Abstract
The totally symmetric stretching mode nu_{1} Ln–(OH2) of the first hydration shells of all the rare earth (RE) ions across the series from lanthanum to lutetium has been measured on dilute aqueous perchlorate solutions at room temperature. An S-shaped relationship has been found between the nu_{1} Ln–(OH2) peak positions and the Ln–(OH2) bond distances of the lanthanide(III) aqua ions. While the light rare earth ions form nona-hydrates, the heavy ones form octa-hydrates and the rare earth ions in the middle of the series show non integer hydration numbers between 9 and 8. A relationship between wavenumber positions nu_{1} Ln–(OH2) and the Ln–(OH2) bond distances of the RE hydrates has been given. Recent quantum mechanical calculations support the given interpretation.
Highlights
The trivalent lanthanides (Ln3+) are part of the longest series of chemically similar metal ions marked by progressive filling of the 4f orbitals going from La3+ to Lu3+
In addition to the 1 Ln–OH2 values, the Ln–O bond distances were included in Table 1 for all the rare earth (RE) ions. tIinonFsigasr.e1parensde2n,tetdh.eTrehperCesleOn−4ta(atiqv)emRoadmeasnhasvpeecbteraenofgdivielunteinErue(cCelnOt 4p)u3balnicdatGiodn(sC(lOfo4r)d3 estoaliulssee refs [28, 34].)
The isotropic band at 367 cm–1 is assigned to the Eu–OH2 symmetric stretching mode oFifga. 20 .4R0a3mmanols·cLa–t1teGridn(gCplOro4f)i3lessoRluptoilo(nu.pTpehre sdpeepcotrluamriz)e, dRdpeeporlch(mloirdadtelebsapnedcstrautm4)6a1ndcmR–i1so and 628 cm–1 are prominent bands in the lower frequency part of the spectrum, while the band of Gd–OH2 is broad and weak and strongly polarized (ρ = 0.004) and present in the isotropic scattering
Summary
The trivalent lanthanides (Ln3+) are part of the longest series of chemically similar metal ions marked by progressive filling of the 4f orbitals going from La3+ to Lu3+. A subsequent decrease of the ionic radii along the series is observed due to the increase of the nuclear electric field. The decrease of the ionic radii of the rare earth ions with increasing atomic number, known as lanthanide contraction [1, 2], was first coined by Goldschmidt [2]. A smooth plot of thermodynamic properties of lanthanide salts in aqueous solution would, be expected. This is especially true because the f-electrons do not take part in the bonding (negligible ligand field effect).
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