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Atomic and molecular electron clouds magnetically screen the nucleus of an atom or molecule from a field B0 through pertubations of the electron orbitals, called nuclear shielding. The angular dependence of nuclear shielding in solids gives the spin Hamiltonian in terms of shielding tensor S and chemical shift tensor σHcs = -S I B0 = -g I σ B0Averaging over all possible molecular orientations within a crystal leads to field dependent line broadening.

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