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Logic Puzzle #12: Torsion Angle and Coupling

January 19, 2011

For a simple case, the torsion angle (or dihedral angle) is described as the angle between 4 contiguous atoms or 3 successive bonds. In NMR, the magnitude of the coupling is directly related to the torsion angle between the vicinal nuclei (see the Karplus equation for more details).

Below are two animations, A and B, for identical fragments differing only in the torsion angle. Assuming the following fragments exhibit a rigid geometry, which torsion angle would generally contribute to a prominent coupling between the red and gold nuclei?

A (torsion angle at 90º)                 B (torsion angle at 35º)

Logic#12Torsion_90deg_Jan                     Logic#12Torsion_35deg_Jan

 

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