Balakrishnarajan, M. M.; Pancharatna, P. D. J. Am Chem. Soc. 2001, 123, 4313-4323.) devised for macropolyhedral boranes. This is also confirmed by electronic structure calculations at the extended Huckel and B3LYP/6-31G∗ levels. The aromaticity of this B57H363+ molecule is on par with the most stable B12H122- itself, as revealed by nuclear independent chemical shift calculations. The B57 skeleton can be made electron precise by adopting a nido arrangement by eliminating an atom from the closo skeleton, so that three valence electrons will be removed. The exact site of elimination, governed by thermodynamic factors, necessitates the removal of a boron atom from any of the six symmetrically equivalent B[13] sites in the unit cell. This leads to partial occupancies, which causes disorder in packing, as revealed by X-ray structure studies. The rest of the boron atoms are distributed in icosahedral B12 fragments, whose two-electron deficiency is satisfied by the capping of extra atoms, distributed statistically in the interstitial sites. These results show that the three-dimensional network of the idealized β-rhombohedral unit cell is not stable, unlike the electron-precise carbon polymorphs such as diamond and graphite. Thus, disorder in the form of partial occupancies, interstitial atoms, alien atoms, etc., is necessary for electron sufficiency and hence for the stability of this polymorphic form. Through these ingenious steps, all components of the unit cell attain electron sufficiency, which explains the high thermodynamic stability of the polymorph. The connection established between boranes and elemental boron in terms of their structure and distribution of electrons has important implications in understanding the structure of boron-rich solids and new strategies to utilize their diverse and technologically important properties

Ab initio predictions on novel stuffed polyhedral boranes

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Electronic requirements of polycondensed polyhedral boranes

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Topological Impact of Delocalization on the Stability and Bandgap of Partially Oxidized Graphene

Strategic perturbations on graphene framework to inflict a tunable energy bandgap promises intelligent electronics that are smaller, faster, flexible, and much more efficient than silicon. Despite different chemical schemes, a clear scalable strategy for micromanaging the bandgap is lagging. Since conductivity arises from the delocalized π-electrons, chemical intuition suggests that selective saturation of some sp2 carbons will allow strategic control over the bandgap. However, the logical cognition of different 2D π-delocalization topologies is complex. Their impact on the thermodynamic stability and bandgap remains unknown. Using partially oxidized graphene with its facile and reversible epoxides, we show that delocalization overwhelmingly influences the nature of the frontier bands. Organic electronic effects like hyperconjugation, conjugation, aromaticity, etc., can be used effectively to understand the impact of delocalization. By keeping a constant C4O stoichiometry, the relative stability of various π-delocalization topologies is directly assessed without resorting to resonance energy concepts. Our results demonstrate that >C=C< and aromatic sextets are the two fundamental blocks resulting in a large bandgap in isolation. Extending the delocalization across these units will increase the stability at the expense of the band gap. The bandgap is directly related to the extent of bond alternation within the π-framework, with forced single/double bonds causing the large gap. Furthermore, it also establishes the ground rules for the thermodynamic stability associated with the π-delocalization in 2D systems. We anticipate our findings will provide the heuristic guidance for designing partially saturated graphene with desired bandgap and stability using chemical intuition

Enigmatic Alternatives for Epoxides in Graphene Oxide

The structural mystery of the long-known graphene oxide (GO) unfolds as one of the formidably abstract conceptual problems among nanomaterials. Generally construed as the oxidized form of graphene, it is variously proposed to host a variety of functional groups with oxygen and hydrogen. Theoretical studies are abundant on the highly-strained epoxides, while larger cyclic ethers having one or more oxygen atoms and vinylogous carbonyls are paid scant attention even though they are predicted by several structural models. The nature of the geometric and electronic structure of these alternative functional groups, the preferred distribution on the graphene lattice, comparative stability, etc., remains unexplored. Our systematic inquiry into the impact of hexagonal and periodic constraints on these mystic functional groups unveils several surprises. Among those that retain the hexagonal carbon backbone, epoxides are surprisingly more stable than larger ethers despite the excessive strain associated with their acute triangular geometry. Epoxidation conserves the planarity of the carbon backbone that allows their optimal distribution on the lattice by reducing the repulsive interactions from oxygen lone pairs and π-electrons. These findings categorically rule out the possibility of 1,3 ethers in GO and settle the long-standing debate on its existence. They face severe steric repulsion even in low dimensional systems that tear down the σ-skeleton of graphene completely apart, reducing its dimensionality. We show that selective breaking of the σ-bonds is preferred over epoxides if backed by cyclic delocalization of electrons. Particularly, 1,6-diones in trans orientation are thermodynamically favored and justify the large holes experimentally observed through microscopic imaging

Missing hydrogens in B<SUB>19</SUB>H<SUB>20</SUB><SUP>-</SUP>? application of electron counting rule for edge-sharing macropolyhedral boranes

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Nature of Interactions between Boron Clusters: Extended Delocalization and Retention of Aromaticity post Oxidation

Polyhedral boron clusters are lauded as 3D-aromatic that frequently form interconnected periodic networks in boron-rich borides with metal and non-metals having high thermodynamic stability and hardness. This leads to the question of whether the spherical delocalization of electrons in these clusters is extended across the network as in organic aromatic networks. These borides also frequently show partial oxidation, having fewer electrons than that is mandated by electron counting rules, whose impact on their aromatic stability and geometry remains mysterious. Understanding the nature of electronic communication between polyhedra in polyhedral borides is largely unknown though it is crucial for the rational design of advanced materials with desirable mechanical, electronic and optical properties. Here we show that electronic delocalization across polyhedral clusters has a significant impact on their structure and stability. Our computational inquiry on closo borane dimers shows substantial variation in conjugation with the ideal electron count. Upon two-electron oxidation, instead of forming exohedral multiple bonding that disrupts the aromaticity, it undergoes subtle geometric transformations that conserve aromaticity. The nature of geometric transformation depends on the HOMO that is decided locally on the polyhedral degree of the interacting vertices. The prevalence of π-type interactions as HOMO in tetravalent vertices encourage conjugation across clusters and turn into a macropolyhedral system hosting a rhombic linkage between clusters upon oxidation. In contrast, the σ-type interactions dominate the HOMO of pentavalent vertices that prefers to confine aromaticity within the polyhedra by separating them with localized 3c-2e bonds. Our findings expose the fundamental bonding principles that govern the interaction between boron clusters and will provide chemical guidance for the design and analysis of polyhedral boride networks with desired properties

Alternatives for Epoxides in Graphene Oxide

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