MAS NMR investigation of molecular order in an ionic liquid crystal

The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline][geranate(H)octanoate], an analogue of Choline And GEranate (CAGE), which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated by magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small-angle X-ray scattering (SAXS), and mass spectrometry. Mass spectrometry and the 1H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)2(H)], and [(choline)(geranate)2(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate, and hydroxyl groups, with rapid hydrogen bond breaking and re-formation observed to average the 1H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ∼293 K. The occurrence of this transition close to room temperature could be utilized as a potential temperature-induced “switch” of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 23% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both 1H–13C dipolar-chemical shift correlation (DIPSHIFT) and 1H double-quantum (DQ) MAS NMR experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and nonpolar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the MAS NMR measurement of the partially averaged 13C–1H dipolar couplings (DCH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and 1H–1H dipolar couplings (DHH) by double-quantum (DQ) build-up curves. The corresponding order parameters, SCH and SHH, are in the range 0–0.2 and are lower compared to those for smectic (i.e., layered) phases of conventional nonionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant–solvent systems

Nitrogen-based ligands for asymmetric chemistry

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Data for MAS NMR investigation of molecular order in an ionic liquid crystal

The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline]+[geranate(H)octanoate]-, an analogue of Choline And GEranate (CAGE) which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated using magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small angle X-ray scattering (SAXS) and mass spectrometry. Mass spectrometry and the 1H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)2(H)] and [(choline)(geranate)2(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate and hydroxyl groups, with rapid hydrogen bond breaking and reforming observed to average the 1H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ~293 K. The occurrence of this transition close to room temperature could be utilized as a temperature-induced ‘switch’ of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 25% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both 1H-13C dipolar-chemical shift correlation (DIPSHIFT) and 1H double-quantum (DQ) experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and non-polar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the measurement of the partially averaged 13C-1H dipolar couplings (DCH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and 1H-1H dipolar couplings (DHH) by DQ build-up curves. The corresponding order parameters, SCH and SHH, are in the range 0 to 0.2, and are lower compared to those for smectic (i.e., layered) phases of conventional non-ionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant-solvent systems

Enhancing the stability of ionic liquid media for cellulose processing: acetal protection or carbene suppression?

Although excellent candidate solvents for cellulose, capa-ble of dissolving 20 wt% of the carbohydrate for electro-spinning processes, dialky-limidazolium carboxylate ionic liquids undergo unde-sirable side reactions with the reducing end of saccharides, terminating in an equilibrium concentration of a 2-(hydroxymethyl)-substituted imidazolium ‘adduct’. The addition of small molar quantities of a benign, non-toxic and inexpensive co-solvent, e.g. glycerol, mini-mises the rate of adduct ac-cumulation, thereby enhanc-ing the long-term thermal stability and recyclability of the expensive ionic liquid component. NMR, UV-vis and mass spectrometry ex-periments reveal that the im-proved stability is likely at-tributable to suppression of the transient dialkylimidazol-2-ylidene carbene, via hy-drogen-donation by the pro-tic co-solvent, rather than by cyclic acetal protection of the carbohydrate. The incor-poration of (up to) 10 wt% of glycerol into the solvent mix-ture does not exacerbate the rate of cellulose depolymeri-sation compared to in the neat ionic liquid, and high solubility of cellulose is main-tained. Furthermore, a col-ourimetric comparison of the recovered solvents, following cellulose re-precipitation, demonstrates that glycerol does not increase the concen-tration of contaminant re-ducing sugars in the organic electrolyte