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2.8.5: The Basis for Differences in Chemical Shift

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    We come now to the question of why nonequivalent protons have different resonance frequencies and thus different chemical shifts. The chemical shift of a given proton is determined primarily by interactions with the nearby electrons. The most important thing to understand is that when electrons are subjected to an external magnetic field, they form their own small induced magnetic fields in opposition to the external field.

    Consider the methane molecule (\(CH_4\)) in which the four equivalent protons have a chemical shift of 0.23 ppm. The valence electrons around the methyl carbon, when subjected to B0, generate their own very small induced magnetic field that opposes B0. This induced field, to a small but significant degree, shields the nearby protons from experiencing the full force of B0, an effect known as local diamagnetic shielding. In other words, the methane protons do not quite experience the full force of B0 - what they experience is called Beff, or the effective field, which is slightly weaker than B0 due to the influence of the nearby electrons.

    clipboard_e49c4ed8805ad03cfed817da2e602ae2d.png

    Because Beff is slightly weaker than B0, the resonance frequency (and thus the chemical shift) of the methane proton is slightly lower than what it would be if it did not have electrons nearby and was feeling the full force of B0. (You should note that the figure above is not to scale: the applied field is generated by a superconducting magnet and is extremely strong, while the opposing induced field from the electrons is comparatively very small.)

    Now consider methyl fluoride, CH3F, in which the protons have a chemical shift of 4.26 ppm, significantly higher than that of methane. This is caused by something called the deshielding effect. Recall that fluorine is very electronegative: it pulls electrons towards itself, effectively decreasing the electron density around each of the protons. For the protons, being in a lower electron density environment means less diamagnetic shielding, which in turn means a greater overall exposure to B0, a stronger Beff, and a higher resonance frequency. Put another way, the fluorine, by pulling electron density away from the protons, is deshielding them, leaving them more exposed to B0. As the electronegativity of the substituent increases, so does the extent of deshielding, and so does the chemical shift. This is evident when we look at the chemical shifts of methane and three halomethane compounds (remember that electronegativity increases as we move up a column in the periodic table, so fluorine is the most electronegative and bromine the least).

    CH4 CH3Br CH3Cl CH3F
    0.23 ppm 2.68 ppm 3.05 ppm 4.26 ppm

    Table 2.4.4.1. 1H NMR chemical shifts for CH3X compounds.

    To a large extent, then, we can predict trends in chemical shift by considering how much deshielding is taking place near a proton.

    The chemical shift of trichloromethane (common name chloroform) is, as expected, higher than that of dichloromethane, which is in turn higher than that of chloromethane.

    CH4 CH3Cl CH2Cl2 CHCl3
    0.23 ppm 3.05 ppm 5.30 ppm 7.26 ppm

    Table 2.4.4.2. 1H NMR chemical shifts for CH4-nCln compounds.

    The deshielding effect of an electronegative substituent diminishes significantly with increasing distance.

    Contributors

    This page titled 2.8.5: The Basis for Differences in Chemical Shift is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Tim Soderberg.