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Logic Puzzle #14: No Bias this Time Please … Solution 2

In addition to spin-spin splitting or J-coupling, the chemical shift difference of two nuclei can offer a clue into the type of coupling present. For unsaturated sp2 carbons, diastereotopic geminal protons tend to exhibit a small chemical shift difference, less than 45 Hz, in comparison to that seen with vicinal protons. Please note that this guide offers significant variations and thus be used cautiously [1-2].  

From the 1H-13C HMQC spectrum (Question and Solution), the two correlations are separated by 75 Hz. If the two nuclei are coupled, then it is less likely the correlations are a result of geminal hydrogen atoms (=CH2).

For one correlation, the J-coupling can be estimated at 16 Hz. This is an indication that there is trans coupling present.

1.     J. Heterocycl. Chem., 2005, v.42, p.293-296.

2.     J. Magn. Reson., 1970, v.2, p.150.