Raman spectroscopic evidence for a discrete change in coordination number of rare earth aquo-ions in the middle of the series

The effects of the coexisting anions on the complex formation of rare earth ions with thiocyanate ions in aqueous solution

“…This indicates that the higher coordination is favored at low temperatures. This trend is in consistent with the previous Raman studies [13,20] in which the anomalous concentration dependence of the hydration number change was clearly observed.…”

“…The extended s-shaped variation is often observed in the series behavior of various complex formation constants for rare earth ions [12]. The frequency change by ∼10 cm −1 in the coordination number change is almost the half of the value for the hydration number change of aqua-rare earth ions [13]. It is a common phenomenon that the coordination number change is associated with the rapid change in various properties (complex formation constant, thermodynamic properties, etc).…”

The hydration properties of Gd(III) and Tb(III): An ab initio quantum mechanical molecular dynamics study

“…The calculations yielded wavenumbers of 340 (Gd 3+ ) and 336 cm −1 (Tb 3+ ), which corresponds to force constants of 99 and 97 N m −1 for the ion-ligand bonds. Raman shifts reported by Kanno et al for glassy aqueous solutions of rare earth chlorides [48,49] and chloride solutions in the glassy state [50] range from 390 to 397 cm −1 for gadolinium(III), and from 388 to 398 cm −1 for terbium(III) [48][49][50]. The deviations of experimentally and theoretically obtained wavenumbers of the Ln(III) water bond vibrations are reasonable, as systems studied in the glassy state at liquid-nitrogen temperature considerably differ from aqueous solutions at room temperature.…”

The thermodynamics of aqueous trivalent rare earth elements. Apparent molar heat capacities and volumes of Nd(ClO4)3(aq), Eu(ClO4)3(aq), Er(ClO4)3(aq), and Yb(ClO4)3(aq) from the temperatures 283 K to 328 K

“…The structure of hydrated lanthanide ions has been investigated using X-ray (30)(31)(32)(41)(42)(43)(44)(45) and neutron diffraction, (33)(34)(35) extended X-ray absorption spectroscopy (EXAFS), (46) Raman spectroscopy, (9,47,48) n.m.r., (49) molecular dynamics simulations, (50) and quantum mechanical calculations. (36) Generally, the results from these investigations suggest that lighter lanthanide ions, from La 3+ (aq) to Nd 3+ (aq) in the series, have a hydration number of nine; those from Tb 3+ (aq) to Lu 3+ (aq) have a hydration number of eight; and those in the middle of the series have a fractional hydration number reflecting an equilibrium between the hydration numbers eight and nine.…”

“…Although their chemical and physical properties are believed to be predominantly electrostatic in nature, and thus are determined by size differences, many structural and thermodynamic studies indicate that some properties of Ln 3+ (aq) do not vary smoothly along the series. (2)(3)(4)(5)(6)(7)(8)(9) This phenomenon has been interpreted as reflecting a change in the number of water molecules in the primary coordination sphere of these ions, the so-called ''gadolinium break''. (10,11) (11)(12)(13) The behaviour of the standard partial molar heat capacities C°p ,2 and volumes V°2 is dominated by configurational hydration effects (''structural'' effects) at temperatures near 298.15 K, and by long-range solvent polarization (''field'' effects) at high temperatures.…”