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 10-40oC have been determined by electron pulse radiolysis/transient absorption spectroscopy techniques. For the free ligand, an activation energy of Ea = 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) x 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)3and Lu(TODGA)3(NO3)3 complexes had faster rate constants of k = (5.30 ± 0.51) x 1010, (4.23 ± 0.18) x 1010, (2.44 ± 0.13) x 1010, (1.68 ± 0.03) x 1010, and (9.1 ± 0.7) x 109 M-1 s-1, respectively. The corresponding Arrhenius activation energies determined for three (La, Gd, Lu) lanthanide-TODGA complexes showed consistent values of Ea = 35 ± 2.2, 35.3 ± 2.0, 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 for the previously reported decreased degradation of TODGA complexes under radiolytic environments.