Nature and strength of intrinsic cation–anion interactions of 1-alkyl-3-methylimidazolium hexafluorophosphate clusters

Abstract

Imidazolium-based cations and the hexafluorophosphate anion are among the most commonly used ionic liquids (ILs). Yet, the nature and strength of the intrinsic cation–anion interactions, and how they influence the macroscopic properties of these ILs are still not well understood. Threshold collision-induced dissociation is utilized to determine the bond dissociation energies (BDEs) of the 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [2C_nmim:PF₆]⁺. The cation, [C_nmim]⁺, is varied across the series, 1-ethyl-3-methylimidazolium [C₂mim]⁺, 1-butyl-3-methylimidazolium [C₄mim]⁺, 1-hexyl-3-methylimidazolium [C₆mim]⁺, 1-octyl-3-methylimidazolium [C₈mim]⁺, to examine the structural and energetic effects of the size of the 1-alkyl substituent of the cation on the binding to [PF₆]⁻. Complementary electronic structure methods are employed for the [C_nmim]⁺ cations, (C_nmim:PF₆) ion pairs, and [2C_nmim:PF₆]⁺ clusters to elucidate details of the cation–anion interactions and their impact on structure and energetics. Multiple levels of theory are benchmarked with the measured BDEs including B3LYP, B3LYP-GD3BJ, and M06-2X each with the 6-311+G(d,p) basis set for geometry optimizations and frequency analyses and the 6-311+G(2d,2p) basis set for energetic determinations. The modest structural variation among the [C_nmim]⁺ cations produces only minor structural changes and variation in the measured BDEs of the [2C_nmim:PF₆]⁺ clusters. Present results are compared to those previously reported for the analogous 1-alkyl-3-methylimidazolium tetrafluoroborate IL clusters to compare the effects of these anions on the nature and strength of the intrinsic binding interactions.