RESUMEN
Ionic liquid viscosity is one of the most important properties to consider for practical applications. Yet, the connection between local structure and viscosity remains an open question. This article explores the structural origin of differences in the viscosity and viscoelastic relaxation across several ionic liquids, including cations with alkyl, ether, and thioether tails, of the imidazolium and pyrrolidinium families coupled with the NTf2- anion. In all cases, for the systems studied here, we find that pyrrolidinium-based ions are "harder" than their imidazolium-based counterparts. We make a connection between the chemical concept of hardness vs softness and specific structural and structural dynamic quantities that can be derived from scattering experiments and simulations.
RESUMEN
Functionalization of the imidazolium (Im+) cationic component of ionic liquids (ILs) with ether chains affords the possibility of tuning their properties through manipulation of the resulting interion and intramolecular interactions. Herein, we quantify these interactions at the molecular level through analysis of the vibrational spectra displayed by size-selected and cryogenically cooled ions. These spectra are obtained using the "tagging" approach carried out with photofragmentation tandem mass spectrometry. In the isolated cations, we find that the oxygen atom on the ether chain binds exclusively to the acidic C(2)H position on the Im+ ring. Upon complexation with BF4- to form the ternary (ether-MIm+)2(BF4-) cation, however, the less acidic C(4,5)H groups also participate as contact points for the ionic assembly, in contrast to the behavior of the closely related (EMIm+)2(BF4-) system. These experimental results support the conclusions derived from earlier X-ray scattering and molecular dynamics results on bulk ILs regarding interactions with the ring CH groups and their implications on tuning the viscosities of this class of functionalized ILs.
RESUMEN
Ionic liquids (ILs) with relatively low viscosities and broad windows of electrochemical stability are often constructed by pairing asymmetric cations with bisfluorosulfonylimide (FSI-) or bistriflimide (NTf2 -) anions. In this work, we systematically studied the structures of ILs with these anions and related perfluorobis-sulfonylimide anions with asymmetry and/or longer chains: (fluorosulfonyl)(trifluoromethylsulfonyl)imide (BSI0,1 -), bis(pentafluoroethylsulfonyl)imide (BETI-), and (trifluoromethylsulfonyl) (nonafluorobutylsulfonyl)imide (BSI1,4 -) using high energy X-ray scattering and molecular dynamics simulation methods. 1-alkyl-3-methylimidazolium cations with shorter (ethyl, Im2,1 +) and longer (octyl, Im1,8 +) hydrocarbon chains were selected to examine how the sizes of nonpolar hydrocarbon and fluorous chains affect IL structures and properties. In comparison with these, we also computationally explored the structure of ionic liquids with anions having longer fluorinated tails.
RESUMEN
Recently, we have reported a systematic study of photoinduced electron-transfer reactions in ionic liquid solvents using neutral and anionic electron donors and a series of cyano-substituted anthracene acceptors [ Wu , B. ; Maroncelli , M. ; Castner , E. W. Jr Photoinduced Bimolecular Electron Transfer in Ionic Liquids . J. Am. Chem. Soc. 139 , 2017 , 14568 ]. Herein, we report complementary results for a cationic class of 1-alkyl-4-dimethylaminopyridinium electron donors. Reductive quenching of cyano-substituted anthracene fluorophores by these cationic quenchers is studied in solutions of acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Varying the length of the alkyl chain permits tuning of the quencher diffusivities in solution. The observed quenching kinetics are interpreted using a diffusion-reaction analysis. Together with results from the prior study, these results show that the intrinsic electron-transfer rate constant does not depend on the quencher charge in this family of reactions.
RESUMEN
Ionic liquids (ILs) incorporating cyclic phosphonium cations are a novel category of materials. We report here on the synthesis and characterization of four new cyclic phosphonium bis(trifluoromethylsulfonyl)amide ILs with aliphatic and aromatic pendant groups. In addition to the syntheses of these novel materials, we report on a comparison of their properties with their ammonium congeners. These exemplars are slightly less conductive and have slightly smaller self-diffusion coefficients than their cyclic ammonium congeners.
RESUMEN
X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis(trifluoromethylsulfonyl)amide (NTf2(-)) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr(1,n(+)) with n = 5 or 7, as systems with linear tails, 1-(2-ethylhexyl)-1-methylpyrrolidinium, Pyrr(1,EtHx(+)), as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr(1,ChxMe(+)), as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr(1,n(+))/NTf2(-) ionic liquids with n = 4, 6, 8, and 10.1,2 General methods for interpreting the structure function S(q) in terms of q-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail.