Structure and dynamics of room temperature ionic liquids with bromide anion: Results from 81Br NMR spectroscopy

We report the results of a comprehensive 81Br NMR spectroscopic study of the structure and dynamics of two room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium bromide ([C4mim]Br) and 1-butyl-2,3-dimethylimidazolium bromide ([C4C1mim]Br), in both liquid and crystalline states. NMR parameters in the gas phase are also simulated for stable ion pairs using quantum chemical calculations. The combination of 81Br spin-lattice and spin-spin relaxation measurements in the motionally narrowed region of the stable liquid state provides information on the correlation time of the translational motion of the cation. 81Br quadrupolar coupling constants (CQ) of the two RTILs were estimated to be 6.22 and 6.52 MHz in the crystalline state which were reduced by nearly 50% in the liquid state, although in the gas phase, the values are higher and span the range of 7-53 MHz depending on ion pair structure. The CQ can be correlated with the distance between the cation-anion pairs in all the three states. The 81Br CQ values of the bromide anion in the liquid state indicate the presence of some structural order in these RTILs, the degree of which decreases with increasing temperature. On the other hand, the ionicity of these RTILs is estimated from the combined knowledge of the isotropic chemical shift and the appropriate mean energy of the excited state. [C4C1mim]Br has higher ionicity than [C4mim]Br in the gas phase, while the situation is reverse for the liquid and the crystalline states. Copyright © 2015 John Wiley & Sons, Ltd

Comparison between Cycloalkyl- and <i>n</i>-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis­(trifluoromethanesulfonyl)­amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective <i>n</i>-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by ¹³C NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents

Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence

The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties. To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis­(fluorosulfonyl)­amide ([Pip_1,4]­[FSA]) was selected as the first sample. In addition to the rotational flexibility of the alkyl group, the [Pip_1,4]⁺ cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations. Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip_1,4]­[FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion. Each phase change is also governed by very slow dynamics. The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations. Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 38.8 kJ/mol. It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings

Effects of Cyclic-Hydrocarbon Substituents and Linker Length on Physicochemical Properties and Reorientational Dynamics of Imidazolium-Based Ionic Liquids

We synthesized a series of alkylimidazolium-based ionic liquids (ILs) incorporating cyclopentyl, cyclohexyl, or phenyl groups as nonionic units substituted on an acyclic alkyl linker and characterized them with respect to physicochemical properties and reorientational dynamics. The effects of the nonionic substituents and linker length on the properties of these ILs were carefully examined. The physicochemical properties of the ILs are found to partially reflect the properties of the nonionic substituents. While the liquid densities showed a similar trend in linker-length dependence of each series of ILs, a distinct trend was observed for the shear viscosities of them. By comparison of correlation times obtained by ¹³C NMR spectroscopy, it is revealed that elongation of the linkers influences the characteristic effects of the nonionic substituents on the reorientational dynamics of the system

Comparison between Cycloalkyl- and <i>n</i>-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis­(trifluoromethanesulfonyl)­amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective <i>n</i>-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by ¹³C NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents

Preparation and Characterization of Conducting Mixed-Valence 9,9′-Dimethyl-3,3′-bicarbazyl Rectangular Nanowires

The facile synthesis of an organic electric conducting nanowire is described. The simple oxidation of 9-methylcarbazole by iron­(III) perchlorate in a methanol/acetonitrile mixture under atmospheric pressure and temperature produces abundant nanowires without using a template. The nanowire consists of 9,9′-dimethyl-3,3′-dicarbazyl and has a rectangular nanowire shape with an average diameter of 397 ± 50 nm and length of 17 ± 5 μm. The results of the elemental analysis, ¹H NMR, FT-IR, XPS, and ESR measurements revealed that the chemical composition of the nanowire is (dicarbazyl)_0.12(dicarbazylium·ClO₄^–)_0.88·H₂O. This result, combined with the UV–vis–NIR measurement, demonstrates that 9,9′-dimethyl-3,3′-dicarbazyl stacks in a mixed valence state. The nanowire is electroactive and has an electric conductivity of 3.0 × 10^–5 S cm^–1