Nuclear magnetic resonance shift reagents. Question of the orientation of the magnetic axis in lanthanide-substrate complexes

The utility of nmr shift reagents which function predominantly by the dipolar mechanism for structural and stereochemical problems is becoming abundantly clear. After a brief flurry of reports which indicated that the magnitude of the shifts induced by lanthanide chelates could be satisfactorily correlated by distance factors alone, the importance of angular factors, as predicted by the expected proportionality of the shifts to (3 cos^2 χ - 1) /r^3 , which would lead to either upfield or downfield shifts with the same reagent-substrate complex, has been clearly established. The (3 cos^2 χ - 1)/r^3 dependence, where χ is the angle between the principal magnetic axis of the complex and the vector of length, r, connecting the metal atom and nucleus under consideration, is justified strictly only for axially symmetric, or effectively axially symmetric, complexes. Nonetheless, excellent fits have been obtained following (3 cos^2 χ - 1)/r^3 proportionality with complexes of shift reagents and alcohols which can hardly be expected to be strictly axially symmetric and, furthermore, with the assumption that χ is, in fact, the angle between the vector r and the O-metal bond of the complex