skip to main content

Title: Nature and strength of intrinsic cation–anion interactions of 1-alkyl-3-methylimidazolium hexafluorophosphate clusters
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 n mim:PF 6 ] + . The cation, [C n mim] + , is varied across the series, 1-ethyl-3-methylimidazolium [C 2 mim] + , 1-butyl-3-methylimidazolium [C 4 mim] + , 1-hexyl-3-methylimidazolium [C 6 mim] + , 1-octyl-3-methylimidazolium [C 8 mim] + , to examine the structural and energetic effects of the size of the 1-alkyl substituent of the cation on the binding to [PF 6 ] − . Complementary electronic structure methods are employed for the [C n mim] + cations, (C n mim:PF 6 ) ion pairs, and [2C n mim:PF 6 ] + 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 more » analyses and the 6-311+G(2d,2p) basis set for energetic determinations. The modest structural variation among the [C n mim] + cations produces only minor structural changes and variation in the measured BDEs of the [2C n mim:PF 6 ] + 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. « less
Authors:
;
Award ID(s):
1709789
Publication Date:
NSF-PAR ID:
10287007
Journal Name:
Physical Chemistry Chemical Physics
Volume:
23
Issue:
23
Page Range or eLocation-ID:
13405 to 13418
ISSN:
1463-9076
Sponsoring Org:
National Science Foundation
More Like this
  1. Ionic liquids (ILs) exhibit unique properties that have led to their development and widespread use for a variety of applications. Development efforts have generally focused on achieving desired macroscopic properties via tuning of the IL through variation of the cations and anions. Both the macroscopic and microscopic properties of an IL influence its tunability and thus feasibility of use for selected applications. Works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been limited to date. Specifically, the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation (TCID) approaches supported and enhanced by electronic structure calculations to determine the bond dissociation energies (BDEs) and characterize the nature of the cation-anion interactions in a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations with the hexafluorophosphate anion, [2C n mim:PF 6 ] + . To examine the effects of the 1-alkyl chain on the structure and energetics of binding, the cation was varied over the series: 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] + . The variationmore »in the strength of binding among these [2C n mim:PF 6 ] + clusters was found to be similar in magnitude to the average experimental uncertainty in the measurements. To definitively establish an absolute order of binding among these [2C n mim:PF 6 ] + clusters, we extend this work again using TCID and electronic structure theory approaches to include competitive binding studies of three mixed 2:1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [C n-2 mim:PF 6 :C n mim] + for n = 4, 6, and 8. The absolute BDEs of these mixed [C n-2 mim:PF 6 :C n mim] + clusters as well as the absolute difference in the strength of the intrinsic binding interactions as a function of the cation are determined with significantly improved precision. By combining the thermochemical results of the previous independent and present competitive measurements, the BDEs of the [2C n mim:PF 6 ] + clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2C n mim:BF 4 ] + and [C n-2 mim:PF 6 :C n mim] + clusters previously examined to elucidate the effects of the [PF 6 ] - and [BF 4 ] - anions on the binding.« less
  2. Ionic liquids (ILs) are gaining attention as protein stabilizers and refolding additives. However, varying degrees of success with this approach motivates the need to better understand fundamental IL-protein interactions. A combination of experiment and simulation is used to investigate the thermal unfolding of lysozyme in the presence of two imidazolium-based ILs (1-ethyl-3-methylimidazolium ethylsulfate, [EMIM][EtSO 4 ] and 1-ethyl-3-methylimidazolium diethylphosphate, [EMIM][Et 2 PO 4 ]). Both ILs reduce lysozyme melting temperature Tm , but more gradually than strong denaturants. [EMIM][Et 2 PO 4 ] lowers lysozyme Tm more readily than [EMIM][EtSO 4 ], as well as requiring less energy to unfold the protein, as determined by the calorimetric enthalpy ΔH. Intrinsic fluorescence measurements indicate that both ILs bind to