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