Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids

Chlorine gas and anion radical reactivity in molten salts and the link to chlorobasicity

Next-generation nuclear power plants may include exciting novel designs in which molten salts are the coolant or a combination of the coolant and fuel. Whereas it is straightforward to see why having a low volatility coolant can be advantageous for safety, much is not understood about the production of volatile halogen gases as a result of radiation and even less is known about the distribution of these species at and away from interfaces. Using first principles molecular dynamics simulations, we investigate the product of the disproportionation reaction between chlorine anion radicals (nominally Cl2˙-) in the bulk and slab configurations. We find that the product depends on the chlorobasicity of the medium. For example, in ZnCl2, Cl2 forms, but in a eutectic mixture of LiCl and KCl, Cl3- is formed as a product. We also find that Cl3- prefers to form at the vapor interface and this may have implications for corrosion and reactivity. Furthermore, the mechanisms of the mobility of Cl2 and Cl3- are radically different, the first one being vehicular and the second Grotthus-like. Chlorobasicity is linked to the electronic structure of the host melt; ZnCl2 forms extended networks along which metal ions and anionic counterions have significant electronic orbital overlap forming long, linear, molecular-like constructs; the opposite is true for the alkali metal eutectic salt.

Dynamics of an Excess Electron in Molten LiF, KF, MgF2, and BeF2

Ab initio molecular dynamics simulations suggest that the dynamics of an excess electron in different types of molten salts are not always the same. In molten LiF, KF, and MgF2, the excess electron localizes in the cavity as a solvated electron for 10 ps, which agrees with the widely accepted theory of Pikaev. In molten BeF2, the excess electron shows a different localization pattern: it mostly exists in localized states but also occurs in many delocalized states. This "localize-delocalize" pattern originates from the high viscosity of BeF2 (16 000 cP at 900 °C), which will lead to slow ionic motion and finally result in slow solvent relaxation. Besides, the species formed by the localization of the excess electron in these four melts are also different. The spectral feature (broad peak in the vis-IR region) of the localized electron in molten alkaline halides was also observed in LiF, KF, MgF2, and BeF2. Both an excess electron and electrons in the bulk liquid could contribute to the spectra, but the excitation of the excess electron makes a bigger contribution to the broad vis-IR peak. Our predicted spectrum of molten LiF/KF qualitatively reproduces the major feature of the experimental spectrum, which partially validates our simulations.

Are High-Temperature Molten Salts Reactive with Excess Electrons? Case of ZnCl2

New and exciting frontiers for the generation of safe and renewable energy have brought attention to molten inorganic salts of fluorides and chlorides. This is because high-temperature molten salts can act both as coolants and liquid fuel in next-generation nuclear reactors. Whereas research from a few decades ago suggests that salts are mostly unreactive to radiation, recent experiments hint at the fact that electrons generated in such extreme environments can react with the melt and form new species including nanoparticles. Our study probes the fate of an excess electron in molten ZnCl2 using first-principles molecular dynamics calculations. We find that on the time scale accessible to our study, an excess electron can be found in one of three states; the lowest-energy state can be characterized as a covalent Zn2Cl5•2- radical ion, the other two states are a solvated Zn•+ species (ZnCl3•2-) and a more delocalized species that still has some ZnCl3•2- character. Since for each of these, the singly occupied molecular orbital (SOMO) where the excess charge resides has a distinct and well-separated energy, the different species can in principle be characterized by their own electronic spectra. The study also sheds light onto what is commonly understood as the spectrum of a transient radical species which can be from the SOMO onto higher energy states or from the melt to pair with the excess electron leaving a hole in the liquid.

Understanding the Photoexcitation of Room Temperature Ionic Liquids

Photoexcitation of (neat) room temperature ionic liquids (RTILs) leads to the observation of transient species that are reminiscent of the composition of the RTILs themselves. In this minireview, we summarize state-of-the-art in the understanding of the underlying elementary processes. By varying the anion or cation, one aim is to generally predict radiation-induced chemistry and physics of RTILs. One major task is to address the fate of excess electrons (and holes) after photoexcitation, which implies an overview of various formation mechanisms considering structural and dynamical aspects. Therefore, transient studies on time scales from femtoseconds to microseconds can greatly help to elucidate the most relevant steps after photoexcitation. Sometimes, radiation may eventually result in destruction of the RTILs making photostability another important issue to be discussed. Finally, characteristic heterogeneities can be associated with specific physicochemical properties. Influencing these properties by adding conventional solvents, like water, can open a wide field of application, which is briefly summarized.

