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How should I label the multiplicity of the aromatic signals in an 1H-NMR spectrum with the following substitution pattern?

NMR

I have seen lots of people labeling both signals in this kind of system as doublets, probably because the resolution of the spectrum was not high enough, or because we are often told, these look like doublets. However, what looks like a doublet doesn't necessarily have to be one and thus, should not be called doublet, because it doesn't reflect the actual couplings in this system:

  • The ortho-coupling (3J) for both signals should be about 6.0 and 9.0 Hz.
  • The meta-coupling (4J) for both signals should be between 0.9 and 3.0 Hz.
  • The para-coupling (5J) for both signals should be between 0.0 and 1.0 Hz.

(These numbers are from the Hesse-Meier-Zeeh, a German textbook about spectroscopy in organic chemistry).

I am inclined to call these signals multiplets rather than doublets, because I can only guess the coupling constants, especially for the right signal at about 7.36 ppm.

Or is it good practice to call them pseudo-doublets? And what would be the symbol (like s for singlet, d for doublet, etc.)?

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    $\begingroup$ First, if R1 and R2 don't produce any kind of steric hindrance in the molecule and they rotate freely around the R-C(Ar) bond, I don't see any reason why H and H' should be different (I mean, should there be any splitting at all for them on them?). Second, if you want to be very precise, you can call them ddd, but that arises the problem with the constant values. Maybe you can try to register this spectrum on an NMR of a higher frequency? But synthetic papers don't really often care about higher coupling constants than 3J. $\endgroup$ – wolphram Oct 18 '17 at 8:32
  • $\begingroup$ @wolphram This is true, but as you can see in the spectrum, this arene gives more than just a doublet. Even the signal on the right has visible shoulders. I could call them ddd, but then I would have to show three coupling constants, which I cannot measure. $\endgroup$ – basseur Oct 18 '17 at 8:49
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    $\begingroup$ @wolphram $\ce{H}$ and $\ce{H'}$ may be chemically equivalent but are not magnetically equivalent, the coupling constants of each to $\ce{H_X}$ (one single, specific proton, not the ‘closer neighbour’) are different for $\ce{H_A}$ and $\ce{H_A'}$. $\endgroup$ – Jan Oct 18 '17 at 9:24
  • $\begingroup$ @Jan so you are saying that if that's, say, p-chlorotoluene, they won't be magnetically equivalent? $\endgroup$ – wolphram Oct 18 '17 at 10:09
  • $\begingroup$ @wolphram Correct. $\endgroup$ – Jan Oct 18 '17 at 12:39
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Many people report these as doublets. However, you are correct that these are not doublets - there are second order effects in AA’XX’ spin systems (note that a first-order analysis will fail, the pattern is not a ddd - see https://www.chem.wisc.edu/areas/reich/nmr/05-hmr-15-aabb.htm for more info).

Using the phrase “apparent doublet” (abbreviate as app d) is probably a good compromise between being technically correct and retaining useful information. You can still report the “coupling constant” between the two largest peaks, but with the caveat that it is not a first-order doublet.

If you want to be very precise, then multiplet is the way to go. However a typical organic chemist would also be interested in the splitting between the two largest peaks in your multiplet (which if I am not mistaken, corresponds to the sum of $^3J_\ce{AX}$ and $^5J_\ce{AX’}$). Since the para coupling is tiny, the splitting is essentially representative of the ortho coupling. In my opinion it is helpful to leave that information in.

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  • $\begingroup$ At our university, we usually have the sentence "First-order principles were applied in the analysis of the spectra" in our theses. So I guess, I will be fine with "app d". Thanks! Adding coupling constants to a multiplet would seem rather wrong. $\endgroup$ – basseur Feb 23 '18 at 9:32

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