Dynamics of Discotic Fluoroalkylated Triphenylene Molecules Studied by Proton NMR Relaxometry

The Larmor frequency and temperature dependence of the proton nuclear magnetic resonance (NMR) spin–lattice relaxation time was measured in the isotropic and columnar phases of both chain-end fluorinated triphenylene disk-like and fully hydrogenated molecules. In the columnar phase, the results are interpreted in terms of the collective motions, due to the deformations of the columns, and individual molecular translational self-diffusion displacements and rotations/reorientacions. In the isotropic phase, local molecular motions and order fluctuations as a pretransitional effect were considered. The activation energies of the molecular motions of the partially fluorinated molecule were found to be higher than those corresponding to the hydrocarbon homologue. Our findings show a clear difference in the relaxation dispersion between the two liquid crystals homologues. In particular it is observed that the columnar undulations have a much stronger contribution to the relaxation rate in the low frequency regime in the case of the fully hydrogenated triphenylene. The effect of fluorination of the pheripheral chain enhances the columnar mesophase’s stability

¹H NMR Relaxation Study of a Magnetic Ionic Liquid as a Potential Contrast Agent

A proton nuclear magnetic relaxation dispersion ¹H NMRD study of the molecular dynamics in mixtures of magnetic ionic liquid [P₆₆₆₁₄]­[FeCl₄] with [P₆₆₆₁₄]­[Cl] ionic liquid and mixtures of [P₆₆₆₁₄]­[FeCl₄] with dimethyl sulfoxide (DMSO) is presented. The proton spin–lattice relaxation rate, <i>R</i>₁, was measured in the frequency range of 8 kHz–300 MHz. The viscosity of the binary mixtures was measured as a function of an applied magnetic field, <b>B</b>, in the range of 0–2 T. In the case of DMSO/[P₆₆₆₁₄]­[FeCl₄] the viscosity was found to be independent from the magnetic field, while in the case of the [P₆₆₆₁₄]­[Cl]/[P₆₆₆₁₄]­[FeCl₄] system viscosity decreased with the increase of the magnetic field strength. The spin–lattice relaxation results were analyzed for all systems taking into account the relaxation mechanisms associated with the molecular motions with correlation times in a range between 10^–11 and 10^–7s, usually observed by NMRD, and the paramagnetic relaxation contributions associated with the presence of the magnetic ions in the systems. In the case of the DMSO/[P₆₆₆₁₄]­[FeCl₄] system the <i>R</i>₁ dispersion shows the relaxation enhancement due to the presence of the magnetic ions, similar to that reported for contrast agents. For the [P₆₆₆₁₄]­[Cl]/[P₆₆₆₁₄]­[FeCl₄] system, the <i>R</i>₁ dispersion presents a much larger paramagnetic relaxation contribution, in comparison with that observed for the DMSO/[P₆₆₆₁₄]­[FeCl₄] mixtures but different from that reported for other magnetic ionic liquid system. In the [P₆₆₆₁₄]­[Cl]/[P₆₆₆₁₄]­[FeCl₄] system the relaxation enhancement associated with the paramagnetic ions is clearly not proportional to the concentration of magnetic ions, in contrast with what is observed for the DMSO/[P₆₆₆₁₄]­[FeCl₄] system

CO₂ in 1-Butyl-3-methylimidazolium Acetate. 2. NMR Investigation of Chemical Reactions

The solvation of CO₂ in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been investigated by ¹H, ¹³C, and ¹⁵N NMR spectroscopy at low CO₂ molar fraction (mf) (<i>x</i>_CO₂ ca. 0.27) corresponding to the reactive regime described in part 1 of this study. It is shown that a carboxylation reaction occurs between CO₂ and Bmim Ac, leading to the formation of a non-negligible amount (∼16%) of 1-butyl-3-methylimidazolium-2-carboxylate. It is also found that acetic acid molecules are produced during this reaction and tend to form with elapsed time stable cyclic dimers existing in pure acid. A further series of experiments has been dedicated to characterize the influence of water traces on the carboxylation reaction. It is found that water, even at high ratio (0.15 mf), does not hamper the formation of the carboxylate species but lead to the formation of byproduct involving CO₂. The evolution with temperature of the resonance lines associated with the products of the reactions confirms that they have a different origin. The main byproduct has been assigned to bicarbonate. All these results confirm the existence of a reactive regime in the CO₂–Bmim Ac system but different from that reported in the literature on the formation of a reversible molecular complex possibly accompanied by a minor chemical reaction. Finally, the reactive scheme interpreting the carboxylation reaction and the formation of acetic acid proposed in the literature is discussed. We found that the triggering of the carboxylation reaction is necessarily connected with the introduction of carbon dioxide in the IL. We argue that a more refined scheme is still needed to understand in details the different steps of the chemical reaction in the dense phase