In the average case one can simply dissolve an analyte in an appropriate deuterated solvent and acquire a simple 1D spectrum to obtain all the required structural information. However, sometimes doing so may not provide you with all of the information you need! It is not uncommon to encounter labile proton peaks in functional groups such as alcohols, amines, amides, and carboxylic acids. These resonances move around the spectrum depending on temperature, pH and solvent choice and can overlap or interfere with the other static signals from the protons attached to the carbon backbone of the organic molecule. Such overlap can convolute a spectrum to varying degrees from affecting integration to completely drowning out a peak. This is vital information that we can obtain in a very simple fashion. In a previous blog post, Matt Zamora discussed how a spectrum of acetic acid in D2O differs from the spectrum of acetic acid in CDCl3 (read it here!) because of the exchangeable nature of the carboxylic acid protons. Additionally, labile protons have a tendency to produce peaks that are substantially broader than protons that are not labile. In this blog entry, I will discuss using what is known as a D2O quench/shake to exchange out labile protons and the valuable information that can be gained.
First, we will look a relatively simple example that highlights instances where you would want to confirm the presence of a labile proton signal. In the spectrum of 4-hydroxypropiophenone (figure 1) you can see that the signal for the hydroxy proton at 10.24 ppm comes in as a nice sharp, well-resolved singlet. Adding D2O to the NMR tube and re-acquiring the 1D 1H spectrum shows complete disappearance of the signal at 10.24 ppm and confirms that assignment to the hydroxy proton.