Logic Puzzle #12: Torsion Angle and Coupling … Solution

Couplings can be affected by the torsion angle. The couplings can be expressed mathematically with a cos θ relationship. At certain values of θ, the couplings are expected to be relatively weaker to non-existent.

Based on the torsion angle, fragment B is expected to exhibit a prominent coupling between the red and gold nuclei. Fragment A, exhibiting a torsion angle of 90º, will generally lead to a weak or non-evident coupling on an NMR spectrum.

At the initial stages of an elucidaton of an unknown structure, the torsion angle(s) is usually not known. As such, an elucidator must be prepared to expect the unexpected coupling(s). 

Logic Puzzle #12: Torsion Angle and Coupling

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 Puzzle #11: Pairwise Correlation Confidence … Solution

Long range 2D NMR experiments do not necessarily provide information about all the connectivities. The following structure elucidation problem set is one such example.

Based on the 1H-13C HMBC shown below, there is no evident correlation between the 1H at 5.31 ppm and the 13C at 21.1 ppm. Note the green box describes the region of interest.

The correlation between the methyl 1H at 2.15 ppm and 13C at 84.7 ppm (indicated by the purple arrow and the red bonds below) is a weak 4J coupling (also denoted as a W-coupling or M-coupling). The coupling arises from the individual methyl protons rotating and interacting through a W-relationship (or M-)with the carbon. This coupling is not evident for the methine 1H at 5.31 ppm and the 13C at 21.1 ppm.

 
Thank you Serge for your comment.

Logic Puzzle #11: Pairwise Correlation Confidence

Typical for long range 2D NMR experiments, spectral data may exhibit more than one correlation for two coupled nuclei (e.g. A to B and B to A). The pairwise correlations offer an extra degree of confidence in the interpretation.  

For the following fragment, an 1H-13C HMBC correlation exists for the 1H 2.15 ppm to 13C 84.7 ppm (represented by the purple arrow in the diagram below).

 
Is the pairwise correlation for the 1H 5.31 ppm to 13C 21.1 ppm evident?

 
A special thanks goes to Gene M. for pointing me to the data.