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I understand that ring protons experience deshielding because the induced magnetic field (outside the ring) has the same direction as the external applied field, but shouldn't electron withdrawing groups that delocalise electrons throughout an extended aromatic system also lead to a smaller induced magnetic field outside the ring (thereby lowering the effective field experienced by the protons and diminishing their chemical shift)?

With electrons spread out in a larger conjugated system (and thus, with potentially smaller ring currents?), I expected to see lower values for the protons; but, by studying multiple spectra, I came to notice that electron-withdrawing groups actually lead to increased chemical shift values (/heightened effective magnetic fields).

I suspect the reasoning behind electron donating groups decreasing the chemical shift is similar, but I can't seem to grasp the logic, since I am correlating electron density with increased ring currents (and increased induced magnetic field). Any help would be appreciated.

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I agree that without data one can reason in both ways. However even if you can assume smaller ring current and therefore less deshielding, the electron density is moved toward the electron withdrawing group, and this deshield the protons as well - especially in an aromatic ring. Reality tells that is the second effect that is prevalent.

Alternatively and shorter, you can see the electron withdrawal as from the hydrogens, not from the ring current. As you know, in delocalised electronic system the inductive effects persist over few bonds.

I think this can rationalise the data, at least at posteriori.

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  • $\begingroup$ Thank you - I didn't know exactly which of the two effects - deshielding versus smaller ring current - had the biggest impact, thereby being predominant. $\endgroup$
    – mateis
    Oct 31, 2021 at 12:03
  • $\begingroup$ @mateis the point is that, let me use the following expressions for sake of brevity, as you suck electron density from the ring, the ring sucks in electron density from the C-H. Hope it helps. $\endgroup$
    – Alchimista
    Oct 31, 2021 at 12:43

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