tryptophan residues in a dynamic mode, and furthermore, molecular dynamics simulations show a high density of [EMIM] + near lysozyme’s Trp62 residue. For both ILs approximately half of the [EMIM] + cations near Trp62 show perfect alignment of their respective rings. The [EMIM] + cations, having a "local" effect in binding to tryptophan,likely perturb a critically important Arg-Trp-Arg bridge through favorable π − π and cation-π interactions. Simulations show that the anions, [EtSO 4 ] - and [Et 2 PO 4 ]more »- , interact in a "global" manner with lysozyme, due to this protein’s strong net positive charge. The anions also determine the local distribution of ions surrounding the protein. [Et 2 PO 4 ] - is found to have a closer first coordination shell around the protein and stronger Coulomb interactions with lysozyme than [EtSO 4 ] - , which could explain why the former anion is more destabilizing. Patching of ILs to the protein surface is also observed, suggesting there is no universal IL solvent for proteins, and highlighting the complexity of the IL-protein environment.« less
  3. The need for a deep understanding of CO2 interactions with other mols. is significant given the importance of supercrit. CO2 (s.c.-CO2) as a green solvent, and interest in design of novel materials for CO2 capture and storage. Soly. of fluorinated compds. in s.c.-CO2 is generally higher than their hydrocarbon analogs and fluorination of hydrocarbon compds. improves "CO2-philicity". Although dissoln. of a compd. in s.c.-CO2 (or any solvent) is a complex process, much can be learned by systematic examn. of solute-solvent interactions as a function of solute mol. properties in dimers and other small clusters. In the present study, Fourier-transform microwave spectroscopy and Symmetry Adapted Perturbation Theory (SAPT) calcns. have been used to examine intermol. interactions between CO2 and a series of fluorinated ethylene mols. with varying degree and position of fluorination (1-3 F atoms). While 1-fluoro-, 1,1-difluoro- and 1,1,2-trifluoroethylene...CO2 complexes are planar (with two isomers obsd. for 1-fluoroethylene), cis-1,2-difluoroethylene...CO2 is nonplanar. Although lowest energy structures predicted by MP2/6-311++G(2d,2p) calcns. are not always in agreement with obsd. configurations, SAPT calcns. provide binding energies consistent with the obsd. structures. We are now extending these studies to trimers contg. one fluorinated ethylene with two CO2 "solvent" mols. in order to gain further insightmore »into structural and energetic changes as a solvation shell is formed.« less
  4. Complexes of 18-crown-6 ether (18C6) with four protonated amino acids (AAs) are examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by the infrared free electron laser at the Centre Laser Infrarouge d’Orsay (CLIO). The AAs examined in this work include glycine (Gly) and the three basic AAs: histidine (His), lysine (Lys), and arginine (Arg). To identify the (AA)H + (18C6) conformations present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311+G(d,p) level of theory. Relative energies of various conformers and isomers are provided by single point energy calculations carried out at the B3LYP, B3P86, M06, and MP2(full) levels using the 6-311+G(2p,2d) basis set. The comparisons between the IRMPD and theoretical IR spectra indicate that 18C6 binds to Gly and His via the protonated backbone amino group, whereas protonated Lys prefers binding via the protonated side-chain amino group. Results for Arg are less definitive with strong evidence for binding to the protonated guanidino side chain (the calculated ground conformer at most levels of theory), but contributions from backbone binding to a zwitterionic structure are likely.
  5. In the title double proton-transfer salt, C 12 H 12 N 2 2+ ·2C 8 H 7 O 4 − , consisting of a 1:2 ratio of 4,4'-(ethene-1,2-diyl)dipyridinium cations ( trans bipyridinium ethylene) to 2-hydroxy-3-methoxybenzoate anions ( o -vanillate), the complete cation is generated by crystallographic inversion symmetry and it is linked to adjacent o -vanillate anions by N—H...O hydrogen bonds, forming trimolecular assemblies. The trimers are linked by C—H...O hydrogen bonds as well as aromatic π–π stacking interactions into a three-dimensional network. The anion features an intramolecular O—H...O hydrogen bond.