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Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion.
Lionel Goodman1, Hongbing Gu, Vojislava Pophristic
1Department of Chemistry and Chemical Biology, Wright and Rieman Laboratories, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901, USA. goodman@rutchem.rutgers.edu
The Journal of Physical Chemistry. A
|July 13, 2006
Summary
Hyperconjugation explains molecular conformation, with both anti and cis interactions influencing the gauche preference in chemical structures. This model accounts for observed angles without invoking steric repulsion.
Area of Science:
- Computational Chemistry
- Molecular Modeling
- Quantum Chemistry
Background:
- Understanding molecular conformation is crucial in chemistry.
- Hyperconjugation is a key electronic effect influencing molecular geometry.
- Previous models often relied on steric repulsion to explain conformational preferences.
Purpose of the Study:
- To investigate the role of hyperconjugation in determining the conformational preference of molecules.
- To explain the observed dihedral angles using a purely electronic model.
- To analyze the contributions of both anti and cis interactions to molecular stability.
Main Methods:
- Natural Bond Orbital (NBO) deletion calculations were employed.
- Analysis focused on hyperconjugative interactions between C-H bonds and C-F* antibonds.
- Torsional angles and their impact on electronic stabilization were systematically studied.
Main Results:
- Vicinal hyperconjugation between anti C-H and C-F* bonds drives the gauche preference.
- Substantial cis C-H/C-F* interactions contribute significantly (25% of anti interaction).
- The equilibrium conformer's dihedral angle (>60 degrees) is explained by maximizing anti and cis hyperconjugation.
Conclusions:
- A hyperconjugation model alone adequately explains observed conformational preferences.
- Repulsive forces are not necessary to rationalize the 72-degree equilibrium angle.
- The trans conformer's stability results from a balance of hyperconjugative stabilization and steric effects, with significant mode coupling.