Evaluation of the Arrhenius behavior of n-dodecane radical cation (RH˙+) reactivity with lanthanide ion-complexed N,N,N′,N′-tetraoctyl diglycolamide (TODGA)

Abstract

Temperature-dependent rate constants for the reaction of the n-dodecane radical cation (RH˙⁺) with trivalent lanthanide ion-complexed N,N,N′,N′-tetraoctyl diglycolamide (TODGA) over the range 10–40 °C have been determined using electron pulse radiolysis/transient absorption spectroscopy techniques. For the free ligand, an activation energy of E_a = 20.4 ± 0.7 kJ mol⁻¹ and pre-exponential factor of ln(A) = 31.23 ± 0.27 were obtained, corresponding to a room-temperature rate constant of k = (9.94 ± 0.52) × 10⁹ M⁻¹ s⁻¹. The RH˙⁺ reactivity with La(TODGA)₃(NO₃)₃, Nd(TODGA)₃(NO₃)₃, Gd(TODGA)₃(NO₃)₃, Yb(TODGA)₃(NO₃)₃ and Lu(TODGA)₃(NO₃)₃ complexes had rate constants of k = (5.30 ± 0.51) × 10¹⁰, (4.23 ± 0.18) × 10¹⁰, (2.44 ± 0.13) × 10¹⁰, (1.68 ± 0.03) × 10¹⁰, and (9.1 ± 0.7) × 10⁹ M⁻¹ s⁻¹, respectively. The corresponding Arrhenius activation energies determined for three (La, Gd, Lu) lanthanide-TODGA complexes showed consistent values of E_a = 35 ± 2.2, 35.3 ± 2.0, and 33.5 ± 3.9 kJ mol⁻¹, respectively. The similar and relatively large barrier energy suggests a common reaction mechanism involving electron abstraction from one of the coordinating nitrate anions, which is consistent with previously reported decreased degradation of TODGA complexes under radiolytic environments.