Applications of lanthanoid shift reagents to ¹³C and ¹H nuclear magnetic resonance spectroscopy

The induction by conventional shift reagents of contact effects in donor substrates containing delocalised π systems has been confirmed by ¹³C, ¹⁹F, and ¹H data. For a given lanthanoid, the magnitude of these effects is dependent on the particular chelate employed. It therefore follows that, when no change in shift ratios is observed as the europium chelate is varied, significant contact effects are not present. The axial symmetry approximation, although clearly not always valid, has given excellent predictions of both ¹H and ¹³C shifts observed with (tropone) Fe(CO)₃. Correction by diamagnetic lanthanoid complexation shifts significantly improves agreement of observed and calculated ¹³C shifts. Attempts to use shift reagents empirically on a series of closely related organometallic molecules have given mixed results, suggesting that the average magnetic axis orientation may be sensitive to small changes in substrate geometry.