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I anion exchanged one of my compounds (Br- to PF6-) and the NMR integrations changed as shown on the two NMRs below. The aliphatic region peaks integrate to smaller values in the second one. Are there any known cases where PF6- somehow influences the relaxation time?

The other possibility is that this product decomposed to a similar compound, which would slightly throw off the integrations...

Original NMR

NMR after anion exchange

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  • $\begingroup$ "Are there any known cases where PF6- somehow influences the relaxation time?" Well, you could just use longer dwell time or some kind of experiment that get's rid of such effects (qNMR). $\endgroup$ – DSVA May 8 '17 at 18:35
  • $\begingroup$ As above - if you think it's a relaxation effect, acquire another spectrum with a longer relaxation delay. $\endgroup$ – Popher May 8 '17 at 21:28
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On face value, there is nothing here that necessarily suggests a change in relaxation times. Before thinking too hard about what chemistry is happening, you should assure us that your data is reliable and consistent. You should provide details of your experimental conditions; are you using a calibrated π/2 pulse for excitation or a shorter pulse approximation? What relaxation delay are you using? Have you measured T1 relaxations for these nuclei? You could consider repeating your experiment using a much greater relaxation rate between scans to see whether the integrations change.

These are more interesting phenomena in the spectra that need rationalising; loss of fine structure for downfield peaks for instance.

As for your question though, there are lots of examples of counterions changing relaxation times. This is a major tool used for probing outer sphere complexation, and can be used to determine how anions and cations interact around the ligand. It is used widely in protein structure (complexation with paramagnetic cations) and investigations with ionic liquids. If this is a counterion-induced relaxation change, you should be able to account for this by repeating your NMR experiment using appropriate parameters.

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