15N NMR studies provide insights into physico-chemical properties of room-temperature ionic liquids

Abstract

Ionic liquids (ILs) have gained a lot of attention as alternative solvents in many fields of science in the last two decades. It is known that the type of anion has a significant influence on the macroscopic properties of the IL. To gain insights into the molecular mechanisms responsible for these effects it is important to characterize these systems at the microscopic level. Such information can be obtained from nuclear spin-relaxation studies which for compounds with natural isotope abundance are typically performed using direct ¹H or ¹³C measurements. Here we used direct ¹⁵N measurements to characterize spin relaxation of non-protonated nitrogens in imidazolium-based ILs which are liquid at ambient temperature. We report heteronuclear ¹H–¹⁵N scalar coupling constants (ⁿJ_HN) and ¹⁵N relaxation parameters for non-protonated nitrogens in ten 1-ethyl-3-methylimidazolium ([C₂C₁IM]⁺)-based ILs containing a broad range of anions. The ¹⁵N relaxation rates and steady-state heteronuclear ¹⁵N–{¹H} NOEs were measured using direct ¹⁵N detection at 293.2 K and two magnetic field strengths, 9.4 T and 16.4 T. The experimental data were analyzed to determine hydrodynamic characteristics of ILs and to assess the contributions to ¹⁵N relaxation from ¹⁵N chemical shift anisotropy and from ¹H–¹⁵N dipolar interactions with non-bonded protons. We found that the rotational correlation times of the [C₂C₁IM]⁺ cation determined from ¹⁵N relaxation measurements at room temperature correlate linearly with the macroscopic viscosity of the ILs. Depending on the selected anion, the ¹⁵N relaxation characteristics of [C₂C₁IM]⁺ differ considerably reflecting the influence of the anion on the physicochemical properties of the IL.