Computational NMR Study of Ion Pairing of 1-Decyl-3-methyl-imidazolium Chloride in Molecular Solvents

Abstract

The 1 H NMR spectra of 10 −5 mole fraction solutions of 1-decyl-3-methyl-imidazolium chloride ionic liquid in water, acetonitrile, and dichloromethane have been measured. The chemical shift of the proton at position 2 in the imidazolium ring of 1-decyl-3methyl-imidazolium (H2) is rather different for all three samples, reflecting the shifting equilibrium between the contact pairs and free fully solvated ions. Classical molecular dynamics simulations of the 1decyl-3-methyl-imidazolium chloride contact ion pair as well as of free ions in water, acetonitrile, and dichloromethane have been conducted, and the quantum mechanics/molecular mechanics methods have been applied to predict NMR chemical shifts for the H2 proton. The chemical shift of the H2 proton was found to be primarily modulated by hydrogen bonding with the chloride anion, while the influence of the solventsthough differing in polarity and capabilities for hydrogen bondingis less important. By comparing experimental and computational results, we deduce that complete disruption of the ionic liquid into free ions takes place in an aqueous solution. Around 23% of contact ion pairs were found to persist in acetonitrile. Ion-pair breaking into free ions was predicted not to occur in dichloromethane.

Structural Features of the [C4mim][Cl] Ionic Liquid and Its Mixtures with Water: Insight from a 1H NMR Experimental and QM/MD Study

“…In systems J and M with low averaged number of chloride anions near the C 2 –H 2 moiety, a substantial increase in the shielding constant is predicted by as much as 1.3 ppm as compared to the value computed for the neat IL, system G . The present results are in line with the previous observations that the presence or the absence of the hydrogen bonding between the C 2 –H 2 group and the chloride anion is the main factor determining the H 2 shielding constant of the imidazolium cation in the liquid phase . Our computational results in Table thus allow concluding that the observed monotonically decreasing chemical shift of the H 2 atom with the increasing content of water in the mixture as illustrated in Figure b is due to the gradual breakdown of the hydrogen bonding between the C 2 –H 2 moiety of imidazolium cations and chloride anions.…”

“…The present results are in line with the previous observations that the presence or the absence of the hydrogen bonding between the C 2 –H 2 group and the chloride anion is the main factor determining the H 2 shielding constant of the imidazolium cation in the liquid phase. 49 Our computational results in Table 2 thus allow concluding that the observed monotonically decreasing chemical shift of the H 2 atom with the increasing content of water in the mixture as illustrated in Figure 1 b is due to the gradual breakdown of the hydrogen bonding between the C 2 –H 2 moiety of imidazolium cations and chloride anions. Notably, imidazolium chloride ILs are known to eventually dissociate into free fully solvated ions at the conditions of infinite dilution in an aqueous solution.…”

“…Notably, imidazolium chloride ILs are known to eventually dissociate into free fully solvated ions at the conditions of infinite dilution in an aqueous solution. 49 …”

“…Our computational results in Table thus allow concluding that the observed monotonically decreasing chemical shift of the H 2 atom with the increasing content of water in the mixture as illustrated in Figure b is due to the gradual breakdown of the hydrogen bonding between the C 2 –H 2 moiety of imidazolium cations and chloride anions. Notably, imidazolium chloride ILs are known to eventually dissociate into free fully solvated ions at the conditions of infinite dilution in an aqueous solution …”

“…Even though the accuracy of predicted NMR spectra based on classical trajectories was questioned previously, an important insight into the molecular structure of the studied IL systems was reached . We have employed classical MD simulations along with a combined quantum mechanics/molecular mechanics (QM/MM) model to study various NMR parameters − and lately addressing ion pairing in the solutions of [C10mim]­[Cl] IL where solvents differ in polarity and capabilities for hydrogen bonding . In this work, this computationally expensive yet accurate computational technique is applied in order to gain molecular-level insight into structural organization of [C4mim]­[Cl]/water mixtures reflecting experimental NMR data in Figure .…”