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Logic Puzzle #39: Coincidental Overlap or Not? … Solution

February 5, 2013

Quaternary carbons can be tricky in identifying. Keep your options open to ensure the bigger picture is not missed.

The expanded region of the 13C NMR spectrum shows 4 signals for an unknown compound. The signal at 172 ppm represents 1 or 2 amide group(s). The higher intensity of the signal at 172 ppm in relation to the other quaternary carbon signals suggests the presence of more than 1 carbon. However, additional experiments are needed to confirm this.

  1. Collect a quantitative 13C NMR spectrum,
  2. Recollect the 13C NMR with C6D6 or other solvent added to try to resolve the carbons,
  3. Collect a 1H-13C HMBC and count the number of correlations associated to the signal at 172 ppm,
  4. Collect MS data and relate it to the carbon atom count,
  5. Collect 13C NMR at different temperatures,
  6. Etc.

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