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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Computational Tools for Calculating log# Values of Geochemically Relevant Uranium Organometallic Complexes Matthew E. Kirby, Alexandra Simperler, Samuel C. Krevor, Dominik Jakob Weiss, and Jason L. L. Sonnenberg J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06863 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018
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The Journal of Physical Chemistry
Computational Tools for Calculating logβ Values of Geochemically Relevant Uranium Organometallic Complexes Matthew E. Kirby1, Alexandra Simperler2, Samuel Krevor1, Dominik J. Weiss1,3*, Jason L. Sonnenberg4* 1
Earth Science and Engineering, Imperial College London, United Kingdom 2
3
Chemistry Department, Imperial College London, United Kingdom
School of Earth, Energy & Environmental Sciences, Stanford University, United States of America 4
Gaussian Inc., Wallingford, CT, United States of America
Corresponding authors email address:
[email protected],
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ABSTRACT
Uranium (UVI) interacts with organic ligands controlling subsequently its aqueous chemistry. It is therefore imperative to assess the binding ability of natural organic molecules. In this paper, we evidence that density functional theory (DFT) can be used as a practical protocol for predicting the stability of UVI organic ligand complexes, allowing for the development of relative stability series for organic complexes with limited experimental data. Solvation methods and DFT settings were benchmarked to suggest a suitable off-the-shelf solution. The results indicate the IEFPCM solvation method should be employed. A mixed solvation approach improves the accuracy of the calculated stability constant (logβ), however, the calculated logβ are approximately five times more favorable than experimental data. Different basis sets, functionals and effective core potentials were tested to check there are no major changes in molecular geometries and ∆rG. The recommended method employed is the B3LYP functional, aug-ccpVDZ basis set for ligands, MDF60 ECP and basis set for UVI, and the IEFPCM solvation model. Using the fitting approach employed in the literature with these updated DFT settings allows fitting of 1:1 UVI complexes with root mean square deviation of 1.38 logβ units. Fitting multiple bound carboxylate ligands indicate a second, separate fitting for 1:2 and 1:3 complexes.
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INTRODUCTION Utilization of nuclear energy has resulted in widespread contamination of the environment with uranium. It generated uranium containing waste which, depending on national policy, will likely be disposed of in a near surface or deep geological repository. Uranium (UVI) can be mobilized from these sites by groundwater as part of an alkaline (pH > 9) or acidic plume (pH 3.5 to 6).1-4 Understanding how the chemical composition of groundwater can influence aqueous UVI chemistry is key to constrain the subsequent mobility of UVI. In particular, it is important to characterize the effect of naturally occurring organic molecules as they are dominant in these environments. One class of naturally occurring organic molecules are siderophores. These are released by plants, bacteria and fungi primarily to solubilize FeIII.5, 6 However, siderophores are known to complex with uranium7-9 leading to uranium leaching
10-12
and desorption from mineral
surfaces.13, 14 Determining the stability of possible UVI siderophore complexes is not practical as there are 500 known siderophores. Fortunately, there are very few functional groups in siderophores. These are the catecholate (1), hydroxamate (2), α-hydroxycarboxylate (3), αaminocarboxylate (4), hydroxyphenyloxazolone (5) and, α-hydroxyimidazole (6) functional groups as seen in Figure 1.5 Characterizing the stability of each functional group across the pH range of interest (pH 3.5-6) will allow us to begin to understand which class of siderophores form the most stable complexes with UVI. The relative stability of UVI-siderophore functional group complexes can be compared by calculating the stability constant, logβ, of the reaction. The experimental determination of stability constants often require multiple experimental techniques and is time consuming and labor intensive. It can also be difficult to use certain experimental techniques such as
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potentiometric titrations in high pH and high ionic strength solutions due to the sodium error when using glass electrodes.15 Density functional theory (DFT) provides a cost-effective and radiation free way to estimate the logβ of UVI complexes as it computes the Gibbs energy (G) directly for each species in metal-ligand complexation reactions, allowing for the subsequent calculation of log β values. The logβ values for UVI ligand reactions were calculated before using DFT. The absolute values, however, were off by 10 logβ units (>60 kJ/mol)16,
17
or >30 log β units.18 Although
absolute values differed from experiment, the relative values estimated chemical trends correctly. The accuracy and computational cost of logβ calculations in aqueous conditions depends on four different model chemistry aspects in the DFT protocol suggested here. These are the solvation method employed, the effective core potential (ECP), basis set and functional. This is represented by equation 1. = ( ℎ, , , )
(1)
The functional employed, ECP (as long as it is a small core ECP), and the basis set used do not significantly affect the solvation energies for closed-shell, f-elements (e.g. UVI) in solution.19-21 However, previous work suggested that the errors in logβ are sensitive to the solvation method employed.19, 20, 22 Computational methods generally model water molecules that are not directly coordinated with UVI implicitly to save on computational cost. Calculated stability constants, however,
are
improved using a mixed implicit-explicit solvation method as it is more successful at incorporating solute-solvent cavity and dispersion terms.23 Nevertheless this approach has to date only been explored for uranyl in detail for water exchange reactions.19, 22, 24 A mixed approach was employed for 1:1 UVI acetate, oxalate and catecholate complexes.25
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To counter the absolute errors in logβ, a fitting approach is frequently employed. In this case the computed logβ of different complexes are plotted against experimental data, and the fitting curve is used to calculate fitted logβ values. This was successfully applied for a variety of metal centres with ligands including siderophores.26-28 Recently this approach was also applied to 1:1 UVI complexes for oxygen donor ligands allowing for prediction of logβ value to within
