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Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides
Sebastian Jung1, Joachim Podlech1
1Institut für Organische Chemie , Karlsruher Institut für Technologie (KIT) , Fritz-Haber-Weg 6 , 76131 Karlsruhe , Germany.
The Journal of Physical Chemistry. A
|June 13, 2018
Summary
The sulfoxide γ-gauche effect in carbon-13 NMR spectroscopy is caused by stereoelectronic interactions, specifically the sulfur lone pair. This effect is not due to steric factors.
Area of Science:
- Organic Chemistry
- Computational Chemistry
- Spectroscopy
Background:
- The 13C NMR gamma-gauche effect describes shielding of carbons beta to a sulfoxide group.
- Previous explanations often cited steric interactions.
Purpose of the Study:
- Investigate the origins of the 13C NMR gamma-gauche effect in sulfoxides.
- Clarify the role of stereoelectronic versus steric factors.
Main Methods:
- Calculated and measured 13C NMR data for various sulfoxides.
- Iso-chemical-shielding surfaces (ICSSs) and current maps.
- Natural Bond Orbital (NBO) and Natural Chemical Shielding (NCS) analyses.
Main Results:
- Upfield shifts occur only when the beta-carbon is anti to the sulfur lone pair.
- Carbons synclinal to the lone pair are unaffected.
- The S=O bond can be described as a reduced triple bond.
Conclusions:
- The gamma-gauche effect is primarily due to stereoelectronic interactions, particularly the sulfur lone pair.
- Steric interactions do not adequately explain the observed shielding.
- The bonding in sulfoxides involves significant antibonding contributions.
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