Structure of ionic liquids with cationic silicon-substitutions

Significantly lower viscosities result when a single alkyl carbon is replaced by a silicon atom on the side chain of an ionic liquid cation. To further explore this effect, we compare liquid structure factors measured using high-energy X-ray scattering and calculated using molecular dynamics simulations. Four ionic liquids are studied that each has a common anion, bis(trifluoromethylsulfonyl)amide (NTf 2 -). The four cations for this series of NTf 2 - -anion ionic liquids are 1-methyl-3-trimethylsilylmethylimidazolium (Si-mim⁺), 1-methyl-3-neopentylimidazolium (C-mim⁺), 1-methyl-3-pentamethyldisiloxymethylimidazolium (SiOSi-mim⁺), and 1-methyl-1-trimethylsilylmethylpyrrolidinium (Si-pyrr⁺). To achieve quantitative agreement between the structure factors measured using high-energy X-ray scattering and molecular dynamics simulations, new transferable parameters for silicon were calibrated and added to the existing force fields.