Exceptionally Long Covalent CC Bonds-A Local Vibrational Mode Study

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• For decades, synthesizing compounds with the longest covalent carbon-carbon (C-C) bonds primarily relied on steric hindrance and strain.
• Electronic effects, such as in electron-deficient species, have also been investigated for their role in elongating C-C bonds.
• Negative hyperconjugation in diamino-o-carborane analogs has been recently implicated in their long C-C bonds.

Purpose of the Study:

• To systematically analyze C-C bonding across a diverse set of 53 molecules, encompassing various elongation strategies.
• To quantitatively assess the influence of steric hindrance, strain, and electronic effects on C-C bond length and strength.
• To establish new guidelines for characterizing and designing molecules with exceptionally long C-C bonds.

Main Methods:

• Computational analysis of 53 molecules, including clamped bonds, diamondoid dimers, electron-deficient species, and di-N,N-dimethylamino-o-carborane.
• Utilization of local vibrational CC stretching force constants (ka(CC)) as an intrinsic bond strength measure.
• Calculation of related bond strength orders (BSO n(CC)) at the ωB97X-D/aug-cc-pVTZ level of theory.

Main Results:

• Steric hindrance and/or strain were confirmed to elongate C-C bonds.
• Electronic effects were found to induce even longer and weaker C-C bonds compared to steric factors.
• The electron deficient ethane radical cation (D3d form) showed the longest C-C bond (1.935 Å), while di-N,N-dimethylamino-o-carborane had the weakest C-C bond (BSO n = 0.209).

Conclusions:

• Electronic effects play a crucial role in achieving extreme C-C bond elongation, potentially surpassing steric contributions.
• The longest bond is not necessarily the weakest, highlighting complex structure-property relationships.
• Local vibrational force constants and bond strength orders provide valuable metrics for characterizing and designing molecules with elongated C-C bonds.