7.13 : Diamagnetic Shielding of Nuclei: Local Diamagnetic Current

An applied magnetic field causes the electrons present in the molecule to circulate, setting up a local diamagnetic current within the molecule. The local diamagnetic current arising from circulating sigma-bonding electrons induces a magnetic field, B_local that opposes the applied magnetic field, B₀. The effective magnetic field experienced by these nuclei is given by the difference between the applied and local magnetic fields in a phenomenon called local diamagnetic shielding. Essentially, B_local increases with the electron density surrounding the nuclei, leading to increased shielding and a lower B_effective. Since electron densities vary within a molecule, nuclei in the same molecule are shielded to different extents and experience different effective fields. A nucleus in an electron-dense environment is well-shielded from the applied magnetic field and experiences a lower B_effective. Consequently, the energy required to flip its spin is less than that required for a poorly shielded nucleus in electron-poor surroundings. Thus, shielded nuclei experience resonance at lower frequencies than deshielded nuclei. Resonance frequencies are plotted on the NMR spectrum, making these spectra sensitive to diamagnetic shielding.