[ISPolPH8789]

The chemical shift The chemical shift is one of the most important observables in nuclear magnetic resonance. It provides valuable information about the chemical environment around a nucleus. In a real spin system, nuclei are surrounded by atomic and molecular electron clouds which interact with the nuclear spin angular moment. The principal influence of the surrounding electrons is the magnetic screening which results when electronic orbitals are perturbed by the applied magnetic field BO. The effect of the magnetic screening (shielding), called nuclear shielding, can enhance or oppose the main field. This shielding interaction is isotropic in liquids but in general it has rotational anisotropy in solids. The spin Hamiltonian describing the shielding interaction is therefore Hcs=-S.I.Bo=-g I. segma . BoWhere S is the shielding tensor and σ is the chemical shift tensor which describes the orientation dependence of the interaction. The averaging of the interaction over all possible random orientations in a crystal sample produces a line-broadening which in this case is field dependent.