Quantum chemical calculations showed to be an excellent method to predict the electrochemical window of ionic liquids with reduction-resistant anions. A good correlation between the LUMO energy and the electrochemical window is observed. Surprisingly simple but very fast semiempirical calculations are in full record with density functional theory calculations and are a very attractive tool in the design and optimization of ionic liquids for specific purposes. 1. Introduction In the last two decades ionic liquids [1–3] have received much interest for use as water-free electrolytes. They might combine the advantages of the conventional high-temperature molten salt electrolytes and aqueous electrolytes. Ionic liquids have wide electrochemical [4–6] and temperature [1, 2] windows, high ionic conductivities [6, 7], can dissolve most metal salts [1–3], and allow several metals conventionally obtained from high-temperature molten salts to be deposited at room temperature without corrosion problems [8–11]. Moreover, they might posses lower toxicity, flammability, and volatility compared to conventional electrolyte systems [12]. Applications include the use of ionic liquids as electrolytes in battery systems [13], solar cells [14], and electrochemical capacitors [15–17]. In principle, it is possible to tune the properties of ionic liquids [2]. However, task-specific design of ionic liquids is not straightforward. Reasons for that are that the synthesis of a large variety of ionic liquids is still cumbersome, and experimental measurements on the properties of ionic liquids are relatively scarce. Molecular modeling can be a useful tool to establish both qualitative and quantitative relations between the properties of ionic liquids and their structure [18]. In this work quantum chemical calculations are used to predict the electrochemical stability of ionic liquids with reduction-resistant anions. The electrochemical window [1] of such ionic liquids depends primarily on the resistance of the cation against reduction and the resistance of the anion against oxidation [12]. Previously, Koch et al. have correlated the electrochemical oxidation potentials of several anions with their respective highest occupied molecular orbital (HOMO) energies [19], and an excellent fit was obtained. In this study the electrochemical stability of a series of ionic liquids is correlated with the energy levels of the lowest unoccupied molecular orbital (LUMO) of the cations. 2. Experimental All calculations were carried out using the Spartan’10 molecular modeling suite of programs [20].
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