Hydration of Hg2+ in aqueous solution studied by neutron diffraction with isotopic substitution

Area of Science:

• Inorganic Chemistry
• Solution Chemistry
• Biophysical Chemistry

Background:

• Mercury(II) (Hg2+) is a toxic heavy metal ion with significant environmental and health impacts.
• Understanding the hydration structure of metal ions is crucial for predicting their behavior in aqueous solutions.
• Previous studies on Hg2+ hydration have yielded varying structural parameters, necessitating further investigation.

Purpose of the Study:

• To determine the precise structural parameters of the first hydration shell of Hg2+ in solution.
• To compare the hydration structure of Hg2+ with that of other divalent metal cations, such as Ca2+.
• To elucidate the relationship between Hg2+ hydration and its observed toxicity.

Main Methods:

• Neutron diffraction with isotopic substitution (199Hg for natHg) was employed to study 0.225 mol/L Hg2+ solutions in DNO3/D2O.
• Analysis of neutron scattering data allowed for the determination of interatomic distances and coordination numbers.
• Comparison with existing X-ray diffraction data was performed to highlight differences in structural determination.

Main Results:

• Hg2+ is coordinated by a first solvation shell of six water molecules.
• The determined Hg-O bond distance is 2.48 ± 0.05 Å, and the Hg-H distance is 3.08 ± 0.05 Å.
• The angle between the water molecule plane and the cation-water oxygen axis (phi) is approximately 35 degrees, indicating a specific water molecule orientation.

Conclusions:

• The hydration structure of Hg2+ closely resembles that of Ca2+, suggesting similar interactions with water molecules.
• The observed structural similarity may contribute to the understanding of Hg2+'s extreme toxicity.
• The difference between neutron and X-ray diffraction results for the Hg-O distance suggests a significant covalent contribution to the Hg-O bond.