Nature of partial sigma bond.

Area of Science:

• Computational chemistry
• Materials science
• Quantum chemistry

Background:

• Biradicals are reactive species with two unpaired electrons.
• Carbon cage structures (C8, C20, C60) offer unique electronic environments.
• Understanding bonding in exotic structures is crucial for novel material design.

Purpose of the Study:

• Investigate partial sigma covalent bond formation in hydrogenated/fluorinated C8, C20, C60 biradicals.
• Assess stability, geometry, and bonding character in singlet and triplet states.
• Clarify the role of cage size, fluorination, and hyperconjugation on bonding.

Main Methods:

• Restricted B3LYP-D3/6-31+G(d,p) density functional theory.
• Natural Bond Orbital (NBO) analysis for electronic structure.
• Complete Active Space Self-Consistent Field (CASSCF) method.

Main Results:

• Partial sigma (σ) C-C bonds observed with Wiberg bond orders 0.38-0.48 and lengths 2.62-5.93 Å.
• Cage size affects Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) characteristics.
• Fluorination increases electron density in bonding orbitals; hyperconjugation stabilizes partial σ-bonds.

Conclusions:

• Partial sigma bonds are formed and stabilized in these biradical systems.
• Cage size and substituents critically influence electronic and bonding properties.
• Caution advised against unrestricted SCF methods for biradicals due to potential spin contamination.