Computational Tools for Calculating log β Values of Geochemically Relevant Uranium Organometallic Complexes

Area of Science:

• Environmental Chemistry
• Computational Chemistry
• Radiochemistry

Background:

• Uranium (U^VI) speciation in aqueous environments is significantly influenced by interactions with organic ligands.
• Understanding these interactions is vital for predicting uranium's environmental fate and transport.
• Natural organic molecules play a key role in complexing and controlling uranium's aqueous chemistry.

Purpose of the Study:

• To evaluate the utility of Density Functional Theory (DFT) as a practical method for predicting the stability of U^VI-organic ligand complexes.
• To establish a reliable protocol for benchmarking DFT settings, including solvation models and basis sets, for accurate U^VI complex stability calculations.
• To develop a relative stability series for U^VI complexes with natural organic ligands, particularly where experimental data is limited.

Main Methods:

• Benchmarking of various DFT functionals, basis sets (e.g., aug-cc-pVDZ for ligands, MDF60 ECP for U^VI), and effective core potentials.
• Evaluation of different solvation models, with a focus on the IEFPCM solvation method and mixed solvation approaches.
• Application of a fitting approach to experimental data using optimized DFT settings to determine stability constants (log β).

Main Results:

• The IEFPCM solvation method is recommended for U^VI complex stability calculations.
• A mixed solvation approach enhances the accuracy of calculated stability constants (log β), though values may still be more favorable than experimental data.
• The recommended DFT protocol (B3LYP functional, aug-cc-pVDZ/MDF60 basis set, IEFPCM solvation) allows fitting of 1:1 U^VI complexes with a root-mean-square deviation of 1.38 log β units.
• Separate fitting is required for 1:2 and 1:3 U^VI complexes with multiple carboxylate ligands.

Conclusions:

• DFT provides a practical and valuable tool for predicting the relative stability of U^VI-organic ligand complexes.
• The established DFT protocol offers a robust method for generating stability data, aiding in the study of uranium's environmental chemistry.
• Further refinement of fitting procedures may be necessary for higher-order U^VI complexes.