Optical and Thermodynamic Basicities: UV Spectra of TI+, Pb2+, and Bi3+ in Molten Chloroaluminate Titrations

Following a recent report on the strong environmental dependence of the ³P₁ ← ¹S₀ transition of Pb²⁺ doped, in trace quantities, into thermodynamically characterized binary chloroaluminate molten-salt systems, we examined two other isoelectronic ions, Tl⁺ and Bi³⁺, in order to gain insight into the factors determining a spectral probe's sensitivity to chemical processes and into the limitations of the use of d¹⁰s² ions as basicity probes. Major differences in the probe ion spectral responses ranging from full-range frequency shifts (Bi³⁺) to no shift at all (Tl⁺) were observed as their host solvent was titrated through a composition region where large thermodynamic changes are known to occur (equivalence point at the A1C1₄^- stoichiometry). These differences are discussed in terms of differences in the polarizing power of the probe ions and the ions composing the solvent medium, and in terms of probe ion coordination symmetries and stoichiometrics.