Electronegativity and Shielding

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NMR Made Easy! Part 1 - Electronegativity and Shielding - Organic Chemistry
6:16 — by Frank W.

Tip: This isn't the place to ask a question because the teacher can't reply.

Key Questions

  • The outermost electrons are screened/shielded from the nucles by the inner electrons which phenomenon is known as shielding/screening effect

  • Answer:

    The more shielding, the lower electronegativity.


    I think I'll explain with an example.

    Fluorine (F) has the highest electronegativity on the whole periodic table with a score of 4.0.

    This is because its nucleus has a very powerful attractive (effective nuclear charge) force directed towards all of its electrons. All of the electron levels are pulled very close to the nucleus, so there is very little shielding between the nucleus and the electrons.

    Compare this to Francium, the element with the lowest electronegativity, weighing in at 0.7.

    Francium is at the very bottom of the first group. This means that it has a LOT of electron levels it has to hold onto compared to fluorine. Therefore, there are more levels causing shielding between the electrons and the positively charged nucleus.

    More on Effective Nuclear Charge!

  • Answer:

    Electronegativity is an important factor in NMR spectroscopy because it affects the shielding of the nuclei.


    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"#.

    (from chemwiki.ucdavis.edu)

    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.

    (from chemwiki.ucdavis.edu)

    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.