Effect of hydration equilibria on the relaxometric properties of Gd(III) complexes: new insights into old systems

We present a detailed relaxometric and computational investigation of three Gd(III) complexes that exist in solution as an equilibrium of two species with a different number of coordinated water molecules: [Gd(H₂O)]³⁺ (q = 8, 9), [Gd(EDTA)(H₂O)]^- and [Gd(CDTA)(H₂O)]^- (q = 2, 3). ¹H nuclear magnetic relaxation dispersion (NMRD) data were recorded from aqueous solutions of these complexes using a wide Larmor frequency range (0.01-500 MHz). These data were complemented with ¹⁷O transverse relaxation rates and chemical shifts recorded at different temperatures. The simultaneous fit of the NMRD and ¹⁷O NMR data was guided by computational studies performed at the DFT and CASSCF/NEVPT2 levels, which provided information on Gd⋯H distances, ¹⁷O hyperfine coupling constants and the zero-field splitting (ZFS) energy, which affects electronic relaxation. The hydration equilibrium did not have a very important effect in the fits of the experimental data for [Gd(H₂O)]³⁺ and [Gd(CDTA)(H₂O)]^-, as the hydration equilibrium is largely shifted to the species with the lowest hydration number (q = 8 and 2, respectively). The quality of the analysis improves however considerably for [Gd(EDTA)(H₂O)]^- upon considering the effect of the hydration equilibrium. As a result, this study provides for the first time an analysis of the relaxation properties of this important model system, as well as accurate parameters for [Gd(H₂O)]³⁺ and [Gd(CDTA)(H₂O)]^-.