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Using Stereochemistry

NMRPredict is the only C13 NMR prediction program which is able to take stereochemistry into account during prediction.

There are three possible settings for stereochemistry - which are found under Predict - Options - C13 Stereochemistry

The options are:
  • If Needed - this is the default setting. When the structure is passed to the C13 prediction engine it evaluates the molecule to see whether there is stereochemistry in the molecule or not. If there is stereochemistry only database entries with identical stereochemical features are used, if there is no stereochemistry in the query molecule all database records are used.

  • Always - Here only molecules from the database with identical stereochemical features are used for prediction. This is only a sub-set of the database and is not a recommended setting

  • Never - All database records are used regardless of stereochemistry. This setting is not recommended if the query molecule has stereochemistry, some very "meaningless" results can be obtained!

Below is an example of a "meaningless" prediction. Stereochemistry was set to "Never" and was not taken into account during prediction. What you can see is that, apart from a few outlyers, most data behind the prediction fall at 33 ppm or 21 ppm. This depends on the stereochemistry but we have chosen not to use it. The program therefore simply adds 33 to 21 and divides by two, giving a prediction of 27 ppm. Which is totally "meaningless".

Below is the result of the prediction for the same atom with stereochemistry set to "If Needed". Now the same atom is predicted at 22 ppm.

In the two screenshots below the distribution for the diastereotopic methyl groups at position 4 are shown. Automatic activation of the stereochemical prediction engine gives correct values for both carbons, showing a difference of about 10 ppm. Additionally a few assignment errors in the literature are immediately visible.