Extremely Long C-C Bonds Predicted beyond 2.0 Å.

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Exploration of conjugated molecules with unusual bond lengths.
• Investigation inspired by recent discoveries of elongated single C-C bonds.

Purpose of the Study:

• To design and analyze conjugated molecules featuring extremely long single carbon-carbon (C-C) bonds exceeding 2.0 Å.
• To evaluate the electronic properties, including diradicaloid character, of these elongated bonds.
• To compare the stability and characteristics of molecules with stretched C-C bonds.

Main Methods:

• Design of novel conjugated molecules with single C-C bonds beyond 2.0 Å.
• Analysis using Wiberg bond index, bond dissociation energy (BDE), and fractional orbital density (FOD).
• Unrestricted density functional theory (DFT) and high-level multireference calculations.

Main Results:

• Computational modeling predicted achievable C-C bonds near 2.2 Å with a BDE of 15-25 kcal mol⁻¹.
• All calculations indicated a lack of diradical character in molecules with elongated C-C bonds.
• Fractional orbital density (FOD) calculations quantified diradicaloid character differences.

Conclusions:

• Extremely elongated C-C bonds in conjugated systems do not inherently lead to diradical character.
• The study provides insights into the limits of C-C bond stretching and molecular stability.
• Computational methods successfully characterized the electronic nature of these unusual molecular structures.