Combining EXAFS spectroscopy and molecular dynamics simulations to understand the structural and dynamic properties of an imidazolium iodide ionic liquid

The structural properties of liquid 1-butyl-3-methylimidazolium iodide [C4mim]I have been investigated using an integrated approach that combines EXAFS spectroscopy and molecular dynamics (MD) simulations. A well defined first coordination shell composed on average of 4.5 I(-) ions around the imidazolium cation has been evidenced, and the structural arrangement of the I(-) ions has been found to be different in the proximity of the most acidic hydrogen atom of the imidazolium ring, as compared to the other two ring protons: in the former case the I(-) ion is not coplanar with the imidazolium ring plane, but it prefers to be above and below the plane itself, while in the latter the anion has the same probability of being or not being coplanar with the plane. A quantitative analysis of the I K-edge EXAFS spectrum of liquid [C4mim]I has been carried out starting from the structural information on the system derived from the MD simulation. This combined approach allows one to reduce the number of correlated model parameters required in the fitting of the experimental data and to increase the reliability of the EXAFS data analysis that represents a non-trivial task when dealing with disordered systems. Moreover, the good agreement between the EXAFS experimental and theoretical spectra of liquid [C4mim]I has proven the reliability of the MD results and force field employed.