Hydration of Oxometallate Ions in Aqueous Solution.

Maciej Śmiechowski,Ingmar Persson

doi:10.1021/acs.inorgchem.0c00594

Abstract

The strength of hydrogen bonding to and structure of hydrated oxometallate ions in aqueous solution have been studied by double difference infrared (DDIR) spectroscopy and large-angle X-ray scattering (LAXS), respectively. Anions are hydrated by accepting hydrogen bonds from the hydrating water molecules. The oxygen atom of the permanganate and perrhenate ions form weaker and longer hydrogen bonds to water than the hydrogen bonds in bulk water (i.e., they act as structure breakers), while the oxygen atoms of the chromate, dichromate, molybdate, tungstate, and hydrogenvanadate ions form hydrogen bonds stronger than those in bulk water (i.e., they act as structure makers). The oxometallate ions form one hydration shell distinguishable from bulk water as determined by DDIR spectroscopy and LAXS. The hydration of oxoanions results in X–O bond distances ca. 0.02 Å longer than those in unsolvated ions in the solid state not involved in strong bonding to counterions. The oxygens of oxoanions with a central atom from the second and third series in the periodic table and the hydrogenvanadate ion hydrogen bind three hydrating water molecules, while oxygens of oxoanions with a heavier central atom only form hydrogen bonds to two water molecules.

Highlights

A limited number of methods are available to study the hydration of anions in aqueous solution due to weak hydration and broad distance distribution of the hydrating water molecules
The strength of the hydrogen bonds between an anion and surrounding water molecules is preferably studied by double difference infrared (DDIR) spectroscopy.[1]
The affected spectra reflect the state of the solute-affected water, and the N parameter is the number of perturbed HDO molecules that are spectrally differentiable using the DDIR analysis

Summary

Introduction

A limited number of methods are available to study the hydration of anions in aqueous solution due to weak hydration and broad distance distribution of the hydrating water molecules. The most commonly applied method to study structures in solution, EXAFS, is normally not applicable as the X−(O···H−)O distance is long and the distance distribution is wide, which causes the contribution to the EXAFS function to be very small and such distances to be hardly observable.[4] the hydrogen atom has too weak backscattering ability to be observed accurately. LAXS is a very suitable method as it is very sensitive to long distances with a wide bond distance distribution according to the LAXS equation.[2] Numerous LAXS studies on aqueous solutions have been reported, while only a very limited number of LANS studies on hydrated chloride, bromide, nitrate, and perchlorate ions have been reported.[5,6]

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Conclusion