Magnetic octopole contributions to the ‘pseudo-contact’ chemical shift

Magnetic multipole contributions to the neighbour-anisotropy or ‘pseudocontact’ shift are examined theoretically. Truncation of the multipolar expansion of the nuclear magnetic shielding at the dipolar terms is shown to be satisfactory for shift reagents relying on paramagnetic anisotropy in metal ions. However, this procedure is unreliable for benzene and macrocyclic shift reagents such as porphyrins. For molecules of this type, the ratio of the octopolar to the dipolar shielding is in the order of a²/R² where a is the ‘molecular radius’ and R the distance between the centre of the ring system and the nucleus of interest.