Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium-Based Dicationic Ionic Liquids

Abstract

Electronic structure, binding energies, and spectral characteristics of functionalized asymmetric dicationic ionic liquids (DILs) composed of quaternary ammonium cations substituted with the ethoxyethyl and allyl/3-phenylpropyl/methoxyethoxyethyl/pentyl functionalities on two different nitrogen centers of the dication and the bis(trifluoromethanesulfonyl)imide (TfN) anion were derived employing the dispersion-corrected density functional theory. DILs based on methoxyethoxyethyl-substituted cation reveal stronger binding toward the TfN anion. The measured glass transition temperatures are found to be strongly dependent on the cation-anion binding facilitated through noncovalent interactions with dominant contributions from the electrostatics and hydrogen bonding. The manifestations of these interactions to vibrational spectra, in particular, to SO and CF stretchings in the complexes are presented. It has been demonstrated that the frequency down (red)-shift of the SO stretching in these DILs with varying substituent follows the order: methoxyethoxyethyl (35 cm) > allyl (23 cm) > pentyl (20 cm) > 3-phenylpropyl (5 cm), which is consistent with the strength of cation-anion binding. The CF stretching of the anion exhibits the frequency shift in the opposite direction with its hierarchy being reversed to that of SO stretchings; the largest upshift (blue shift) of 60 cm was predicted for the DILs composed of 3-phenlpropyl substituted dications. The direction of such frequency shift has been rationalized through the difference molecular electron density maps in conjunction with the electron density at the bond critical point in the quantum theory of atoms in molecules. The underlying cation-anion binding has been analyzed through charge distribution analysis characterized in terms of molecular electrostatic potential topography. Furthermore, the observed decomposition temperatures of DILs are shown to correlate well with the maximum surface electrostatic potential parameters in quantum theory of atoms in molecules.

Characterization and Solution Properties of Quaternary-Ammonium-Salt-Type Amphiphilic Gemini Ionic Liquids

“…20,21 However, these amphiphilic ionic liquids are mostly imidazolium-based 20,21 and amine-salt-containing protic-type compounds. 22 So far, only gemini surfactants with hydrophobic, hydrophilic, and spacer groups have been reported, and there have been few reports on gemini ionic liquids, 23−25 including quaternary-ammonium salt-type amphiphilic gemini ionic liquids. 25…”

“…22 So far, only gemini surfactants with hydrophobic, hydrophilic, and spacer groups have been reported, and there have been few reports on gemini ionic liquids, 23−25 including quaternary-ammonium salt-type amphiphilic gemini ionic liquids. 25…”

“…For instance, it has been reported that amphiphilic monomeric ionic liquids of 1-alkyl-3-methylimidazolium and 1-alkylpyridinium salts show surface activities in aqueous solutions similar to those of conventional surfactants. , However, these amphiphilic ionic liquids are mostly imidazolium-based , and amine-salt-containing protic-type compounds . So far, only gemini surfactants with hydrophobic, hydrophilic, and spacer groups have been reported, and there have been few reports on gemini ionic liquids, − including quaternary-ammonium salt-type amphiphilic gemini ionic liquids …”

“…22 So far, only gemini surfactants with hydrophobic, hydrophilic, and spacer groups have been reported, and there have been few reports on gemini ionic liquids, 23−25 including quaternary-ammonium salt-type amphiphilic gemini ionic liquids. 25 In this study, quaternary-ammonium-salt-type amphiphilic gemini compounds with alkyl chains of identical and nonidentical lengths and bulky counterions (C m -2-C n X, where m and n represent the alkyl chain length; m = 4, 6, 8, 10; n = 2, 4, 6, 8, 10; m ≥ n, X indicates counterion, which is one of the following: BF 4 , PF 6 , OTf, FSA, or NTf 2 ; see Figure 1a) were designed and synthesized along with the corresponding monomeric compounds (C n X, n = 2, 4, 6, 8, 10; see Figure 1b). This was done with the aim of developing novel amphiphilic ionic liquids that show the properties of both ionic liquids and surfactants as well as those of gemini surfactants.…”

Ion Pair Structures and Hydrogen Bonding in RnNH4–nAlkylammonium Ionic Liquids with Hydrogen Sulfate and Mesylate Anions by DFT Computations

“…The ability to forecast the physicochemical properties of an unknown ionic liquid will largely depend on understanding the interactions at the molecular level. And density functional theory methods are most often employed to predict the structural features of ILs and to unveil the bonding interactions between the ions. − Nowadays, most quantum chemical calculations are used to estimate the interactions between the ions in the ion pairs of aprotic ionic liquids − as compared to protic ionic liquids (PILs). Works , show that the ion–ion interaction in the latter is definitely stronger than that in aprotic ILs, which can be attributed to highly directional hydrogen bonds.…”