LANTHANIDE ION NUCLEAR MAGNETIC-RESONANCE PROBE STUDIES OF BENZOIC-ACIDS - AGREEMENT WITH AN AXIALLY-SYMMETRIC MODEL

The lanthanide-induced shift ratios measured in the presence of various lanthanide (3+) ions and the spin-lattice relaxation rates induced by Gd3+ were determined for benzoate, o-toluate, and p-toluate at pH 6.0. The data indicate that the shifts originate primarily from a pseudocontact mechanism with axial or near-axial symmetry. The shift ratios for given sets of protons were found to be consistent with this interpretation but suggest that the binding of the lighter Ln3+ ions differ slightly from that of the heavier members of the series. The best agreement between observed and predicted shift ratios for the lighter ions that could be obtained with a bidentate, axially symmetric model was one in which the principal symmetry axis lies in the plane of the carboxylate group and bisects the OCO bond angle. The proton T1 ratios for Gd3+ complexes of o-toluate and p-toluate are consistent with the appropriate distance relationships with one exception. The relaxation time for the ortho proton relative to the ortho methyl proton of o-toluate is shorter than expected and suggests that either the Gd3+ ion is preferentially binding to the carboxylate oxygen nearer the ortho proton or the internal rotation of the methyl group is affecting the correlation time for the ortho methyl proton. © 1978.