Electronegative elements directly attached to a carbon atom bearing hydrogens (protons) pull electron density away from the protons. These protons are "deshielded" because the valence electrons normally "shielding" these protons from the external magnetic field are being inductively drawn away from those protons by the presence of the electronegative element.
Figure 8. Electronegativity effects for fluoromethane (red - high electron density, blue - low electron density).
The following table shows how chemical shifts for methyl protons are affected by the presence of an electronegative element.
Table 2. Electronegativity effects on proton chemical shifts.
Compound CH3X |
CH3H | CH3I | CH3Br | CH3Cl | CH3F |
Element X |
H | I | Br | Cl | F |
Electronegativity of X |
2.1 | 2.5 | 2.8 | 3.1 | 4.0 |
Chemical Shift d |
0.23 | 2.16 | 2.68 | 3.05 | 4.26 |
sp3 Hydrogens
Hydrogens attached to sp3 hybridized carbon atoms resonate between 0 - 2 ppm.
Figure 9. sp3 hydrogens.
sp2 Hydrogens
Hydrogens attached to sp2 hybridized carbon atoms resonate farther downfield than for normal aliphatic protons. The shift from TMS is dependent on the type of sp2 hybridized carbon atom:
Vinylic Hydrogens
Hydrogens attached to carbon-carbon double bonds resonate between 4.5 - 7 ppm. The sp2 hybridized carbon atom of the double bond has increased s-character, and is therefore more electronegative than an sp3 hybridized carbon atom.
Aromatic Hydrogens
Aromatic hydrogens resonate between 7 - 8 ppm.
Aldehyde Hydrogens
Aldehyde protons resonate between 9 - 10 ppm. This further downfield shift is due to the additional effect of the electron withdrawing oxygen atom nearby.
sp Hydrogens
Acetylenic hydrogens resonate between 2 - 3 ppm due to the anistropy of the carbon-carbon triple bond.
Hydrogen bonding causes a further deshielding of protons, and a further downfield shift for these proton resonances. Hydrogen bonding effects are concentration and temperature dependent, and this results in a wide range of possible resonance frequencies for these protons. In general, protons attached to oxygen and nitrogen resonate between 0.5 - 5 ppm.
All groups in a molecule with p electrons will have an effect on the local magnetic field due to the induced circulation of these p electrons.
Aromatic Rings
The p electrons in an aromatic ring are induced to circulate around the ring in response to an applied magnetic field. This "ring current" generates a local magnetic field which opposes the applied magnetic field. However, on the periphery of the ring, the flux lines are in the direction of the applied magnetic field. Consequently, protons attached to the aromatic ring "feel" a larger magnetic field than protons elsewhere in the molecule. Aromatic ring protons will therefore resonate at higher frequency and exhibit a downfield shift (7 - 8 ppm).
Figure 10. Magnetic anisotropy of an aromatic ring.
Acetylenic Hydrogens
The p electrons in a triple bond circulate around the bond axis to produce a magnetic field directly opposing the applied magnetic field. The acetylenic hydrogen is shielded by this induced field, and will therefore resonate at lower frequency (2 -3 pmm).
Figure 11. Magnetic anisotropy of a C,C-triple bond.
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