Photodetachment and Electron Dynamics in 1-Butyl-1-methyl-pyrrolidinium Dicyanamide

The ultrafast transient absorption spectrum of 1-butyl-1-methyl-pyrrolidinium dicyanamide, [Pyr_1,4⁺][DCA^-], was measured in the visible and near-infrared (IR) spectral regions. Excitation of the liquid at 4.6 eV created initially delocalized and highly reactive electrons that either geminately recombined (69%) or localized onto a cavity with a time constant of ∼300 fs. Electron localization was reflected in the evolution of the TA spectrum and the time-dependent loss of reactivity with a dichloromethane quencher. The delocalized initial state and spectrum of the free electrons were consistent with computational predictions by Xu and Margulis [ J. Phys. Chem. B, 2015, 119, 532-542] on excess electrons in [Pyr_1,4⁺][DCA^-]. The computational study considered two possible localization mechanisms for excess electrons, localization on ions, and localization on cavities. In the case of photogenerated electron-hole pairs, the results presented here demonstrate localization to cavities as the dominant channel. Following localization onto a cavity, the free electrons underwent solvation and loss of reactivity with the quencher with rates that slowed in time. The dynamics were similar to an analogous prior study on the related liquid [Pyr_1,x⁺][NTf₂^-]. One significant difference was the larger yield of free electrons from photoexcitation of [Pyr_1,4⁺][DCA^-]. This was found to primarily reflect more efficient localization onto cavities rather than a slower geminate recombination rate.

Theoretical Probing of Weak Anion-Cation Interactions in Certain Pyridinium-Based Ionic Liquid Ion Pairs and the Application of Molecular Electrostatic Potential in Their Ionic Crystal Density Determination: A Comparative Study Using Density Functional Approach

A comprehensive study on the structure, nature of interaction, and properties of six ionic pairs of 1-butylpyridinium and 1-butyl-4-methylpyridinium cations in combination with tetrafluoroborate (BF₄^-), chloride (Cl^-), and bromide (Br^-) anions have been carried out using density functional theory (DFT). The anion-cation interaction energy (ΔE_int), thermochemistry values, theoretical band gap, molecular orbital energy order, DFT-based chemical activity descriptors [chemical potential (μ), chemical hardness (η), and electrophilicity index (ω)], and distribution of density of states (DOS) of these ion pairs were investigated. The ascendancy of the -CH₃ substituent at the fourth position of the 1-butylpyridinium cation ring on the values of ΔE_int, theoretical band gap and chemical activity descriptors was evaluated. The ΔE_int values were negative for all six ion pairs and were highest for Cl^- containing ion pairs. The theoretical band gap value after -CH₃ substitution increased from 3.78 to 3.96 eV (for Cl^-) and from 2.74 to 2.88 eV (for Br^-) and decreased from 4.9 to 4.89 eV (for BF₄^-). Ion pairs of BF₄^- were more susceptible to charge transfer processes as inferred from their significantly high η values and comparatively small difference in ω value after -CH₃ substitution. The change in η and μ values due to the -CH₃ substituent is negligibly small in all cases except for the ion pairs of Cl^-. Critical-point (CP) analyses were carried out to investigate the AIM topological parameters at the interionic bond critical points (BCPs). The RDG isosurface analysis indicated that the anion-cation interaction was dominated by strong H_cat···X_ani and C_cat···X_ani interactions in ion pairs of Cl^- and Br^- whereas a weak van der Waal's effect dominated in ion pairs of BF₄^-. The molecular electrostatic potential (MESP)-based parameter ΔΔV_min measuring the anion-cation interaction strength showed a good linear correlation with ΔE_int for all 1-butylpyridinium ion pairs (R² = 0.9918). The ionic crystal density values calculated by using DFT-based MESP showed only slight variations from experimentally reported values.