The low frequency modes of solvated ions and ion pairs in aqueous electrolyte solutions: iron(ii) and iron(iii) chloride.

We have investigated the hydration dynamics of solvated iron(ii) and iron(iii) chloride. For this, THz/FIR absorption spectra of acidified aqueous FeCl2 and FeCl3 solutions have been measured in a frequency range of 30-350 cm(-1) (≈1-10 THz). We observe a nonlinear concentration dependence of the absorption, which is attributed to the progressive formation of chloro-complexes of Fe(ii) and Fe(iii), respectively. By principal component analysis of the concentration dependent absorption spectra, we deduced the molar extinction spectra of the solvated species Fe(2+) + 2Cl(-) and FeCl(+) + Cl(-), as well as FeCl(2+) + 2Cl(-) and FeCl2(+) + Cl(-). In addition, we obtain ion association constants log KFeCl2 = -0.88(5) and log KFeCl3 = -0.32(16) for the association of Fe(2+) and Cl(-) to FeCl(+) and the association of FeCl(2+) and Cl(-) to FeCl2(+), respectively. We performed a simultaneous fit of all the effective extinction spectra and their differences, including our previous results of solvated manganese(ii) and nickel(ii) chlorides and bromides. Thereby we were able to assign absorption peaks to vibrational modes of ion-water complexes. Furthermore, we were able to estimate a minimum number of affected water molecules, ranging from ca. 7 in the case of FeCl(+) + Cl(-) to ca. 21 in the case of FeCl(2+) + Cl(-).