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There are two singlet signals for NH2 group around 2.4 and 3.05 ppm. Spectra is predicted (from MNova), so there was no water present. Why the signal of NH2 group isn't a broad singlet?

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    $\begingroup$ What is your question? $\endgroup$
    – PAEP
    Nov 5, 2023 at 14:55
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    $\begingroup$ @PAEP Plausible option: why the signal of the amino group isn't a broad singlet (as in e.g., aniline, example in SDBS 905), but split into two. $\endgroup$
    – Buttonwood
    Nov 5, 2023 at 15:03
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    $\begingroup$ @Lucia Boldišová There are multiple tools (as in programs) and methods (as in what parameters, level of theory, etc) to predict NMR spectra. Can you please extend your question by a note to disclose at least the source, name (and because it evolves, the version / year of publication) of the software used to compute the data displayed? There is a hunch (MNova), but to know for sure would provide future readers an avenue in lines of «... and I have already seen this kind of prediction provided by so-and-so». $\endgroup$
    – Buttonwood
    Nov 7, 2023 at 11:22
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    $\begingroup$ Sorry I forgot to mention it, we used MNova for prediction. $\endgroup$ Nov 7, 2023 at 17:24

1 Answer 1

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The amino group can be conformationally locked by an intramolecular hydrogen bond:

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by formation of a five-membred cycle. (And in organic chemistry, the formation of five, and to some lesser degree of six membered cycles anyway is favourable.)

Contrasting to a terminal amino group in e.g. n-butylamine, the two hydrogens of said amino group then no longer experience the same intensity of the magnetic field / averaged over time no longer are magnetically equivalent. Though it depends on the solvent used, temperature and concentration of the sample, and the spectrometer parameters how much of this effect eventually is observed in the actual experiment, I consider this as a good feature in the algorithm to predict the H NMR spectrum.

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    $\begingroup$ This sounds plausible; but I wonder whether the algorithm is too smart for its own good. I am not sure if the C(O)–N bond is configurationally stable, and if so, whether the isomer shown (R1 and R2 cis) is favoured. Even in N-methylacetamide (R1 = R2 = Me) the trans isomer is significantly more stable (e.g. doi.org/10.1021/jp8106556), and here R1 and R2 are even bigger... $\endgroup$ Nov 5, 2023 at 23:04
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    $\begingroup$ @orthocresol In the structure posted by the OP, O (of carbonyl) and H (of amino group) present a path of 5 atoms; O-C-N-N-H. In N-methylacetamide your paper describes, it however only equates to 4; O-C-N-H (with a ratio of the trans conformation over the cis conformation greater than 95:5 for H2O, CHCl3, C6H12). How does the energy surface look like for a rotation round the torsion angle O=C-N-N? So I speculate «the click» by the intramolecular hydrogen bond plus a little pi-pi interaction of the aryls yields a conformation not too costly (energy wise) vs the trans conformers. $\endgroup$
    – Buttonwood
    Nov 7, 2023 at 10:53
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    $\begingroup$ @orthocresol Re algorithm: The OP doesn't disclose the origin of the prediction, Mestre/MNova however were a plausible reference (the optional plugin does cost some extra, eventually can help / automatically assigns heteronuclear correlation spectra, too). Though the companies tend to keep their algorithms closed source, there is e.g. «NMR signal processing, prediction, and structure verification with machine learning techniques» as a review by Carlos Cobas on behalf of Mestrelab in Magn. Reson. Chem. (doi.org/10.1002/mrc.4989, note the public review on publons straddles ACD). $\endgroup$
    – Buttonwood
    Nov 7, 2023 at 11:09
  • $\begingroup$ Of course, that is very reasonable speculation. :) This is probably Mnova, yeah. $\endgroup$ Nov 7, 2023 at 16:52

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