The State of Water in "Ionic Liquid [bmim]Cl/AlCl3/H2O" Systems.

The unique physical and chemical properties of ionic liquids (ILs) determine their numerous applications in "green" chemistry and material science. Recently, systems based on ILs have been considered to be promising for use in a new generation of electrochemical devices. The results of a nuclear magnetic resonance (NMR) study of the microstructure of 1-butyl-3-methylimidazolium chloride (IL)/water mixtures in the presence of Al3+ cations are presented. For the first time, the splitting of spectral lines of water in such systems has been recorded. Comparing the 1H and 27Al NMR data, we have detected the existence of different solvate complexes of Al3+ with Cl- and estimated the characteristic times of exchange processes. For the system under study, a model of the Al3+ cation environment and its evolution with temperature and water content has been described. Quantum-chemical calculations have been performed to substantiate the model.

Translational Diffusion in a Set of Imidazolium-Based Ionic Liquids [bmim]+A- and Their Mixtures with Water.

As the development of the work (J. Phys. Chem. B 2019, 123 (10), 2362-2372), we have investigated the translational mobility in the same set of dried imidazolium-based ionic liquids (ILs) [bmim]A (A = BF4-, NO3-, TfO-, I-, Br-, and Cl-) in a wide temperature range using the NMR technique. It is shown that for the [bmim]+ cation, the temperature dependencies of product Dη do not follow the Stokes-Einstein relation for most systems studied, that is, the so-called "diffusion-viscosity decoupling" was realized. The correlation between local and translational mobility in pure IL of the [bmim][A] type was investigated using the data on NMR relaxation rates and diffusion coefficients. The most recent hypothesis of "water pockets" in mixtures of IL with water is critically discussed. Considering the totality of data in the literature and obtained here, we propose a specific model of the microstructure which may be applied up to water concentrations of 80-90 mol % (the structure of water-rich solutions is out of our current consideration). To confirm the model, molecular dynamics simulations of "IL-water" mixtures were also carried out.

Molecular Mobility in a Set of Imidazolium-Based Ionic Liquids [bmim]+A- by the NMR-Relaxation Method.

The detailed investigation of the local mobility in a set of dried imidazolium-based ionic liquids (1-butyl-3-methylimidazolium) in a wide temperature range and varying anions (BF4-, I-, Cl-, Br-, NO3-, TfO-) is presented. The measurements of temperature dependencies of the spin-lattice relaxation times of 1H and 13C nuclei are motivated by the need to obtain a fundamental characterization of molecular mobility of the substances under study, namely, to estimate the correlation times, τc, for the motion of individual molecular groups. In particular, it follows from obtained results that the mobility of the hydrocarbon "tail" is higher (smaller τc) than that of the imidazole ring, and this expected tendency is quantified. The effect of the influence of an anion type on the cation mobility is also analyzed.

Unexpected Temperature Behavior of Polyethylene Glycol Spacers in Copolymer Dendrimers in Chloroform.

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The dendrimer macromolecules were investigated in a dilute chloroform solution by (1)H NMR methods (spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is "frozen" at high temperatures (above 260 K), but it unexpectedly becomes "unfrozen" at temperatures below 250 K (i.e., melting when cooling). The transition between these two states occurs within a small temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1 and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by dendrimer conformations, in which the PEG group concentration in the layer increases with growing G. We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practical applications such as nanocontainers or nanoreactors.

"Hydration Shells" of CH2 Groups of ω-Amino Acids as Studied by Deuteron NMR Relaxation.

Hydration phenomena play a very important role in various processes, in particular in biological systems. Water molecules in aqueous solutions of organic compounds can be distributed among the following substructures: (i) hydration shells of hydrophilic functional groups of molecules, (ii) water in the environment of nonpolar moieties, and (iii) bulk water. Up to now, the values of hydration parameters suggested for the description of various solutions of organic compounds were not thoroughly analyzed in the aspect of the consideration of the total molecular composition. The temperature and concentration dependences of relaxation rates of water deuterons were studied in a wide range of concentration and temperature in aqueous (D2O) solutions of a set of ω-amino acids. Assuming the coordination number of the CH2 group equal to 7, which was determined from quantum-chemical calculations, it was found that the rotational correlation times of water molecules near the methylene group is 1.5-2 times greater than one for pure water. The average rotational mobility of water molecules in the hydration shells of hydrophilic groups of ω-amino acids is a bit slower than that in pure solvent at temperatures higher that 60 °C, but at lower temperatures, it is 0.8-1.0 of values of correlation times for bulk water. The technique suggested provides the basis for the characterization of different hydrophobic and hydrophilic species in the convenient terms of the rotational correlation times for the nearest water molecules.

13C NMR relaxation and reorientation dynamics in imidazolium-based ionic liquids: revising interpretation.

The temperature dependencies of (13)C NMR relaxation rates in [bmim]PF6 ionic liquid have been measured and the characteristic times (τc) for the cation reorientation have been recalculated. We found the origin of the incorrect τc temperature dependencies that were earlier reported for ring carbons in a number of imidazolium-based ILs. After a correction of the approach (13)C T1, the relaxation data allowed us to obtain the characteristic times for an orientation mobility of each carbon, and a complicated experiment, such as NOE, was not required. Thus the applicability of (13)C NMR relaxation rate measurements to the calculation of the characteristic times for reorientation of all the carbons of the [bmim](+) cation was confirmed and our findings have shown that a (13)C NMR relaxation technique allowed its application to ionic liquids to be equally successful as for other liquid systems.