NMR Spectroscopy and Stereo Chemistry

Stereo Chemical Information from coupling constant

Distinction between cis and trans isomers Trans coupling is greater than for cis.

From the value the isomers can be identified as in the following example.

stereochemical Information

In some molecules the cis-trans, distinction is made even from the position of the absorption, as in cis and trans stilbenes. In trans case the hydrogens are deshielded by two aromatic rings while in the cis case it is by one ring.

Stereochemical Information

Variation of dihedral angle with coupling constant Dihedral angle

Consider the following Newman and Sawhorse projections for ethane

Dihedral angle, Newman and sawhorse projections for ethane

Stereochemistry of cyclohexane derivatives

Stereochemistry of cyclohexane derivatives

The information from the above can be used to assign the configuration of the anomeric carbon in cyclic sugars. In the case of α- glucose at the anomeric carbon the equatorial proton is has a chemical shift of δ 5.2 which is down field by 0.5 for the corresponding axial proton in β-glucose.

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