Teaching 1H NMR
Thursday 15 May 2025
My first Blog post!
Students find getting to grips with proton NMR to be a real challenge. However, I find that if I can help them break it down into all the component pieces of information that together make up a proton NMR spectrum, then it becomes much easier for them. As one of my students said a couple of weeks ago after some analytical chemistry revision, “NMR is easy when all the pieces of the jigsaw are in place!”
I’ve tried out a few strategies to teach this over the years, and by far the most successful I've found has been to take them, step by step, through the process of drawing an 1H NMR spectrum from scratch.
I don't want to teach my grandmother to suck eggs, but here is what I do with the students when beginning to explore proton NMR.
The Basics
Hydrogen Environments
Before we even begin the ‘drawing from scratch’ process, I will always work with the students on identifying different hydrogen environments, using molecular modelling kits to help. (I usually save the theory of nuclear spin for later and tell them that hydrogen atoms in different environments experience different effects in the NMR machine.)
I talk about ‘sitting on the crossbar’ of each hydrogen atom in turn: If we ‘see’ exactly the same thing from different hydrogen atoms, then we know they are in the same hydrogen environment – they are chemically equivalent.
I give them a list of relatively simple organic molecules to draw and work out the number of different hydrogen environments, such as ethanol, propanoic acid, ethane, propane, ethoxyethane, 2-chloropropane, and 2-bromobutane. I then go through each one in turn, explaining the environments; they often miss symmetry in the molecule.
The ‘Components’
I introduce a checklist of ‘components’ (below) and explain that we will use it to draw a proton NMR spectrum for ethanol. We use mini whiteboards for this exercise, as I want the students to continually adjust their spectrum as we piece together the different proton NMR components.
Drawing the 1H NMR spectrum of ethanol
I begin by asking the students how many hydrogen environments we have – they have already encountered ethanol – so they begin confidently ‘Three!’. I ask them to draw three signals (which I always refer to as ‘peaks’ - old habits!) on their mini whiteboards. I nudge them to include a zero signal on the right, but I’m not worried about the size of the signals and the chemical shift positions as yet:
Then I explain that the area under each signal is proportional to the number of hydrogen atoms in each environment (or, in fact, the ratio of hydrogen atoms in each environment, which is often the same thing). I ask them to take their whiteboard erasers and adjust the sizes of their signals. I’m looking for the approximately 1:2:3 ratio of area (I point out that this is not the same as height). I still don’t worry about where the signals are :
Then I explain about the splitting patterns – that each signal is split by any neighbouring hydrogen atoms according to the n+1 rule. This is where it gets more complicated, so we work through each signal in turn, beginning with the CH3 signal. I draw a labelled diagram of ethanol on my classroom whiteboard:
They can see that the two neighbouring HB atoms split the HA signal into a triplet. For the moment, I help them with the triplet signal shape (after they've had a go drawing it):
Time now to tell them that the O–H hydrogen is ‘labile’; rapidly exchanging with its environment, so a hydrogen atom attached to an oxygen tends not to split or be split. I ask them to finish the splitting patterns. They now know that the HC signal is a singlet, and the HB signal with three nearest neighbours is a quartet (I help them with the quartet signal shape).
Although students would not be asked to explain the details of why the patterns are as they are in an examination, it is still great chemistry! Geoff explains the origin of the shapes of the splitting patterns on this page of the site: S3.2 (AHL) Mass spectrometry, IR & 1H NMR spectroscopy.
Lastly, we refer to the data booklet to add some chemical shifts and ask ourselves if our signals are in the right places!? I explain that chemical shift is highly variable and not to be relied upon, but that hydrogen atoms near electronegative atoms are likely to be ‘shifted downfield’ (meaning they are de-shielded by the electronegative atom pulling away electron density and so are more affected by the magnet - so move to higher ppm). Any reasonable guess with chemical shift here is acceptable, using the data booklet as a guide. They may need to adjust their peaks significantly, though, depending on where they were initially placed!
Then I show them a real ethanol spectrum. We note the sharpness of the signals; the numbers indicating the integration trace; that the hydroxy hydrogen is shifted a long way downfield (12.0 ppm in this spectrum!).
Then I might have them draw a few more spectra using their whiteboards for the molecules we explored earlier, when looking at hydrogen environments.
The hope with this exercise is that by drawing the spectrum in stages, the students will begin to realise all the information that is contained in a single 1H NMR spectrum (and how amazing proton NMR really is)!