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Shielding Effect

Electrons surrounding a nucleus induce a magnetic field that opposes the applied field, leading to a downfield shift in the NMR spectrum.
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The statement of the theorem

Consider the applied magnetic field B0B_0 and the induced local magnetic field BindB_{ind} generated by the electron cloud surrounding a nucleus. The effective magnetic field BeffB_{eff} experienced by the nucleus is given by: \nBeff=B0−Bind=B0(1−σ)B_{eff} = B_0 - B_{ind} = B_0 (1 - \sigma) \nWhere σ\sigma is the shielding constant, a dimensionless quantity quantifying the degree of magnetic field opposition, and BeffB_{eff} determines the resonance frequency ωobs=γBeff\omega_{obs} = \gamma B_{eff}.