Local Structure in Ionic Liquids Investigated by Hyper-Rayleigh Scattering

A series of ionic liquids involving imidazolium, pyrrolidinium, and alkyl ammonium cations with different anions (namely, [BF4], [PF6], [TFSI], [SCN], and [(CN)2N]) have been studied by hyper-Rayleigh scattering (HRS), which is a powerful technique to probe the local structure of liquids in a multipolar description. The interpretation of the HRS measurements in terms of an elementary structural (ES) entity has revealed a dominating octopolar nature of ES scatterers. By combining the HRS analysis with density functional theory calculations for different-sized ion pair clusters, we show that the octopolar nature to ES hyperpolarizability in ionic liquids (ILs) originates from a complex local structure due to the formation of ‘transient’ ion clusters within the time of observation of HRS (∼10−12 to 10−14 s). We emphasize that such a structural organization puts clearly into evidence the influence of nonadditive interaction processes within first shell of neighbors (