NMR study of intramolecular chemical exchange in the La(III) complex of triethylenetetraaminehexaacetic acid in aqueous solution

Abstract

An intramolecular ligand rearrangement taking place via a bond-breaking process in the lanthanum(III) complex of triethylenetetraaminehexaacetic acid was probed by dynamic NMR experiments. Evidence that the ligands are exchanging derives from the observation of temperature- and mixing time (τm)-dependent cross peaks between the diastereotopic 1 and 2 protons on each side of the complex in the 2D EXSY experiment. The exchange rate coefficients, k, were determined to be 7.1 × 10−2, 29.1 × 10−2, 70.0 × 10−2 and 140.4 × 10−2 s−1 at 295, 310, 320 and 330 K, respectively, corresponding to the activation parameters Ea = 69.6 kJ, ΔH≠ = 66.9 kJ mol−1 and ΔS≠ = −39.6 J mol−1. Copyright © 2000 John Wiley & Sons, Ltd.