In a paramgnetic material the induced dipole moment of atoms or (molecules) of the material aligns with the external field. I can understand this by Zeeman splitting. A term symbol splits into states with different energy. According to Boltzmann distribution the lower energy states should have more population. These states would correspond to dipole moments aligned with the external field therefore the field inside the material should be enhanced. Why then the nuclei in NMR "feel" a lower magnetic field? I read that it was according to Lenz law. Is that because we take into account only the field in the region near to the nucleus? I mean if we had a paramagnetic material then the field should be greater.
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$\begingroup$ According to my understanding, resonant frequencies are effectively a measure of the effective (felt) magnetic field on the magnetic dipole moment of a nucleus. Therefore you don't include the nuclear magnetic dipole in your calculation of the effective field. As for shielding, it is caused by the electron density surrounding the nucleus. The applied magnetic field causes electrons to precess, generating an opposing magnetic field and lowering the effective magnetic field on the nucleus. $\endgroup$– JabbamangaCommented Jan 29, 2021 at 21:41
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$\begingroup$ Lenz' law is about electrons, specifically about the direction of the field that moving electrons induce! $\endgroup$– KarlCommented Jan 30, 2021 at 8:40
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$\begingroup$ @Karl I can't understand though why in paramagnetic materials the induced field is in same direction with the applied. Are these two phenomena (NMR shielding and paramagnetism) related at all? $\endgroup$– AntonCommented Feb 1, 2021 at 19:42
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$\begingroup$ They are not. The polarisation of spins in a field gives a very small static net magnetisation in the direction of the field. Chemical shift is caused by induction of moving electrons, and creates a field against $B_0$. $\endgroup$– KarlCommented Feb 4, 2021 at 8:03
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