IUPAC definition : In the context of NMR spectroscopy shielding is the effect of the electron shells
of the observed and the neighbouring nuclei on the external magnetic field. The external
field induces circulations in the electron cloud. The resulting magnetic moment is
oriented in the opposite direction to the external field, so that the local field
at the central nucleus is weakened, although it may be strengthened at other nuclei
(deshielding). The phenomenon is the origin of the structural dependence of the resonance
frequencies of the nuclei.;
In the context of NMR spectroscopy shielding is the effect of the electron shells
of the observed and the neighbouring nuclei on the external magnetic field. The external
field induces circulations in the electron cloud. The resulting magnetic moment is
oriented in the opposite direction to the external field, so that the local field
at the central nucleus is weakened, although it may be strengthened at other nuclei
(deshielding). The phenomenon is the origin of the structural dependence of the resonance
frequencies of the nuclei.