Dissecting the single-electron C-C bond: NBO and AIM perspectives

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• Spiro-dibenzocycloheptatriene (1p) is a tricyclic compound with a unique oxidation product.
• The interaction between ipso carbon atoms (C1 and C2) has been proposed as a single electron sigma C-C bond.

Purpose of the Study:

• To perform a comprehensive electronic structure analysis of the oxidation product of spiro-dibenzocycloheptatriene (1p).
• To characterize the nature of the non-covalent interaction between ipso carbon atoms (C1 and C2).
• To determine if the C1-C2 interaction can be classified as a single electron sigma bond.

Main Methods:

• Non-covalent interaction (NCI) analysis.
• Atoms in Molecules (AIM) theory.
• Natural Bond Orbital (NBO) and Natural Resonance Theory (NRT) analyses.
• Spin density population analysis.
• NBO deletion analysis.

Main Results:

• NCI analysis indicates a weak, predominantly van der Waals interaction between the tricyclic moieties.
• AIM theory reveals a bond critical point between C1 and C2, but with low electron density.
• Laplacian density contour shows charge depletion between C1-C2, unlike the charge accumulation in a typical sigma bond.
• NBO analysis shows a minimal contribution of a single-electron sigma bond, with a natural bond order of 0.066 for C1-C2.
• The C1-C2 interaction contributes only ~9.5% to the total pi-pi interaction energy.

Conclusions:

• The C1-C2 interaction is a weak, highly delocalized interaction, not a single electron sigma bond.
• The interaction is governed by subtle electronic effects, primarily van der Waals forces and donor-acceptor interactions.
• The findings challenge the previous characterization of the C1-C2 bond.