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Methyl rotational barriers in amides and thioamides
Kenneth B Wiberg1, Daniel J Rush
1Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA.
The Journal of Organic Chemistry
|February 22, 2002
Summary
Methyl rotational barriers in amides and thioamides were calculated. Steric repulsion and torsional arrangements significantly impact these barriers, especially in transition states.
Area of Science:
- Computational Chemistry
- Organic Chemistry
Background:
- Amides and thioamides are crucial functional groups in organic chemistry.
- Understanding rotational barriers is key to predicting molecular conformation and reactivity.
Purpose of the Study:
- To calculate methyl rotational barriers for N-methyl-substituted amides and thioamides.
- To investigate the influence of steric and electronic factors on these barriers.
Main Methods:
- Ab initio calculations using the MP2/6-311+G** level of theory.
- Analysis of torsional arrangements and steric interactions.
Main Results:
- The H-C(Me)-N-H eclipsed conformation destabilizes amides by ~0.8 kcal/mol.
- Steric repulsion between sulfur and methyl hydrogen is significant in Z-thioamides.
- N,N-dimethyl amides exhibit higher rotational barriers in transition states due to smaller bond angles.
Conclusions:
- Methyl rotational barriers are sensitive to molecular structure and substituents.
- Steric and torsional effects play critical roles in determining amide and thioamide stability and dynamics.