SOMO-HOMO Level Inversion in Biologically Important Radicals.

Area of Science:

• Quantum Chemistry
• Molecular Orbital Theory
• Free Radical Chemistry

Background:

• Traditionally, the singly occupied molecular orbital (SOMO) is assumed to be the highest occupied molecular orbital (HOMO) in radical species.
• This assumption is challenged by observations of SOMO-HOMO level inversion in various radical systems.

Purpose of the Study:

• To investigate the phenomenon of SOMO-HOMO level inversion in a diverse range of radical species.
• To determine the factors influencing the energy gap between SOMO and HOMO.
• To elucidate the implications of this inversion on radical reactivity and electronic configurations.

Main Methods:

• Computational electronic structure calculations using the B3LYP/6-31++G** method.
• Inclusion of aqueous phase effects using the integral equation formalism of the polarized continuum model (PCM) solvation model.
• Analysis of electronic configurations for natural DNA bases, substituted bases, and diatomic molecules.

Main Results:

• SOMO-HOMO level inversion is identified as a prevalent phenomenon across various radical species, including diatomic anions and oxidized DNA bases.
• A greater localization of spin density in a σ-orbital on a single atom correlates with an increased energy gap between the HOMO and SOMO.
• In systems exhibiting SOMO-HOMO inversion, one-electron oxidation occurs from the HOMO, leading to triplet ground states, as exemplified by superoxide anion oxidation to triplet oxygen.

Conclusions:

• The study reveals that SOMO-HOMO level inversion is a common characteristic of radical species, contrary to traditional assumptions.
• The findings provide critical insights into the electronic structure and reactivity of radicals, particularly concerning oxidation processes.
• The results highlight differences between conventional radicals and distonic radical anions regarding SOMO-HOMO inversion and its dependence on localization and solvent effects.