Logic Puzzle #13: Extra Correlations on the HSQC Spectrum … Solution

The intensity of a correlation in relation to other correlations on a 2D NMR spectrum can be just as important in a structure elucidation as assessing the position and phase of a correlation. The example below presents one such case.

From the 1H-13C HSQC in the previous post, the correlations differ significantly in intensity. The weaker correlations can be attributed to long-range couplings, more specifically, 2J coupling responses. In addition to the 1H integrals, the carbons at 121 and 134 ppm are probably CH’s (4 CH’s in total) and probably not CH2’s. Two potential fragments are shown below and here.

 
I would like to send out a special thanks to Sarah T. for discussing the idea with me.

Logic Puzzle #13: Extra Correlations on the HSQC Spectrum

A 1H-13C HSQC is frequently used to assist in assigning a carbon as CH, CH2 or CH3 (additional blog here). The purpose of this puzzle is to examine a non-standard assignment.

The 1H-13C HSQC below shows 2 correlations for each carbon at 121 and 134 ppm. Given also the information on the 1H integrals on the F2 axis, are the carbons at 121 and 134 ppm really CH2’s?

 

Blog on Hiatus from February 7 to 18, 2011

It’s been a busy week with work and I'm expecting the same for next week. The blog P2C2E will be on hiatus for 2 weeks from February 7 to 18. Posts will resume the following week.

Stay tuned for more logic puzzles.

It’s Been Three Years … Already

February 1, 2011 marks the third year anniversary of the weblog P2C2E. This milestone is dedicated to you the loyal readers. Thank you.

I am happy to write that the blog has grown linearly in site visits per year.

Although the number of posts has dropped, due to workload and travel, the number of email subscribers has not. Readership comments have been fairly constant. The top blog in 2010 reached 368 visits.

 
The statistics are based on results from Google Feedburner and Typepad.

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.