Oxygen exchange in uranyl hydroxide via two "nonclassical" ions
Michael Bühl1, Georg Schreckenbach
1School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, United Kingdom. buehl@st-andrews.ac.uk
Inorganic Chemistry
|March 26, 2010
Summary
This study refines the pathway for oxygen atom scrambling in uranyl hydroxide complexes using advanced molecular dynamics. Results show a deprotonation and solvent-assisted proton transfer mechanism with a calculated barrier.
Area of Science:
- Computational Chemistry
- Inorganic Chemistry
- Physical Chemistry
Background:
- The mechanism of oxygen atom scrambling in uranyl complexes is crucial for understanding their reactivity.
- Previous models proposed a pathway for axial and equatorial oxygen atom rearrangement in [UO(2)(OH)(4)](2-).
- Computational simulations are essential for elucidating complex reaction pathways in solution.
Purpose of the Study:
- To refine the proposed pathway for oxygen atom scrambling in the uranyl hydroxide complex [UO(2)(OH)(4)](2-).
- To investigate the role of deprotonation and solvent-assisted proton transfer in the scrambling mechanism.
- To accurately calculate the energy barriers associated with these transformations.
Main Methods:
- Car-Parrinello molecular dynamics (CPMD) simulations were employed using the BLYP functional.
- Simulations were performed in explicit water solvent with ammonium (NH(4)(+)) counterions.
- Thermodynamic integration and constrained CPMD were used to calculate energy differences and barriers.
Main Results:
- Deprotonation of [UO(2)(OH)(4)](2-) to [UO(3)(OH)(3)](3-) was found to have an energy cost (DeltaA) of 7.1 kcal/mol.
- A solvent-assisted proton transfer mechanism was identified, proceeding via a cis-[UO(2)(OH)(4)](2-).OH(-) complex.
- The overall activation barrier (DeltaA‡) for oxygen scrambling was calculated to be 12.5 kcal/mol at the BLYP level.
Conclusions:
- The refined pathway involves deprotonation followed by solvent-assisted proton transfer.
- Pure functionals like BLYP slightly underestimate the overall barrier.
- Higher-level calculations (B3LYP, CCSD(T)) yield barriers of approximately 16-17 kcal/mol, agreeing well with experimental data.
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