Charge shift bonding concept in radical π-dimers.

Area of Science:

• Chemical Bonding Theory
• Quantum Chemistry
• Supramolecular Chemistry

Background:

• The charge shift (CS) bonding concept explains unusual sigma-bond features in molecules with lone pairs.
• Radical π-dimers, particularly those involved in π-stacking, present unique intermolecular bonding characteristics.

Purpose of the Study:

• To investigate pancake bonding in radical π-dimers through the lens of charge shift (CS) bonding.
• To explore the role of nonbonding and slightly bonding π-electron pairs in these dimers.
• To extend the CS bonding concept to a new class of molecules.

Main Methods:

• Utilized computational evidence and theoretical arguments.
• Employed the tetracyanoethylene anion dimer ([TCNE](2)(2-)) as a model system.
• Performed calculations including selected intrapair excitations of SOMO-SOMO bonding and nonbonding π-orbitals, compared to CAS(2,2) calculations.

Main Results:

• Pancake bonding in radical π-dimers demonstrates features consistent with CS bonding.
• The overlap of singly occupied molecular orbitals (SOMOs) is crucial for intermolecular bonding in these dimers.
• Significant binding contributions were recovered by including specific excitations, supporting the CS bonding analogy.

Conclusions:

• The CS bonding concept effectively captures essential features of intermolecular bonding in radical π-dimers.
• This study establishes an analogy between pancake bonded radical π-dimers and other CS bonded molecules.
• A novel application of the lone pair bond weakening effect (LPBWE) was identified, where doubly occupied π-orbitals mimic lone pairs.