1H NMR Broad peaks

Question

Is there any particular reason that peaks in proton NMR can appear with a larger integral ratio than expected, that is to say one specific proton absorption stretches over a wide ppm range?

I have a very broad singlet that is integrating to nearly 5x expected. The rest of the spectrum is fine in terms of integrating and the position of the peaks. This particular proton is a singlet from 9.95 to 10.25 ppm and is the alpha proton of a carbene hydrogen chloride salt. Either that or it is impure, but so far my spectrum is very clean whenever I run it.

Many thanks! :)

Answer 1

The most common process that can cause both peak broadening and an increase in the value of a peak integral is exchange broadening between two different protons. The following diagram represents a typical case where the protons in water are exchanging with the hydroxyl proton in an alcohol.

If the rate of exchange is fast on the nmr timescale, then a single, sharp peak of increased amplitude and a weighted average of the individual chemical shifts is observed. If the rate of exchange is slow compared to the nmr timescale, then two distinct peaks, both with accurate integrals, are seen. In the case where the rate of exchange is comparable to the timescale of the nmr experiment, then a broadened signal with integral intensity equal to the two combined signals and located at a weighted average of the two separate signals is observed.

One way you can test to see if a small amount of water in your sample is the culprit is as follows. Add a small amount of $\ce{H2O}$ to your sample and see if the peak for your sample shifts to a new location and the integral value further increases.

Answer 2

One reason for broadening is chemical exchange.

For example, if the proton you are referring to is in exchange with another population of protons, for example from a solvent, the integrated area could represent the total population that is exchanging.

Coordination of a paramagnetic metal ion is another source of broadening. Integrated area should be unaffected if there is sufficient time between each scan. However, since the paramagnetic metal ion can cause faster relaxation of nearby protons, it is possible that the integration is distorted by protons near the paramagnetic ion fully (or closer to fully) relaxing between scans, while the other protons do not. This effect is often used intentionally to enhance detection of protons near a paramagnetic metal center.