When the electrons in a molecule are subjected to an external magnetic field #B_0#, they generate their own magnetic field that opposes #B_0#.
This induced field shields the nearby protons from the full force of #B_0#.
This effect is called diamagnetic shielding.
The nuclei experience only the effective field #B_"eff"#.
Their resonance frequency is slightly lower than what it would be if they did not have electrons shielding them.
An electronegative atom pulls valence electrons away from the atom and effectively decreases the electron density around the nuclei.
Thus, a lower value of #B_0# is needed to reach the resonance frequency of the nucleus.
The #"Cl"# atom removes enough electron density to shift the frequency of an adjacent proton from δ 0.9 to δ 3.4 ppm.
We see the same effect in #""^13"C"# NMR spectra.
The carbon atom in ethane has a resonance frequency at δ 7 ppm.
In ethanol, the electronegative #"O"# atom deshields the #"C"# atom, and its resonance frequency shifts to δ 58 ppm.