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Symmetrization of INADEQUATE spectra

INADEQUATE spectra contain a special kind of symmetry which is difficult to exploit directly to increase S/N, like with proton correlation experiments. Isotopic shifts also disrupt the ideal symmetry.

iNMR adopts a non-linear filter that removes the signals (and the noise) from the regions where no genuine signal can appear. The program starts from the known frequencies of the 1-D peaks. Then calculates the DQF frequency of every possible pair of nuclei that can be formed. The 2-D spectrum is scanned at these frequencies. Only the peaks falling there and that come in regular pairs are retained. The spectrum becomes much easier to inspect because of the drastic cleaning. As with any symmetrization method (and any non-linear filter in general) there are risks of false positives (artifacts created by the filter) and false negatives (genuine peaks that are suppressed). Both can be spotted through the interactive interface of iNMR.

Being that this filter, among its many tricks, takes advantage of the symmetry of the experiment, we can call it “symmetrization”, though this is only part of the truth.

How to Safely Symmetrize an INADEQUATE:

Step 1

Process the 1-D spectrum. Perform 1-D peak-picking. Do not include solvents and impurities into the list of peaks.

Step 2

Process the 2-D spectrum. Add the 1-D spectrum as an external projection. Verify that the frequency scales of both spectra are correct and accurate.

Step 3

Select, from the main menu, the visualization mode Format > Contours. Adjust the intensity of the plot. Ideally all the peaks should be visible and random noise should be hidden. If this is not possible because the signals are weak, choose a compromise: show all the plausible signals, at the cost of including a little of noise. The lowest contour will be the initial threshold of the filter we are going to apply. Peaks below the threshold will (initially) disappear.

Step 4

Choose the command Process > Symmetrize > Inadequate. You will see the filtered spectrum and a dialog sheet over it. The dialog contains a switch called “filter”. Use this switch to compare the spectrum before and after the cleaning. It is important to verify what happens with a different threshold: try moving the slider. Peaks the appears only with lower levels of threshold could be noise. Treat them with suspicion. Only a complete analysis of the spectrum can solve these doubts.

Step 5

Apart from the threshold, the filter requires 3 parameters. They are less critical, yet it's worth verifying if their values are correct for your compound and your spectrum. To see the effect on the plot after changing a value, click twice the “filter” switch.

Splitting: this is the coupling constant. You can take an approximate value from similar known compounds. If the same molecule exhibits a wide range of couplings, set this parameter to the smallest value.

Linewidth: the average linewidth of a peak. If you can't measure it, set this parameter to twice the digital resolution of the directly revealed (single-quantum) axis.

Spread: the complete width, along the double-quantum axis, of the largest cross-peak.

Step 6

Continue playing with the threshold until you get the best result. Close the dialog.

This filter can alter the apparent coupling constants.

Related Topics

Adding Your Annotations

Assigning the Cross-Peaks

Suppressing Signals