Evidence for hydrogen bonding in solutions of 1-ethyl-3-methylimidazolium halides, and its implications for room-temperature halogenoaluminate(III) ionic liquids

Abstract

Multinuclear N M R spectroscopy and conductivity measurements showed that the 1 -ethyl-3-methylimidazolium cation, [emim] +, not only forms strong hydrogen bonds (using all three ring protons H2, H4 and H5) with halide ions in polar molecular solvents (e.g. ethanenitrile) and ionic liquids, but that it exists in a quasi-molecular state, [emim]X, in non-polar solvents (e.g. trichloro-and dichloromethane), showing a conventional aromatic stacking phenomenon.Over the past ten years the perception of the ability of the 1ethyl-3-methylimidazolium cation, [emim] + , 1,2 to enter into hydrogen bonds with halide ions has developed from not p ~s s i b l e , ~ through contr~versial,~ to widely a ~c e p t e d . ~. ~ However, the strongest evidence for these claims has always rested upon solid-state X-ray crystallographic determinations of the structures of both [emim]+ 4,7 and [edmim]+ (edmim = 1ethyl-2,3-dimethylimidazolium) salts.* A number of first-class solution studies have been reported, 5*6*9 yet no incontrovertible evidence has yet been presented that the [emim]' cation is capable of forming hydrogen bonds in solution. Proton NMR spectra of room-temperature [emim]C1-A1C13 ionic liquids are known to be very composition dependent, particularly in the basicIn particular, the H2 proton is very sensitive, moving significantly upfield with increasing acidity. This has been widely interpreted in terms of a stacked oligomeric model 3,5 in which anions and cations alternate, the anions being associated with the imidazolium ring both directly above and below its plane. Nevertheless, a 'H NMR study l 1 of several imidazole and imidazolium derivatives in CD,Cl, has shown that the chemical shift of the H2 proton was sensitive to the nature of the anion present, and hydrogen bonding between discrete ion pairs was postulated to explain this result. We thus decided to undertake a definitive study of the NMR characteristics of [emim]X (X = CI, Br or I) in molecular solvents, free of the aporia associated with ionic liquids. We demonstrate here, using principally the techniques of multinuclear NMR spectroscopy and conductivity measurements, that [emim] + not only forms strong hydrogen bonds (using all three ring protons H2, H4 and H5) with halide ions in polar molecular solvents (e.g. ethanenitrile, MeCN) and ionic liquids, but that it exists in a quasi-molecular state, [emim]X, in non-polar solvents (e.g. CHCl, and CH,Cl,), showing a conventional aromatic stacking phenomenon. Experiment a1N M R Studies.-Deuterium oxide (99%) was distilled in O ~U O before use; CD3CN and CD2C12 were distilled from calcium hydride under dry dinitrogen. Samples of [emim]X (X = C1, Br

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…All three ring protons H2, H4, and H5, see Fig. 2, form strong hydrogen bonds to e.g., halide ions [60][61][62][63][64][65]. From measurement of the 13 C dipole-dipole relaxation rate, Huang et al [66] found that the hydrogen atom attached to the ring C2 is hydrogen bonded to the [BF 4 ]…”

Azolium-based systems: application of an anion exchange resin (A- form) method and 1H NMR analysis of the charged-assisted (C–H)+•anion hydrogen bonds

“…Evidence for hydrogen bonding in the solid phase of the simple 1-ethyl-3-methylimidazolium salt [emim]I was first reported by Seddon and co-workers in 1986, 32 and later in [emim]A using single-crystal X-ray diffraction analysis, [33][34][35] and confirmed in the solution-phase by multinuclear NMR spectroscopy. 36 Moreover, Ludwig and co-workers reported direct spectroscopic evidence for an enhanced cation-anion interaction driven by (C-H) + ···anion hydrogen bonds in pure ILs, which gave a stronger the role to hydrogen bonds in imidazolium ILs. [19][20] Depending on the structure of the imidazolium-based frameworks, other noncovalent intermolecular interactions can also take place between cations and counteranions, a case in point being the usually weaker CH/π noncovalent interactions that can be rather significant for anions bearing aromatic units, e.g.…”

Aggregation of Imidazolium Ionic Liquids in Molecular Liquids Studied by Small-Angle Neutron Scattering and NMR

“…The crystal X-ray diffraction and molecular vibration studies have shown the hydrogen bonds between the ring hydrogens and halide ions in 1-alkyl-3-methlyimidazolium halide, such as BMI + Cl -. [23][24][25][26] Additionally, the neutron diffraction investigation with empirical potential structure refinement has revealed that the hydrogen bonds between the ring hydrogens and chloride ions are formed even in the liquid state of DMI + Cl -. 27 In the EMI + Cl --CH3CN mixtures below the break point xCH3CN < ~0.8, the inherent structure of EMI + Cl -may be kept even in the solutions because of the hydrogen bonds between cations and anions.…”

“…22 In the weak solvent of acetonitrile, thus, EMI + and Cl -interact with each other to form clusters by both charge-charge interaction and hydrogen bonds. [23][24][25][26][27] On the other hand, acetonitrile molecules aggregate with themselves by the dipole-dipole interactions among them to form clusters in the mixtures. 28 To quantitatively clarify aggregation of EMI + Cl -in the three systems, the Ornstein-Zernike fits were made on the scattering intensities through…”