Structure of a prototypic ionic liquid: ethyl-methylimidazolium bromide.

High-energy X-ray diffraction measurements have been carried out on 1-ethyl-3-methylimidazolium bromide and complemented with molecular-dynamics simulations. Because the structure of the corresponding crystal is known, both the liquid and the crystal phases are simulated numerically. The liquid structure factor is dominated by an intense peak at 1.7 A(-1), associated mainly with the packing of the anions around the large cations. Analysis of the real-space correlations of the liquid shows that the Br(-) ions are distributed more symmetrically around the cation ring and move closer to the ring atoms compared with the crystal. Although the distribution of the anions around the cation in the first coordination shell of the liquid exhibits clear analogies with the crystal, the cation-cation partial distribution function of the liquid shows a significant component with lower distances between ring centers, with some pairs coming as close as 3.5 A in either parallel or antiparallel configurations. Finally, the presence of topological short-range order and charge ordering in the liquid is clearly demonstrated.