Hydration and ion pair formation in common aqueous La(iii) salt solutions-a Raman scattering and DFT study

Abstract

Raman spectra of aqueous lanthanum perchlorate, triflate (trifluorosulfonate), chloride and nitrate solutions were measured over a broad concentration (0.121-3.050 mol L-1) range at room temperature (23 °C). A very weak mode at 343 cm-1 with a full width at half height at 49 cm-1 in the isotropic spectrum suggests that the nona-aqua La(iii) ion is thermodynamically stable in dilute perchlorate solutions (∼0.2 mol L-1) while in concentrated perchlorate solutions outer-sphere ion pairs and contact ion pairs are formed. The La3+ nona-hydrate was also detected in a 1.2 mol L-1 La(CF3SO3)3(aq). In lanthanum chloride solutions chloro-complex formation was detected over the measured concentration range from 0.5-3.050 mol L-1. The chloro-complexes in LaCl3(aq) are fairly weak and disappear with dilution. At a concentration <0.1 mol L-1 almost all complexes disappeared. In LaCl3 solutions, with additional HCl, a series of chloro-complexes of the type [La(OH2)9-nCln]+3-n (n = 1-3) were formed. The La(NO3)3(aq) spectra were compared with a spectrum of a 0.409 mol L-1 NaNO3(aq) and it was concluded that in La(NO3)3(aq) over the concentration range from 0.121-1.844 mol L-1, nitrato-complexes, [La(OH2)9-n(NO3)n]+3-n (n = 1, 2) were formed. These nitrato-complexes are quite weak and disappear with dilution <0.01 mol L-1. DFT geometry optimizations and frequency calculations are reported for a lanthanum-nona-hydrate with a polarizable dielectric continuum in order to take the solvent into account. The bond distances and angles for the cluster geometry of [La(OH2)9]3+ with the polarizable dielectric continuum are in good agreement with data from recent structural experimental measurements and high quality simulations. The DFT frequency of the La-O stretching mode at 328.2 cm-1, is only slightly smaller than the experimental one. This journal is