Labile Protons and J-Coupling

One of the most basic rules we all learn during our undergrad NMR course is how to explain the splitting pattern of signals in ¹H NMR – number of neighboring protons plus one. In general, the multiplicity of a signal is calculated using the 2nI+1 rule, where n is the number of neighboring NMR active nuclei with nuclear spin I (when looking at ¹H nuclei only, I = ½ and the formula shortens to n+1). With this in mind it is possible to understand more complicated examples of signal multiplicities like residual proton signals from deuterated solvents (coupling with deuterium, I = +1) or the origin of carbon satellites (https://www.nanalysis.com/nmready-blog/2021/12/6/unsymmetric-carbon-satellites-in-fluorine-19-nmr).

However, labile protons don’t always follow this rule: oxygen- or nitrogen attached protons are often observed as (broad) singlets, apparently not coupling with their neighboring groups. Let’s have a look at ethanol in deuterated chloroform (Figure 1). In this first sample, the spectrum looks like what most people would have expected. The OH signal is observed as a singlet and apparently does not couple with the neighboring CH₂ group.

Figure 1. 60 MHz ¹H NMR spectrum of ethanol in chloroform-d, 1:1 (v/v).

While it is relatively easy to accept that OH is an exemption of the rule mentioned above and does not add to the multiplicity of neighboring protons, this is only true for the most common cases, but of course there are exemptions of that exemption and we can actually observe signal splitting for hydroxyl groups in some cases.

Let’s start to tackle this topic by a brief discussion on what labile proton exchange means and what effect this has on the NMR spectrum. The OH bond is covalent, but due to the higher electronegativity of oxygen it is polarized and the two electrons of the bond are “drawn towards” the oxygen atom, which facilitates cleavage of the proton. Thus, if we could label individual protons (which we can, experiments with deuterium-labeled molecules can be very insightful for mechanistic studies, but let’s keep this for another blog post) we would observe that while the hydrogen atoms of the CH₃ and CH₂ group in the ethyl fragment of ethanol would remain at their position over a given time period, the oxygen bound H atoms of the hydroxyl groups exchange with other labile hydrogen atoms like in water (Scheme 1). These exchange reactions can be faster than the NMR time scale which reduces the likelihood of observing the signal splitting from the OH group.^[1]

Scheme 1. Proton exchange of labile protons in a mixture of ethanol and water.

But, what if there was no water present in the sample? In Figure 2 you’ll find the spectrum of a sample of dry ethanol in dry CDCl₃ prepared in a glove box under inert atmosphere.

Figure 2. 60 MHz ¹H NMR spectrum of dry ethanol in dry chloroform-d, 1:120 (v/v).

Here, the OH signal of ethanol is observed as a (broadened) triplet from J coupling with the neighboring CH₂ group, which itself appears as a doublet of quartets from coupling with the CH₃ group and the OH group.^[2] Also, all signals are shifted due to the concentration change from 1:1 to 1:120 (v/v), with the OH signal showing the largest shift from 4.42 ppm to 2.52 ppm.^[3]

Interestingly, at high concentrations even under dry conditions, the splitting information gets lost again, due to intermolecular proton exchange between the ethanol molecules, which becomes faster/statistically more likely to happen at higher concentrations (Figure 3).

Figure 3. 60 MHz ¹H NMR spectrum of dry ethanol in dry chloroform-d, 1:1 (v/v).

In summary, everything that increases the exchange rate of labile proton leads to observing the average signal of the labile proton during this process via NMR spectroscopy. Thus, the signal of the OH group of ethanol appears as a singlet in its NMR spectrum if the sample concentration is high or water or acids/bases are present in the sample.

If you want to discuss this in more detail or if you have any questions about signal shapes in benchtop NMR, please don't hesitate to contact us.

References

[1] a) Weinberg, I.; Zimmerman, J. R. J. Chem. Phys. 1955, 23, 748; b) Roberts, J. D. ”Nuclear Magnetic Resonance: Applications to Organic Chemistry” https://chem.libretexts.org/@go/page/366657 (accessed 2023-02-06).
[2] Samuel P. P. “NMR Spectrum of Ethanol : BSc S5 Chemistry_Kerala University” https://www.youtube.com/watch?v=58DBEk4R8i0 (accessed 2023-02-06).
[3] Facey, G. “Protic Samples in Aprotic Solvents” http://u-of-o-nmr-facility.blogspot.com/2012/03/protic-samples-in-aprotic-solvents.html?m=1 (accessed 2023-02-06).