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Logic Puzzle #29: Adjusting the Threshold … Solution

April 27, 2012

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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