title: Water Exchange on [Ln(DO3A)(H2O)2] and [Ln(DTTA-Me)(H2O)2](-) Studied by Variable Temperature, Pressure, and Magnetic Field NMR.
abstract: Water exchange kinetics of [Ln(L)(H2O)2](x) complexes (Ln = Pr, Nd, Dy, Tm, and Yb; L = DO3A and DTTA-Me) were studied by (17)O NMR spectroscopy as a function of temperature, pressure, and frequency and by (1)H nuclear magnetic relaxation dispersion. Water exchange rate constants of both complexes show a maximum at dysprosium. Water exchange on negatively charged complexes of the acyclic DTTA-Me ligand is much faster than on the neutral complexes of the macrocyclic DO3A. Small activation volumes |ΔV(⧧)| < 1 cm(3) mol(-1) measured for water exchange on [Ln(DO3A)(H2O)2] indicate an interchange type of mechanism (I) for the lanthanide complexes studied. In the case of [Ln(DTTA-Me)(H2O)2](-), a change in mechanism is detected from a dissociative mechanism (D, ΔV(⧧) = 7 cm(3) mol(-1)) for complexes with larger ions (Pr to Gd) to an interchange mechanism (Id, I; ΔV(⧧) = +1.8 and +0.4 cm(3) mol(-1)) for complexes with smaller ions (Dy and Tm).
authors.affiliation: Laboratoire de Chimie Inorganique et Bioinorganique, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH , CH-1015 Lausanne, Switzerland.
authors.forename: Shima
authors.initials: S
authors.lastname: Karimi
authors.affiliation: Laboratoire de Chimie Inorganique et Bioinorganique, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH , CH-1015 Lausanne, Switzerland.
authors.forename: Lothar
authors.initials: L
authors.lastname: Helm