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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, [BF(4)], [PF(6)], [TFSI], [SCN], and [(CN)(2)N]) 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 (<1 nm) leading to a coherent HRS structure factor in ILs.