Gas-Phase Binding Energies and Dissociation Dynamics of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters

Abstract

Ionic liquids (ILs) have become increasingly popular due to their useful and unique properties, yet there are still many unanswered questions regarding their fundamental interactions. In particular, details regarding the nature and strength of the intrinsic cation–anion interactions and how they influence the macroscopic properties of ILs are still largely unknown. Elucidating the molecular-level details of these interactions is essential to the development of better models for describing ILs and enabling the purposeful design of ILs with properties tailored for specific applications. Current uses of ILs are widespread and diverse and include applications for energy storage, electrochemistry, designer/green solvents, separations, and space propulsion. To advance the understanding of the energetics, conformations, and dynamics of gas-phase IL clustering relevant to space propulsion, threshold collision-induced dissociation approaches are used to measure the bond dissociation energies (BDEs) of the 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate, [2C n mim:BF4]+. The cation, [C n mim]+, is varied across the series, 1-ethyl-3-methylimidazolium [C2mim]+, 1-butyl-3-methylimidazolium [C4mim]+, 1-hexyl-3-methylimidazolium [C6mim]+, and 1-octyl-3-methylimidazolium [C8mim]+, to examine the structural and energetic effects of the size of the 1-alkyl substituent on binding. Complementary electronic structure calculations are performed to determine the structures and energetics of the [C n mim]+ and [BF4]− ions and their binding preferences in the (C n mim:BF4) ion pairs and [2C n mim:BF4]+ clusters. Several levels of theory, B3LYP, B3LYP-GD3BJ, and M06-2X, using the 6-311+G­(d,p) basis set for geometry optimizations and frequency analyses and the 6-311+G­(2d,2p) basis set for energetics, are benchmarked to examine their abilities to properly describe the nature of the binding interactions and to reproduce the measured BDEs. The modest structural variation among these [C n mim]+ cations produces only minor structural changes and variation in the measured BDEs of the [2C n mim:BF4]+ clusters. Present findings indicate that the dominant cation–anion interactions involve the 3-methylimidazolium moieties and that these clusters are sufficiently small that differences in packing effects associated with the variable length of the 1-alkyl substituents are not yet significant.

Numerical Characterization of Fragmentation in Ionic Liquid Clusters

“…presented fragmentation analysis of EMI-BF4, BMI-BF4, HMI-BF4, and OMI-BF4 using the AMBER force field [47]. The fragmentation rates agreed with those collected from CID experiments.…”

“…Temperatures between 422 K and 431 K were examined. More recently, Roy et al used CID to measure dissociation rates of ionic liquid clusters such as EMI-BF 4 , BMI-BF 4 , HMI-BF 4 , and OMI-BF 4 [47]. These results agreed with quantum mechanical calculations.…”

“…In particular, the time dependent relative position of components of the clusters during the fragmentation process will be investigated to improve the analytical physical model developed in this work. This will included determining whether there is a temperature and electric field dependence of the orientation of the molecules in order to compare with the work done by Prince et al and Roy et al [56,47]. In addition to improving the analytical physical model of dimers presented here, this geometry characterization will be used to extrapolate to behaviors for larger clusters, which could reduce the need to perform computationally expensive MD simulations for each type of cluster emitted.…”

Absolute Trends and Accurate and Precise Gas-Phase Binding Energies of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters from Combined Independent and Competitive TCID Measurements

“…The resulting snapshot structures were sorted by energy, and ∼20–40 structures were sampled for follow-up density functional theory calculations. In addition, 20 of the lowest-energy structures previously found for the [2C n mim:BF 4 ] + clusters for n = 4, 6, and 8 were selected and used to generate additional candidate structures for the [C n –2 mim:BF 4 :C n mim] + clusters by shortening the 1-alkyl chain of one of the cations by an ethyl moiety. This procedure generated two new candidate structures for the mixed IL cluster from each [2C n mim:BF 4 ] + structure, thereby providing ∼40 additional unique structures that were subjected to density functional theory calculations.…”

“…A variety of ILs are being investigated as potential fuels for space propulsion using electrospray thrusters including 1-alkyl-3-methylimidazolium tetrafluoroborate ILs, C n mim-BF 4 , where n describes the number of carbon atoms in the 1-alkyl chain. , The C n mim-BF 4 ILs with ethyl, butyl, hexyl, and octyl 1-alkyl substituents have been studied in greatest detail as their properties are best matched to space propulsion needs. In previous work, we employed threshold collision-induced dissociation (TCID) approaches combined with electronic structure calculations to characterize the nature of the cation–anion interactions and determined the bond dissociation energies (BDEs) of a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [2C n mim:BF 4 ] + for n = 2, 4, 6, and 8 . The structures of the cations: 1-ethyl-3-methylimidazolium, [C 2 mim] + , 1-butyl-3-methylimidazolium, [C 4 mim] + , 1-hexyl-3-methylimidazolium, [C 6 mim] + , and 1-octyl-3-methylimidazolium, [C 8 mim] + , (including atom numbering) and the [BF 4 ] − anion are shown in Figure .…”