Validation of a 13C spectrum
The command View > Comparison can validate a 1-D spectrum against a list of predicted chemical shifts. This page explains how to validate a 13C spectrum. Click here for the 1H validation workflow.
Carbon chemical shifts can be predicted with more or less accuracy either with formulae or with neural networks.
You can do this in many ways, for example looking it up in a book.
iNMR offers a fast and simple workflow.
The chemical shifts are calculated by nmrdb.org
(Universidad del Valle, Colombia).
With a little extra effort you can substitute it with the predictor of your choice.
To Validate a 13C Spectrum:
- Process the spectrum as always. Correct the phase and reference the frequency scale if necessary.
- Insert the SMILES code of your compound into the Edit > Metadata dialog. If the option “Call Open Babel” is on, the SMILES code is automatically generated when you paste a ChemDraw formula or drag a mol file.
- Invoke the menu command View > Prediction. This will open your preferred browser at the page of the NMR predictor.
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Copy the list of predicted chemical shifts. This is an example of what you can copy:
13C NMR: δ 20.7 (1C, s), 21.2 (1C, s), 21.6 (1C, s), 22.2 (1C, s), 26.0 (1C, s), 28.7 (1C, s),....
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If you are not satisfied by this Predictor,
you can alternatively copy the list of peaks from NMRShiftDB.
The format is different in this case. Here is an example:
1 30.77 2 20.28 3 49.24 4 68.92.......
- Return to iNMR. Perform the peak-picking. This step is optional. The number of displayed frequencies must be the same as the number of carbon atoms in your molecule. Do not select the solvent, the impurities, or the TMS signal. If you skip this step and the spectrum contains no picked frequencies, iNMR will do the job automatically and will ignore peaks from common solvents (CDCl3 and DMSO-d6, D2O can't be an issue!). If a peak of your molecule falls at the same frequency of the solvent peaks, it will be ignored too. In this case, a careful manual peak-picking is recommended.
- Invoke the menu command View > Comparison. This will compare the peaks of your sample with the peaks in the clipboard.
- The results quantify the agreement, but the final scientific judgment remains with the user. iNMR cannot tell if the prediction is accurate. What iNMR measures is the agreement between the two lists.
When you connect to the above sites you get more than chemical shifts. You get the assignments. You can store this additional information inside your iNMR document, either with comments or with the J Manager.