To Decouple or Not to Decouple, that is the question. Also…Acids

Acidity is something that you encounter on a daily basis, probably without even realizing it. The tangy taste of an orange (citric acid), that Vitamin C tablet you took this morning (ascorbic acid), those terrible jeans from the 80’s that you still wear (acid wash). My favourite acid is acetic acid. This is the chemical responsible for the smell when you stick your head into a bag of Salt and Vinegar chips (Fig. 1a, a Canadian delicacy!), or take a sniff of my plate of Fish and Chips (Fig. 1b). White vinegar is an aqueous solution that is ~3–9% acetic acid, which gives it its pungent odor and sour taste. The chemical formula of acetic acid is CH₃COOH, with that last proton being the “acidic” one. It is part of what is known as a carboxylic acid functional group, hence its acidity. The structure is shown in Figure 1c.

In chemistry speak, aqueous acetic acid is a weak acid compared to something like HCl_(aq) (hydrochloric acid), which is a strong acid. As a weak acid, CH₃COOH can liberate its acidic proton onto water to a certain degree as regulated by its equilibrium in water (Scheme 1) and its acid dissociation constant (K_a or pK_a), forming H₃O⁺. Many general chemistry laboratory courses explore the analysis of weak acids, correlating their K_a values to the pH of solution as a function of how much acid you add. For example, acetic acid has a pK_a value of 4.76,^[3] so adding CH₃COOH to H₂O will gradually decrease the pH, making the solution more acidic over time. With acids possessing a stronger (lower) pK_a, the pH would plummet a lot faster.

SCHEME 1. EQUILIBRIUM BETWEEN ACETIC ACID/WATER WITH ACETATE/HYDRONIUM.

But, Nanalysis manufactures benchtop NMR instruments, not pH meters, so let’s take a look at the ¹H NMR spectrum of CH₃COOH in deuterated water, or D₂O (Fig. 2). Considering the chemical formula of acetic acid, we would expect to see two peaks: One for the methyl group (CH₃, integration = 3H) and one for the acidic proton (COOH, integration = 1H).

FIGURE 2

Examining Figure 2, we see two resonances. Perfect, right? Well, you have to be careful and not jump to conclusions too quickly! There are a few points to discuss here: The first is the concept of solvent residual peaks. But wait a minute, we used D₂O to dissolve the CH₃COOH. Why would we still see an H₂O peak? Well, when you buy D₂O from an isotope laboratory, 99.9% of the H₂O has been converted to D₂O. But, the process isn’t perfect, so there is still some H₂O in there, which is why you see a bit of a residual solvent in the spectrum. It is commonly marked with an asterisk (*), and labeled as a solvent residual peak, as shown in Figure 3. Solvent residuals, as well as common impurities, are compiled in an awesome article from my favourite ACS Journal, Organometallics, and it is a great resource when you’re trying to nail down peaks.^[4]

FIGURE 3

The second question might be “I clearly see the methyl peak at 2.01 ppm, but where’s the COOH peak?” This is an excellent question, and it all goes back to what acidity “means”. Recall that the acidic proton can leave acetic acid and this is because the acetate ion (CH₃COO^–) is pretty good at handling the negative charge once the H+ leaves. This H⁺ will then go and find a water molecule, but odds are it’s going to find D₂O instead of H²O. The result is HD²O⁺. Since the reaction is in equilibrium, acetate will eventually want something back. Again, odds are it’s going to get a deuteron back instead of a proton. Considering this process, and the wealth of deuterons in this system compared to protons, the carboxylic acid in solution is going to end up being primarily COOD, which is invisible to ¹H NMR, and any and all protons are going to get stashed away in the solvent residual. Make sense? See Scheme 2 for a graphical representation.

SCHEME 2. H/D EXCHANGE BY CARBOXYLIC ACID PROTON IN DEUTERATED SOLVENT.

FIGURE 4

So, the ¹H NMR of acetic acid has been solved (Fig. 4, see above). Now, let’s look at the ¹³C NMR spectrum (Fig. 5). A ¹³C NMR experiment will look at all carbon environments in the tube, so we should expect to see two peaks: One for the methyl carbon of acetic acid, and one for the carboxylic acid carbon. Fortuitously, our solvent has zero carbon atoms, so it is invisible in ¹³C NMR. No need to identify a solvent residual this time! But, with other solvents, the aforementioned compilation article in Organometallics is equally as handy for ¹³C NMR as well.^[4]

FIGURE 5

If we look at the spectrum (Fig. 5), we see way more than two peaks. We see four pretty squished together around 180 ppm, and four between 0 and 40 ppm. But, being the expert NMR spectroscopist that you are, you’ll easily identify them as very "multiplet-looking". To solve this mystery, think back to how multiplets are formed. In ¹H NMR, we know that non-equivalent neighbouring protons will cause a resonance to split into a multiplet. If protons cause splitting of resonances in ¹H NMR, why shouldn’t they “split” carbon resonances in ¹³C NMR too?! The methyl carbon has 3 directly-bound protons, so it will split three times into a quartet with a J-coupling value that is fairly large, since the responsible protons are directly attached to this methyl carbon (¹J_C-H = 128 Hz). In contrast, we’re used to the smaller 3-bond J-values of ¹H NMR, since protons are almost never directly bound to each other. The carbonyl carbon of the carboxylic acid functional group is further away from those protons, so this J-coupling constant is much weaker (²J_C-H = 6 Hz). We can now fully label the ¹³C NMR spectrum of CH₃COOH (Fig. 6).

FIGURE 6

As you can imagine, in organic chemistry, there are so many carbons that are directly bonded to hydrogen atoms, so ¹³C spectra can get pretty complicated very quickly. Acetic acid is a pretty simple molecule, and things are already starting to get pretty crowded! Thankfully, you can do something that is known as “broadband decoupling”. While still “observing” ¹³C, you can “decouple” ¹H, which cancels out any coupling/splitting/multiplicity caused by ¹H nuclei. As a result, any multiplets arising from coupling to ¹H will collapse into a singlet. This broadband decoupling experiment can be denoted with curly brackets as in ¹³C, which means “carbon observed, proton broadband decoupled”. So, let’s acquire the ¹³C spectrum (notice the curly brackets this time) of CH₃COOH (Fig. 7). Now that we know about decoupling, what would you expect in the spectrum? We should see two singlets.

FIGURE 7

You can see that the decoupled spectrum of acetic acid is simplified significantly, with singlets showing up at the center of where the multiplets used to be. See how much easier it is to look at a ¹³C spectrum instead of a ¹³C one? For this reason, the typical practice is to run a decoupled spectrum. Decoupled spectra also inherently take less time. For example, the resonance in the ¹³C spectrum representing the methyl group is spread over 4 peaks of a quartet. If you were to take those “heights”, and add them all together on top of each other, you would get the singlet you see in the ¹³C spectrum. So you can see how you would really only need less than half the time to run a ¹³C spectrum compared to a carbon observed, proton coupled ¹³C spectrum.

If you would like to learn more about our decoupling modes in a ¹³C spectrum, you can refer to our blog about it here. As always, if you have any questions, please don’t hesitate to contact us.

References

[1] i5.walmartimages.ca/images/Large/167/190/999999-66343167190.jpg [Viewed Jan 13, 2017].
[2] static.independent.co.uk/s3fs-public/styles/story_large/public/thumbnails/image/2015/04/13/17/web-fish-chips.jpg [Viewed January 9, 2017].
[3] Bordwell, F. G.Acc. Chem. Res. 1988,21, 456-463.
[4] Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I.Organometallics 2010,29, 2176-2179.