8.5: π Electron Effects on Chemical Shift: Overview
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in magnetic anisotropy.
In alkenes, the induced field is parallel to the applied field near the vinylic protons. This amplifies the deshielding by the sp2-hybridized carbon and vinylic protons appear downfield between 4.5–6.1 ppm. Similarly, the induced magnetic field of the carbonyl π-electrons promotes the deshielding of aldehydic protons, which appear between 9.5–10.5 ppm.
In alkynes, the deshielding caused by electronegative sp-hybridized carbon is countered by the shielding effect of cylindrical π-electron cloud in the vicinity of acetylenic protons. The induced field is oriented against that of the applied field at the hydrogen atoms, because of which lower frequency radiation is required to bring them into resonance. Consequently, acetylinic protons appear upfield between 2.0–3.2 ppm.
