An Analysis of N.M.R. Shifts in Lanthanide Complexes

A method is developed to analyse a range of n.m.r. shifts in lanthanide complexes and to estimate the relative contact and pseudocontact contributions to the n.m.r. shifts for a specific n.m.r. nucleus in the lanthanide complexes. where 2, = 2gNpNpB(-ihZ. rN X Vi/rN3 + 3(rN si)(rN. z)/rN5 -si 'z/rN3)p0/4~ (la) i 22 = C (~~PNPB~~(YN~)s~.Z/~IPO/~~ i (lb) Here rN is the radius vector of the n.m.r. nucleus. The contribution of 2, to the isotropic n.m.r. screening constant is frequently referred to as the pseudocontact contribution to distinguish it from the isotropic contact contribution represented by 2,. Thus the total shift may be of contact or pseudocontact origin, or a combina- tion of both. In some studies, for example resolution enhancement, the shift mechan- ism is unimportant. However, knowledge of the relative contributions is necessary to gain information about spin delocalization (contact) or coordination geometry (pseudocontact). In general, the two contributions cannot be separated experimentally and theoretical calculations are required to estimate them. However, in some specific cases, the temperature dependence of the shift may be a useful guide as to its origin.'