'Hop' off the Diagonal: COSY spectrum of α-humulene

NMR spectroscopy is by far the most useful characterization technique in organic chemistry, especially if you have to elucidate the structure or configuration of your products. Arguably, 2D experiments such as COSY, HSQC, and HMBC have simplified this task tremendously. In this post I wanted to highlight the COSY of α-humulene.

Why α-humulene? Well, despite the fact that this natural product is a monocyclic sesquiterpene – containing the ever impossible to draw 11 membered ring (horror, I know) - it’s from a class of compounds our customers have had a lot of interest in lately! You may recognize it from the characteristic taste of your favourite hoppy beer, or from the distinctive cannabis aroma.

The 60 MHz 1D ¹H spectrum and structure are shown in figure 1. You can see there is some overlapping of signals, which make structural assignment quite difficult.

Figure 1: 60 MHz 1H NMR Spectrum of alpha-humulene

Using the COSY spectra, however, paired with some chemical intuition, we were able to pretty much assign all the peaks in this relatively complex molecule! Figure 2 shows this beautiful COSY spectrum along with the off-diagonal correlations.

Figure 2: 60 MHz COSY of alpha-humulene

While some correlations are a little bit weak - for instance H²–H⁴ – some are more intense than we would expect, such as H¹-H⁴. In my opinion these protons are a little bit far apart to display such a strong correlation, and don’t really have a good suggestion to explain this behavior at further except to suggest virtual coupling? If you have a better idea please discuss them with us!

How does this compare to high-field data? Well, for your reference I have included the COSY spectrum of α-humulene acquired in a 700 MHz instrument in Figure 3.^[1] Of course at low-field the peaks are broader and we see more second order effects, but the same information is present!

Figure 3: 700 MHz COSY of alpha-humulene [1]

Feel free to contact us if you have any questions or if you want to see whether our instrument will be suitable for your chemistry!

References
^[1] Neuenschwander, U.; Czarniecki, B.; Hermans I. J. Org. Chem., 2012, 77, 2865.