Reactions of hexa-aquo transition metal ions with the hydrated electron up to 300°C
journal contribution
posted on 2024-11-07, 20:10authored byK. Kanjana, B. Courtin, A. MacConnell, D. M. Bartels
Reactions of the hydrated electron with divalent aqueous transition-metal ions, Cd2+, Zn2+, Ni2+, Cu2+, Co2+, Fe2+, and Mn2+, were studied using a pulse radiolysis technique. The kinetics study was carried out at a constant pressure of 120 bar with temperatures up to 300 °C. The rate constants at room temperature agree with those reported in the literature. The reaction of Cd2+ is approximately diffusion-limited, but none of the first-row transition-metal ion reactions are diffusion-controlled at any temperature studied. The activation energies obtained from the Arrhenius plots are in the range 14.5–40.6 kJ/mol. Pre-exponential factors are quite large, between 1 × 1013 and 7 × 1015 M–1 s–1. There appears to be a large degree of entropy–enthalpy compensation in the activation of Zn2+, Ni2+, Co2+, and Cu2+, as the larger pre-exponential factors strongly correlate with higher activation energy. Saturation of the ionic strength effect suggests that these reactions could be long-range nonadiabatic electron "jumps", but Marcus theory is incompatible with direct formation of ground state (M+)aq ions. A self-consistent explanation is that electron transfer occurs to excited states derived from the metal 4s orbitals. The ionic strength effect in the Mn2+ and Fe2+ reactions suggests that these proceed by short-range adiabatic electron attachment involving breakdown of the water coordination shell.