On the large σ-hyperconjugation in alkanes and alkenes

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• The conventional understanding in organic chemistry posits that sigma (σ) bonds in alkanes and unsaturated hydrocarbons are highly localized.
• Non-steric interactions of these σ-bonds are often considered negligible, leading to the assumption of
• Purpose of the Study
• Main Methods
• Main Results
• Conclusions

Purpose of the Study:

• To scrutinize the conventional view of localized σ-bonds in hydrocarbons using advanced computational methods.
• To quantify the extent of σ-electron delocalization and its energetic contributions in various hydrocarbon molecules.
• To investigate the impact of σ-delocalization on molecular stability and structural properties, particularly in cyclic and polycyclic systems.

Main Methods:

• Application of the block-localized wavefunction (BLW) method to calculate σ-delocalization energies (DEs).
• Analysis of geminal and vicinal hyperconjugative interactions within hydrocarbon frameworks.
• Comparison of σ-delocalization energies with π-conjugation effects in unsaturated systems.

Main Results:

• Molecules conventionally considered
• strain-free
• exhibit surprisingly large total σ-BLW-delocalization energies.
• Significant σ-delocalization energies were computed for n-alkanes, cyclohexane, and adamantane, ranging from 11.6 to 91.0 kcal mol(-1).
• Substantial σ-BLW-DEs were also found in unsaturated hydrocarbons like ethylene, allene, butadiene, benzene, and cyclobutadiene, with values up to 28.3 kcal mol(-1).

Conclusions:

• σ-electron delocalization in hydrocarbons is significant and contributes substantially to molecular stabilization, often underestimated in conventional analyses.
• The presence of numerous σ-hyperconjugative interactions can lead to total σ-stabilization energies that may surpass those from π-interactions.
• Cyclic and polycyclic hydrocarbons like cyclohexane and adamantane experience intramolecular repulsions that reduce σ-hyperconjugation, contrary to the notion of being strain-free.