Classical and distonic radical cations: a valence bond approach

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Physical Chemistry

Background:

• Radical cations are key intermediates in chemical reactions.
• Understanding their electronic structure and stability is crucial for reaction mechanism elucidation.
• Previous studies have explored various radical cation systems, but a comprehensive analysis of ethyl-based radical cations with diverse substituents is needed.

Purpose of the Study:

• To investigate the electronic structure and stability of conventional radical cations and their distonic isomers derived from CH(3)CH(2)X (X=F, OH, NH(2), Cl, SH, PH(2)).
• To elucidate the nature of bonding in these radical cations using advanced computational methods.
• To compare the relative stabilities of conventional and distonic forms and analyze geometric variations.

Main Methods:

• Standard Møller-Plesset (MP) and G2 computational methods were employed.
• Ab initio valence bond (VB) theory was utilized for detailed electronic structure analysis.
• Geometrical optimizations and electronic state characterizations were performed.

Main Results:

• Conventional radical cations exist in two states (c' and c''), with c' states generally more stable due to resonance.
• Some c' states exhibit significantly elongated C-C bonds, indicative of a two-center, one-electron bond.
• Distonic radical cations are typically more stable than conventional ones, except for X=SH and PH(2).
• For X=F and Cl, distonic radical cations resemble ion-molecule complexes.
• The electronic structure of the distonic radical cation with X=OH is intermediate.

Conclusions:

• The study provides a detailed understanding of the electronic structure and bonding in ethyl-based radical cations and their distonic isomers.
• The findings highlight the importance of considering different electronic states and bonding descriptions (e.g., one-electron bonds, ion-molecule complexes) for accurate representation.
• Computational chemistry, particularly valence bond theory, is essential for rationalizing observed structural and electronic properties.