Use of Lanthanide Nuclear Magnetic Resonance Shift Reagents in Determination of Molecular Configuration

Previous determinations of molecular geometry by use of lanthanide shift reagents have been based on the (tacit) assumption that the complex is rigid. However, whenever rapid internal rotations are present it is necessary to average the entire quantity, [(3 cos²θ_i, — l)/r_i³], before comparing observed and calculated shift ratios. This paper presents several models for free or hindered internal rotation and tests the models on three organic substrates which are rigid except at the point of attachment to the lanthanide. While diacetoneglucose appears to be rigid at the point of attachment (so that the configuration of the complex can be established with confidence), both aniline and a rigid bicyclic alcohol (1) exhibit considerable internal rotation. In fact, the shift ratios for aniline cannot be accounted for by any “static” model for the complex. Moreover, examples are provided which show that “good” fits between observed and calculated shift ratios are not in themselves evidence for the existence of that conformation (for both aniline and 1, some of the best “fits” occur at chemically unreasonable values for the lanthanide-donor atom separation). However, the use of several models for internal rotation, interpreted by means of contour plots of “fits” as a function of geometry of the complex, can provide a means for sorting out the correct from the spurious calculated configurations.