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.