Effects of Lanthanoid Cations on the First Electronic Transition of Liquid Water Studied Using Attenuated Total Reflection Far-Ultraviolet Spectroscopy: Ligand Field Splitting of Lanthanoid Hydrates in Aqueous Solutions

The effects of the lanthanoid cations (Ln3+) on the first electronic transition (à ← X̃) of liquid water were studied from the attenuated total reflection far-ultraviolet (ATR-FUV) spectra of trivalent Ln3+ electrolyte solutions (1 M), except Pm3+. The à ← X̃ transition energies of the Ln3+ electrolyte solutions show a distinct tetrad in their dependence on the number of 4f electrons of the Ln3+ cations. For the half occupation period of the 4f electrons, the à ← X̃ transition energies decrease from La3+ (4f0, 8.0375 eV) to Nd3+ (4f3, 8.0277 eV) and increase from Sm3+ (4f5, 8.0279 eV) to Gd3+ (4f7, 8.0374 eV). For the complete occupation period, there are two local minima at Dy3+ (4f9, 8.0349 eV) and Yb3+ (4f13, 8.0355 eV). The à ← X̃ transition energies of the tetrad nodes (La3+, Gd3+, Ho3+ (4f10), and Lu3+ (4f14)) increase slightly, as the nuclear charge increases in accordance with the hydration energies of the Ln3+ cations. The energy difference (ΔE) between the à ← X̃ transition energies and the line between La3+ and Lu3+ is largest at Nd3+ (80.5 cm–1) for the half occupation period and at Dy3+ (26.1 cm–1) and Yb3+ (24.5 cm–1) for the complete occupation period. The order of magnitude of ΔE is comparable to the ligand field splitting (LFS) of the ground state multiplets of Ln3+ complexes. The observed tetrad trend of the à ← X̃ transition energies of the Ln3+ electrolyte solutions across the 4f period reflects the hydration energies of the Ln3+ cations and the LFS induced by water ligands.