Excess Electron Solvation in an Imidazolium-Based Room-Temperature Ionic Liquid Revealed by Ab Initio Molecular Dynamics Simulations

Abstract

We present the first approach to the excess electron solvation in a novel medium, room-temperature ionic liquid, using ab initio molecular dynamics simulation techniques in this work. Results indicate that an excess electron can be solvated in the [dmim](+)Cl(-) IL as long-lived delocalized states and two short-lifetime localized states, one a single-cation-residence parasitical type and the other a double-cation-based solvated type state. The presence of a low-lying pi*-LUMO as the site of excess electron residence in the cation moiety disables the C-H unit as a H-bond donor, while the aromaticity requirement of the rings and the effect of the counterion Cl(-)'s make the resulting ion pairs a weak stabilizer for an excess electron. Although no large solvent reorganization in IL was found at the picosecond scale, the IL fluctuations sufficiently modify the relative energy levels of the excess electron states to permit facile state-to-state conversion and adiabatic migration. The binding energy of the excess electron is only approximately 0.2 eV, further indicating that it is in a quasi-free state, with a large drift mobility, suggesting that ILs are unreactive and promising mediators for transport of excess electrons, in agreement with the experimental findings. The present study provides insight into the novel electron solvation character in a new class of promising media for physical and chemical processes, which are fundamental for understanding of electron migration mechanisms in IL-based applications.