Why does the 1H NMR spectrum of methacryloyl chloride show two singlets instead of two doublets for vinyl protons?

Question

Based on what I have understood, for methacryloyl chloride there is a geminal coupling of the vinyl protons since they are not chemically equivalent. Why are two singlets observed for vinyl protons instead of two doublets?

Answer 1

The geminal $^2\!J_\ce{HH}$ coupling in terminal alkene $\ce{CH2}$ can be pretty small; often on the order of $\pu{2 Hz}$. The late Hans Reich's NMR website has a lot of data which you can check out.

(And yes, this is in stark contrast to geminal couplings in diastereotopic, $\mathrm{sp^3}$ $\ce{CH2}$ groups, which often have large magnitudes!)

So, to observe it in the spectrum, you will need to increase the resolution. The easiest way would be to perform extra zero-filling and see if that helps. The next simplest way is to process the data using a different window function / apodisation function: to increase the resolution, you can use a Gaussian window function, see e.g.

• Why is the Gaussian Window Function used to enhance resolution in 1D NMR spectra?

• and some practical instructions at the bottom of this answer: Multiplet shape in proton NMR of morpholines

• and there are numerous resources online which you can find as well.

If none of that works, it's possible that you might need to rerun the spectrum and increase the acquisition time of the spectrum, but I'd consider that to be quite unlikely for a routine $\ce{^1H}$ spectrum.

[That said, this appears to be a spectrum you've taken from someone else's work, so... maybe all you can do in this situation is to take a magnifying glass, and hope that the spectrum was processed and the image created with sufficiently high resolution. The suggestions above obviously only make sense if you are the one who acquired the spectrum.]