Spectroscopic experimental and theoretical study of Uranyl(VI) in an aqueous system - Molecular modelling meets environmental protection

TL;DR

This study combines experimental TRLFS spectroscopy with theoretical molecular modeling to accurately analyze uranyl(VI) speciation in aqueous environments, improving interpretation of luminescence spectra for environmental monitoring.

Contribution

It introduces a methodology to simulate TRLFS spectra using first-principles calculations, bridging experimental data with theoretical models for uranyl complexes in water.

Findings

Achieved minimal theory-experiment deviation in spectral peak positions.

Validated the simulation approach with experimental spectra of uranyl aquo complexes.

Provided insights into ligand effects on luminescence spectra.

Abstract

Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS) and cryo-TRLFS are sensitive tools for \textit{in situ} speciation of low-level uranium in aqueous samples, including natural waters. To tailor, often ill-conditioned (i.e., leading to ambiguous interpretations), multi-linear analysis, first-principles based theoretical computation (Molecular Dynamics and Quantum Chemistry) of luminescence spectra would be beneficial. We present a methodology to simulate TRLFS and cryo-TRLFS spectra and present vibrationally resolved luminescence spectra for aquo complex [UO$_2$(H$_2$O)$_5$]$^{2+}$(aq). Comparison to experimental data, interpretation of spectra in the terms of a minimal non-redundant set of spectroscopic parameters (peak spacing \omega, the 0' $\rightarrow$ 0 peak position T$_{00}$, envelope shape parameter \Delta R, average peak width \sigma and luminescence life-times) and shifts due to ligand coordination are discussed. The minimum theory-experiment deviation in T$_{00}$ (exp. $\sim$ 20 485 cm$^{-1}$) has been reported for SORECP/TD-DFT with LB$\alpha$ functional - 20 cm$^{-1}$, 90 cm$^{-1}$ and 100 cm$^{-1}$ for different models, a similar level of agreement has been met for \omega$_{gs}$. The RMS along configuration space sampling CMD trajectory for T$_{00}$ corresponds well to \sigma. Preliminary predictivity study for a small set of uranyl complex species and comparison of pseudo-potential (SORECP) and all-electron results for [UO$_2$(H$_2$O)$_5$]$^{2+}$(g) are appended.