Spectroscopic Evidence for Equilibrium between Eight- and Nine-Coordinate Eu³⁺(aq) Species in 0.1 M EuCl₃(aq)

Abstract

Crystalline europium bromate, [Eu(H 2 O) 9 ][(BrO 3 ) 3 ], europium ethyl sulfate, [Eu(H 2 O) 9 ][(C 2 H 5 SO 4 ) 3 ], and europium decavanate, [Eu(H 2 O) 8 ] 2 [(V 10 O 28 )]‚8H 2 O, are used as models for nine-and eight-coordinate Eu 3+ aquo species in solution. A comparison of the 5 D 1 r 7 F 0 laser excitation spectra and D 0 f 7 F 1,2 emission spectra of the model crystal systems with that of 0.1 M EuCl 3 (aq) clearly indicates a significant presence of the Eu(H 2 O) 8 3+ species in aqueous solution. Modeling the 0.1 M EuCl 3 (aq) spectra using weighted sums of the eight and nine-coordinate crystal spectra suggest that approximately two-thirds of Eu 3+ ions exist as Eu(H 2 O) 8 3+ in aqueous solution. The current study takes advantage of the symmetry of the Eu(H 2 O) n 3+ complexes and lends further support, from this perspective, that an equilibrium exists between Eu(H 2 O) 8 3+ and Eu(H 2 O) 9 3+ , with significant representation from both species.

Solution Chemistry of Europium(III) Aqua Ion at Micromolar Concentrations as Probed by Direct Excitation Luminescence Spectroscopy

“…The NaNO 3 sample showed an enhancement of all the detectable emission bands ðaqÞ to give a hydroxide complex which is substantially more luminescent than the complex at acidic pH. The observed pH dependence of the excitation spectra between pH 6 and 7 shows that the the Eu III H 2 O ligand deprotonates , which is in equilibrium with the nine coordinate Eu III complex [26]. The q number for Eu 3þ ðaqÞ at pH 5.0 and 6.5 (Table) is most consistent with nine coordinate Eu III complexes.…”

Complexation of Light Trivalent Lanthanides with N-(2-Hydroxyethyl)ethylenediamine-N,N′,N′-triacetic Acid in Aqueous Solutions: Thermodynamic Analysis and Coordination Modes

“…Measurements based on the luminescence lifetime, thermodynamic measurements, and X-ray diffraction investigations strongly support that hydration number of the Eu 3+ aquo ion in H 2 O is 8–9. ,,− So, the average number of H 2 O oscillators per free Eu 3+ in the solutions calculated to be 8.80 is reasonable.…”

“…39 It is known that the quenching of the luminescence of Eu 3+ in aqueous solution is due to the efficient energy transfer from the excited state ( 5 D 0 ) of the Eu 3+ ion to the O− H vibration of inner-sphere-coordinated water molecules. 40,41 There is a correlation between the inner-sphere hydration number and luminescence lifetime of the Eu 3+ ion. 42 It is evident that τ becomes longer as −log [H + ] of the Eu 3+ / HEDTA system was increased (Figure 2), suggesting that the inner-sphere hydration number (or the number of O−H oscillators) of the Eu 3+ ion in the more basic solution was reduced because of a change of the coordination sphere of the Eu 3+ ion by complexation with a HEDTA molecule.…”

Tb3+ and Eu3+ luminescence in imidazolium ionic liquids

“…Due to the low luminescence signal and interference by luminescence from the ionic liquid, attempts to precisely determine the observed 5 D 0 luminescence decay rate constant of EuBr 3 in BMIBr/water were unsuccessful. However, estimates based on luminescence decay measurements show that k obs > 10000 s −1 for EuBr 3 in BMIBr/water, which is larger than the decay constant measured for aqueous EuBr 3 (presumably europium octa-or nonahydrate [17]), k obs = 6700 s −1 . Luminescence decay measurements exciting into this 5 D 0 → 7 F 0 transition of EuCl 3 in BMIBr/water yield k obs = 10600 s −1 , which is consistent with the estimated decay constant for EuBr 3 in BMIBr/water.…”