[U. of Ottawa NMR Facility Blog] Chemical Exchange Agents to Simplify NMR Spectra

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Chemical Exchange Agents to Simplify NMR Spectra

One can simplify 1H NMR spectra by eliminating exchangeable proton signals. This is most commonly done by adding a drop or two of D2O to the NMR sample. An example of this can be seen in a previous post. The deuterium from the D2O replaces the exchangeable protons (-OH, -NH, -NH2, -COOH) of the sample and their 1H signals disappear. The disadvantage of this technique is the introduction of a strong HDO signal which may overlap with other signals in the spectrum and thereby hinder the interpretation.

An alternative of the "D2O shake" is to add a drop or two of concentrated trifluoroacetic acid (TFA) to the sample. TFA has a single exchangeable proton at ~ 14 - 16 ppm. The -COOH proton of the TFA exchanges with the exchangeable protons in the sample. The exchange rate is usually fast enough on the NMR time scale such that the resultant spectrum has a single broad resonance representing all of the exchangeable protons at a chemical shift between the chemical shift of the pure TFA and that of the exchangeable protons in the sample (usually >10 ppm depending on the sample and the amount of TFA added). The broad peak at a shift > 10 ppm is not likely to overlap with other resonances in the spectrum and therefore will not hinder the interpretation of the data. An example of the use of TFA is shown in the figure below.

The bottom spectrum is that of sucrose dissolved in DMSO-d6. One can observe all of the -OH protons in addition to all of the other sugar protons. The middle spectrum is that of pure TFA in DMSO-d6. The -COOH resonance appears at ~ 15.6 ppm. The top spectrum is that of sucrose in DMSO-d6 with a drop of TFA added. One can see that all of the -OH protons of the sugar (highlighted in yellow) have combined with the -COOH resonance of the TFA yielding a single broad resonance at ~ 13 ppm as a result of the exchange. In addition to moving the -OH resonances out of the way, one can see simplifications to the other sugar protons as the result of loosing the J coupling between the -OH protons and the remaining sugar protons.

A comparison of the use of TFA compared to D2O as an exchange agent is shown in the figure below.

Both methods produce similar results except that the spectrum with added D2O has a large HDO peak (off-scale in the figure) which overlaps with other signals.



Source: University of Ottawa NMR Facility Blog