Kinetic study of dissociation of Eu(III) complex with H8dotp (H8dotp = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylphosphonic acid))

The dissociation kinetics of the europium(III) complex with H8dotp ligand was studied by means of molecular absorption spectroscopy in UV region at ionic strength 3.0 mol dm−3 (Na,H)ClO4 and in temperature region 25–60 °C. Time-resolved laser-induced fluorescence spectroscopy (TRLIFS) was employed in order to determine the number of water molecules in the first coordination sphere of the europium(III) reaction intermediates and the final products. This technique was also utilized to deduce the composition of reaction intermediates in course of dissociation reaction simultaneously with calculation of rate constants and it demonstrates the elucidation of intimate reaction mechanism. The thermodynamic parameters for the formation of kinetic intermediate (ΔH0 = 11 ± 3 kJ mol−1, ΔS0 = 41 ± 11 J K−1 mol−1) and the activation parameters (Ea = 69 ± 8 kJ mol−1, ΔH≠ = 67 ± 8 kJ mol−1, ΔS≠ = −83 ± 24 J K−1 mol−1) for the rate-determining step describing the complex dissociation were determined. The mechanism of proton-assisted reaction was proposed on the basis of the experimental data.