NMR and Quantum Chemistry Study of Mesoscopic Effects in Ionic Liquids

H-1, C-13, and Br-81 NMR spectra of the neat room-temperature ionic liquid (RTIL), namely, 1-decy1-3-methylimidazolium bromide ([C(10)mim][Br])) as well as its solutions in acetonitrile, dichloromethane, methanol, and water have been investigated. The most important observation of the present work is the significant broadening of Br-81 NMR signal in the solutions of [C(10)mim][Br] in organic solvents, which molecules tend to associate into hydrogen bond networks and the appearance of the complex contour of Br-81 NMR signal in the neat RTIL as well as in the liquid crystalline (LC) ionogel formed in RTIL/water solution. The complex structure of Br-81 signal changes upon heating and dilution in water. It disappears at ca. 353 K and in the aqueous solution below ca. 0.1 mol fraction of RTIL. Several new H-1 NMR signals appear at the [C(10)mim][Br]/water compositions just before the solidification of the sample (similar to 0.3 mol fraction of [C(10)mim][Br]). These additional peaks can be attributed to the H2O protons placed in inhomogeneous regions of the sample or due to the appearance of nonequivalent water sites in LC ionogel, the exchange between which is highly restricted or even frozen. The complex shape of Br-81 NMR signal can originate from the presence of supra-molecular structures (mesoscopic domains) that live over the period of the NMR time-scale due to a very high viscosity of [C(10)mim][Br]. These domains exhibit some features of partially disordered solids (liquid- or plastic crystals). To evaluate the static and dynamic contributions into the relaxation rate of Br-81 nuclei, the quantum chemistry calculations of the electronic structure, magnetic shielding, and electric field gradient (EFG) tensors of [C(10)mim][Br] and related model systems (Br-center dot 6H(2)O cluster, with addition of the dipoles (hydrogen fluoride) and charged particles - cations: H+ or C(1)mim+) were performed.