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Presumably the amine acts as an electron donor, but that would suggest extra electron density of the adjacent carbons, making them more shield, which isn't seen in the results in the image below.

indole with 13C-NMR chemical shifts in ppm

Specifically I'm wondering what causes the carbon with a chemical shift of 111 ppm to be less deshielded than the carbon with 123 ppm, as they look like they should be similar electronically, and I can't work out a resonance form that gives different results. Any help with this, and a more general rule set for chemical shift in aromatic compounds, would be much appreciated.

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    $\begingroup$ +1 Good question. Although note that in my experience, carbon shifts are much more difficult to analyse and interpret with our usual intuitive notions of electronegativity, resonance etc. You may want to look at the proton NMR and see if you can interpret that with the resonance structures, inductive effects etc. $\endgroup$
    – S R Maiti
    Apr 26, 2021 at 20:34
  • $\begingroup$ Why so many close votes? This does not look like a homework question in any way. $\endgroup$
    – S R Maiti
    Apr 27, 2021 at 11:32

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