In the past, when shifting focus from NMR theory to structure elucidation using an NMR spectrum, I would teach students to construct the molecule incrementally using information from the spectrum. Typically, I would work through some examples to show students how to repeatedly return to the spectrum, building as much of one portion of the molecule as possible before switching to another portion. I would then challenge students to independently work through as many of these types of structure elucidation problems as possible.
In office hours, when students would come for help on interpreting NMR spectra, many would seem intimidated by what they felt to be daunting problems. But we would continue with the same strategy: use the spectrum to build as much of one portion of the molecule as possible before starting to build another portion. Occasionally this would leave us with multiple fragments of one molecule, at which point we would simply try putting them together in various ways to give us a handful of complete molecules as candidates. Finally, it was a matter of returning to the NMR spectrum once more to see which of our candidates were consistent with the entire spectrum and which led to contradictions.
It wasn’t long before I noticed students in office hours would often experience the same “eureka” moment— Seeing that we were occasionally left with molecular fragments that we had to piece together, students would almost invariably say, “So this is just like a jigsaw puzzle!” I would say yes, it is. Somehow, that affirmation of an association to a much more familiar (and probably relaxing) experience made the task seem much less intimidating.
To take advantage of this familiarity students might have working with jigsaw puzzles, I modified parts of Section 16.15 (Structure Elucidation Using NMR Spectroscopy) in the second edition of my textbook. Instead of asking students to construct as much as possible of one portion of a molecule, I now provide students with a set of questions to apply toward each signal, compelling them to construct multiple smaller fragments. Later, students are asked to piece those fragments together.
To see how it works, consider this proton NMR spectrum of C10H12O2.
From each signal’s chemical shift, splitting pattern and integration, students are shown how to construct the four fragments in the following table. For each fragment, the protons responsible for the signal are shown in red without parentheses, and the protons in black inside parentheses are responsible for splitting the signal.
Now, to put these “puzzle pieces” together, students need to see that a portion of one fragment can double as a portion of another fragment (already, the 13 carbons in the above table exceed the 10 carbons given in the molecular formula!). In this case, the first three entries can make up a propoxy group, as shown below.
Finally, the molecule can be completed by realizing two things. First, the propoxy group and the phenyl group (Entry 4) contain a total of nine carbons, 12 hydrogens, and one oxygen, leaving one carbon and one oxygen yet to account for. Second, the propoxy and phenyl groups account for an IHD of four, but the molecular formula given has an IHD of five, so the remaining part of the molecule needs to add one to the IHD, too. Both are accomplished by a C=O group, giving us propyl benzoate as the completed molecule.
Last year, I had success applying this strategy with a small handful of students. This year, we begin our unit on NMR spectroscopy in a couple weeks, at which point I plan to introduce and emphasize this “jigsaw puzzle” approach to the entire class. I’m eager to see it work.
