Local Structure in Ionic Liquids Investigated by Hyper-Rayleigh Scattering

• Published in: The journal of physical chemistry. B Vol. 114; no. 46; pp. 15057 - 15065
• Main Authors: Rodriguez, V., Grondin, J., Adamietz, F., Danten, Y.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 25-11-2010
• Subjects: B: Statistical Mechanics, Thermodynamics, Medium Effects; Chemical Sciences; Environmental Sciences; Physics
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp107165k

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 (<1 nm) leading to a coherent HRS structure factor in ILs.