Hydration number absorption spectroscopy

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

Keller, D.E., Koningsberger, D.C., and Weckhuysen, B.M., Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy Correlation between the V---V coordination number and distance and the point of zero charge of the support oxide, Phys. Chem. Chem. Phys., 8, 4814, 2006. 

Infrared and Raman spectroscopy have very wide applications and have proved an important source of information on ion-solvent interactions and hydration numbers. In general, the position of an absorption enables a guess to be made for the hydration numbers, h+ and h-. Since spectroscopic techniques deal with frequencies associated with coordinative bonding between the ion and the solvent, such techniques furnish information about the strength of the ion-solvent interactions in the primary solvation. 

We define the hydration number as the average number of water molecules in the first sphere about the metal ion. The residence time of these molecules is determined generally by the nature of the bonding to the metal ion. For the f-element cations, ion-dipole interactions result in fast exchange between the hydration layer and the bulk solvent. The techniques for studying the nature (number and/or structure) of the hydration shell can be classified as either direct or indirect methods. The direct methods include X-ray and neutron diffraction, luminescence and NMR (nuclear magnetic resonance) relaxation measurements. The indirect methods involve compressibility, NMR exchange and absorption spectroscopy measurements. 

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