The formation of aqueous 111In complexes

Time differential PAC measurements have been made in carrier free solutions of 111InCl3 at values of pH between 0.8 and 7.4 and temperatures between 78 and 350 K. At pH 0.8, our results may be explained by the free rotational diffusion of a molecular unit of mean radius 0.26 ± 0.4 nm, thereby confirming the results of x-ray scattering and potentiometric experiments. The mean radius of the molecular complex is found to increase with pH, until at pH 7.4 the attenuation of the angular correlation becomes dominated by the contribution from a damped static interaction having a correlation time in excess of 24 ns. Such a long correlation time is inconsistent with the free rotational diffusion of a complex no larger than In4(OH)66+ as suggested by x-ray and potentiometric measurements. If the latter are correct, then our investigation points to the involvement either of hindered rotation of the above polynuclear complex or possibly to the effects of low frequency vibrations of its InO6 octahedra. No evidence can be found to suggest that electron capture decay after-effects are contributing to the attenuation of the angular correlation in these systems. (author)