The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro‐ methylsulfonyl)imide Ionic Liquids

Abstract

Extensive atomistic simulations demonstrated that a gradual substitution of hexyl chains with phenyl groups in tetraalkylphosphonium cations results in remarkable changes in hydrogen bonding interactions, liquid structures and scattering structural functions, and rotational dynamics of hexyl chains and phenyl groups in tetraalkylphosphonium bis(trifluoromethylsulfonyl)imide ionic liquids. Hydrogen donor sites in hexyl chains present competitive characteristics with those in phenyl groups in coordinating anions, as well as their continuous and intermittent hydrogen bonding dynamics. Cation‐cation and anion‐anion spatial correlations show concomitant shift to short distances with decreased peak intensities with variations of cation structures, whereas cation‐anion correlations have a distinct shift to large radial distances due to decreased associations of anions with neighboring cations. These microstructural changes are qualitatively manifested in shifts of prominent peaks for prevalent charge alternations and adjacency correlations between ion species in scattering structural functions. Meanwhile, rotational dynamics of hexyl chains speed up, which, in turn, slow down rotations of phenyl groups, whereas anions exhibit imperceptible changes in their rotational dynamics. These computational results are intrinsically correlated with conformational flexibilities, molecular sizes, and steric hindrance effects of phenyl groups in comparison with hexyl chains, and constrained distributions of anions around cations in heterogeneous ionic environments.

Effects of Nitridation and Vinylation of Imidazolium Rings on Hydrogen Bonding Interactions, π–π-Stacking Structures, and Dynamical Heterogeneities in Imidazolium and Triazolium Ionic Liquids

“…Such an observation corresponds to the cooperative HB dynamics between cations and anions with a systematic variation of cation structures, which is similar to those observed in atomistic simulations of imidazolium [BOB] and tetraalkylphosphonium [NTF 2 ] ILs at varied temperatures. 28,65 For triazolium [NTF 2 ] ILs, the intermittent H5−NS and H4−NS HB dynamics, as shown in Figure 11, present distinct features in comparison with those for H5 and H4 atoms in coordinating [NTF 2 ] anions via O atoms, as shown in Figure 10. The residence time for intermittent H5−NS and H4−NS HB dynamics is approximately one order of magnitude shorter than those for intermittent H5−O and H4−O HB dynamics (Table 1).…”

“…For [NTF 2 ] anions, both O and central N atoms (as labeled in Figure 1) are preferential HB acceptors but with different HB capabilities. 28,69 In addition, the introduced NS atoms in triazolium cations act as potential HB acceptors to stabilize ion structures in local ionic environments. 3) are slightly wider than those for methyl-based triazolium cations (upper panels in Figure 3).…”

“…3,7,9,15,16 Therefore, the microstructural and dynamical quantities of ion species, the self-assembled mesoscopic liquid morphologies, and the macroscopic functionalities of imidazolium ILs can be widely tuned via a judicious selection of ion moieties, an introduction of a controllable amount of molecular solvents (solutes) into IL matrices, and a specific mutation of constituent ions with atoms and molecular groups to meet specific requirements in diverse applications. 17−28 It has been well documented that the functionalization of alkyl and ring moieties in imidazolium cations with specific units, such as hydrogen, 21,26,29 fluorine, 20,30 silicon 24,31,32 and selenium atoms, 25 hydroxyl and ether groups, 18,33,34 and even phenyl moieties, 28,35,36 results in distinct microstructural arrangements and dynamical heterogeneities of IL ions in varied chemical environments. The mutation of alkyl chains in imidazolium cations using either ether or hydroxyl groups disturbs a subtle balance between Coulombic, hydrogen bonding (HB), and dispersion forces, contributing to enhanced intermolecular interactions among constituent ions.…”

“…18,40 Terminating alkyl chains in imidazolium cations with aromatic moieties (phenyl groups) results in a systematic increase in glass transition temperatures and melting points arising from preferential π−π-stacking interactions among aromatic moieties and imidazolium rings and additional anion-π interactions of [NTF 2 ] anions with π electrons of phenyl groups. 28,35,36 In addition, the decoration of imidazolium-ring moieties with specific atoms can mediate HB interactions between constituent ions, leading to remarkable changes in microstructures and mesoscopic liquid morphologies, as well as hydrodynamic behaviors of imidazolium ILs. 21,26,29,41 The protonation of N(3) position on the imidazolium ring (Figure 1) can tune cation's hydrophilicity from aprotic to protic type.…”

“…Room-temperature ionic liquids (ILs) are fascinating molten salts composed solely of ion species with distinct molecular symmetry and charge delocalization, having their melting points below 100 °C. − Recent years have seen a great enthusiasm for ILs regarding their utilities as facilitating functional materials in diverse applications including material synthesis and catalysis, , microlubrication and nanotribology, , gas adsorption and separation, , and electrochemical devices for energy storage and harvesting. ,,, Among the whole IL community, imidazolium ILs hold peculiar significance because of their multifaceted physicochemical characteristics, such as low volatility and flammability, reasonable viscosity–conductivity features, high thermal- and electrochemical-oxidative stabilities, wide electrochemical windows, and exceptional affinities to molecular compounds having varied polarities. − ,, These remarkable properties of imidazolium ILs are intrinsically correlated with inherent ion structures and delicate interactions among constituent ions at the nanoscopic level. ,,,, Therefore, the microstructural and dynamical quantities of ion species, the self-assembled mesoscopic liquid morphologies, and the macroscopic functionalities of imidazolium ILs can be widely tuned via a judicious selection of ion moieties, an introduction of a controllable amount of molecular solvents (solutes) into IL matrices, and a specific mutation of constituent ions with atoms and molecular groups to meet specific requirements in diverse applications. − …”