Richard C. Shiery’s research while affiliated with Pacific Northwest National Laboratory and other places

Due to their unique physiochemical properties that may be tailored for specific purposes, ionic liquids (ILs) have been investigated for various applications, including chemical separations, catalysis, energy storage, and space propulsion. The different cations and anions comprising ILs may be selected to optimize a range of desired properties, such as thermal stability, ionic conductivity, and volatility, leading to the designation of certain ILs as designer “green” solvents. The effect of counterions on the properties of ILs is of both fundamental scientific interest and technological importance. Herein, we report a systematic experimental and theoretical investigation of the size, charge, stability toward dissociation, and geometric/electronic structure of 1-ethyl-3-methyl imidazolium (EMIM)-based IL clusters containing two different atomic counterions (i.e., bromide [Br⁻] and iodide [I⁻]). This work extends our studies of EMIM⁺ cations with atomic chloride (Cl⁻) and molecular tetrafluoroborate (BF4⁻) anions reported previously by Baxter et al. [Chem. Mater. 34, 2612 (2022)] and Zhang et al. [J. Phys. Chem. Lett. 11, 6844 (2020)], respectively. Distributions of anionic IL clusters were generated in the gas phase using electrospray ionization and characterized by high mass resolution mass spectrometry, energy-resolved collision-induced dissociation, and negative ion photoelectron spectroscopy experiments. The experimental results reveal anion-dependent trends in the size distribution, relative abundance, ionic charge state, stability toward dissociation, and electron binding energies of the IL clusters. Complementary global optimization theory provides molecular-level insights into the bonding and electronic structure of a selected subset of clusters, including their low energy structures and electrostatic potential maps, and how these fundamental characteristics are influenced by anion substitution. Collectively, our findings demonstrate how the fundamental properties of ILs, which determine their suitability for many applications, may be tuned by substituting counterions. These observations are critical in the sub-nanometer cluster size regime where phenomena do not scale predictably to the bulk phase, and each atom counts toward determining behavior.

Rapid and accurate approaches to characterizing the coordination structure of an ion are important for designing ligands and quantifying structure-property trends. Here, we introduce AFICS (Analysis of the First Ion Coordination Sphere), a tool written in Python 3 for analyzing the structural and geometric features of the first coordination sphere of an ion over the course of molecular dynamics simulations. The principal feature of AFICS is its ability to quantify the distortion a coordination geometry undergoes compared to uniform polyhedra. This work applies the toolkit to analyze molecular dynamics simulations of the well-defined coordination structure of aqueous Cr3+ along with the more ambiguous structure of aqueous Eu3+ chelated to ethylenediaminetetraacetic acid. The tool is targeted for analyzing ions with fluxional or irregular coordination structures (e.g., solution structures of f-block elements) but is generalized such that it may be applied to other systems.

The aqueous hydration structure of the Bi3+ ion is probed using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) simulations of ion-water clusters and condensed-phase solutions. Anomalous features in the EXAFS spectra are found to be associated with a highly asymmetric first-solvent water shell. The aqueous chemistry and structure of the Bi3+ ion are dramatically controlled by the water stabilization of a lone-pair electronic state involving the mixed 6s and 6p orbitals. This leads to a distinct multimodal distribution of water molecules in the first shell that are separated by about 0.2 Å. The lone-pair structure is stabilized by a collective response of multiple waters that are localized near the lone-pair anti-bonding site. The findings indicate that the lone-pair stereochemistry of aqueous Bi3+ ions plays a major role in the binding of water and ligands in aqueous solutions.

In liquid, temperature affects the structures of lanthanide complexes in multiple ways that depend upon complex interactions between ligands, anions, and solvent molecules. The relative simplicity of lanthanide aqua ions (Ln3+) make them well suited to determine how temperature induces structural changes in lanthanide complexes. We performed a combination of ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure (EXAFS) measurements, both at 25 and 90 °C, to determine how temperature affects the first- and second-coordination spheres of three Ln3+ (Ce3+, Sm3+, and Lu3+) aqua ions. AIMD simulations show first lanthanide coordination spheres that are similar at 25 and 90 °C, more so for the Lu3+ ion that remains as eight-coordinate than for the Ce3+ and Sm3+ ions that change their preferred coordination number from nine (at 25 °C) to eight (at 90 °C). The measured EXAFS spectra are very similar at 25 and 90 °C, for the Ce3+, Sm3+, and Lu3+ ions, suggesting that the dynamical disorder of the Ln3+ ions in liquid water is sufficient such that temperature-induced changes do not clearly manifest changes in the structure of the three ions. Both AIMD simulations and EXAFS measurements show very similar structures of the first coordination sphere of the Lu3+ ion at 25 and 90 °C.