Interpreting Ring Currents from Hückel-Guided σ- and π-Electron Delocalization in Small Boron Rings

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Aromaticity in small boron clusters is debated due to conflicting magnetic and electronic data.
• Existing models struggle to consistently explain the aromatic behavior of boron rings.
• Understanding boron cluster aromaticity is crucial for designing novel materials.

Purpose of the Study:

• To resolve inconsistencies in the description of aromaticity for small boron clusters (B3-, B3+, B4, B42+, B42-).
• To introduce a new theoretical framework for understanding σ-aromaticity based on delocalization topologies.
• To establish a coherent link between electronic structure and magnetic response in boron systems.

Main Methods:

• Employed a multidimensional approach integrating Adaptive Natural Density Partitioning (ANDP) and Electron Density of Delocalized Bonds (EDDB).
• Analyzed magnetically induced current density and the z-component of the induced magnetic field.
• Developed a model distinguishing radial and tangential σ-delocalization pathways.

Main Results:

• B3- exhibits robust double aromaticity through cooperative radial/tangential σ-delocalization and a π-bond.
• B42- shows fully doubly aromatic character with delocalized π- and σ-circuits.
• The study successfully correlates electronic structure, σ-framework topology, and magnetic properties across various boron clusters.

Conclusions:

• A new model of σ-aromaticity, considering radial and tangential delocalization, reconciles electronic and magnetic descriptors for boron clusters.
• Hückel's rule remains applicable for interpreting aromaticity in small boron rings when σ-framework topology is properly resolved.
• The findings provide a unified understanding of aromaticity evolution in boron clusters upon oxidation or reduction.