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I have completely unpacked the NMR spectrum for the drug aminocaproic acid (structure below) and have found a peak that I myself can not explain. The signals for the drug are simple to understand what corresponds to what but the reason behind one of the peaks having six peaks is unknown to me. I had made the assumption that the furthermost up-field signal would have 5 peaks and be a multiplet, due to it having four nonequivalent neighbors and N+1 rule. If there is a reason behind this can some one please explain.

Molecular structure

NMR spectrum

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  • $\begingroup$ Could you please specify what ppm region should one look at? The peaks at 2.2 ppm have some kind of marker above, but I'm not sure you are talking about them. Also, check out SDBS entry for aminocaproic acid, at least you can check your assignments. $\endgroup$ – andselisk Aug 3 '17 at 12:55
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    $\begingroup$ Coupling to exchangeable protons like OH or NH2 is often not seen ("exchange decoupling") - see chemistry.stackexchange.com/q/48232/16683 and this is covered in most organic-NMR texts, usually under the section on spin-spin coupling $\endgroup$ – orthocresol Aug 3 '17 at 13:23
  • $\begingroup$ You mean why the multiplett below 1.4 ppm has six peaks? Four non-equivalent neighbours make not five peaks, but a dddd signal, which can be up to eight peaks, if nothing overlaps. $\endgroup$ – Karl Aug 3 '17 at 21:22
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    $\begingroup$ Sorry, dddd could have up to sixteen peaks, of course. You could also think about a tt signal. But generally, longer alkyl chains just make multipletts, because the two protons in a methylene group are usually not magnetically equivalent. $\endgroup$ – Karl Aug 3 '17 at 21:33
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The N+1 rule, in its ideal form, assumes that for multiplets arising from $\mathrm{^3J}$ couplings between neighbouring $\ce{CH_2}$ groups,

  1. shifts for the spins within a $\ce{CH_2}$ group are identical
  2. all couplings between spins in two different groups are identical

While condition 1 usually holds for substituted alkanes of the form $\ce{A-(CH_2)_n-B}$, condition 2 does not. This leads to the title "Is the n+1 rule ever obeyed?" in a section of a textbook [1], like a cry of despair. The text elaborates:

At other times, when there is a large difference between $\mathrm{^3J_{AB}}$ and $\mathrm{^3J_{BC}}$, distinct peaks, more than five in number, can be seen. Deviations of this type are most common in a chain of the type $\ce{X-(CH_2)_3-Y}$, where X and Y are widely different in character.

So, the exception is actually closer to the rule, and that is because true magnetic (unlike chemical) equivalence is relatively rare.

[1] Pavia, Lampman and Kriz, Introduction to Spectroscopy, 2nd Ed., Saunders Publishing

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