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Logic Puzzle #29: Adjusting the Threshold … Solution

Part of the process of interpreting data from 2D NMR spectra is to adjust the intensity threshold in order to differentiate visually the signals from the noise. A certain level of cautious peak picking is needed so not to overlook a weak signal nor confuse a noisy region for a real signal.

For an unknown compound, the 1H-13C HSQC spectrum shows several overlapping 1H multiplets equating to 4 non-exchangeable H atoms, four C atoms with one C signal more intense than the other two and two 1H-13C correlations with the third correlation in question. If all H atoms are to be accounted for, there are three scenarios to consider:

  1. 1 CH at 123 ppm, 2 CH’s at 120 ppm, 1 CH at 122 ppm = 4 H and 4 C atoms
  2. 1 CH at 123 ppm, 2 CH’s and 1 CH2 at 120 ppm, 1 C at 122 ppm, 1 artefact correlation = 4 H and 5 C atoms
  3. 1 CH at 123 ppm, 3 CH’s at 120 ppm, 1 C at 122 ppm, 1 artefact correlation = 4 H and 5 C atoms

Unfortunately, the data at hand may not be sufficient to rule out any of the three scenarios.

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