The Local Structure of Ionic Liquids: Cation–Cation NOE Interactions and Internuclear Distances in Neat [BMIM][BF₄] and [BDMIM][BF₄]

Abstract

The increasing interest in room-temperature ionic liquids (RTILs) is related to their possible exploitation as environmentally friendly neoteric solvents because of their vanishing vapor pressure, thermal and chemical stability, air and moisture stability, wide liquidus range, solvent capability. Suitable applications of RTILs in synthesis, catalysis, biocatalysis, material science, and chemical engineering have been reported.[1] The rapidly increasing number and relevance of applications stimulated deeper understanding of the structure of RTILs in terms of intermolecular interactions that take place in the bulk liquid at the atomic level.[2] In such a context, the nuclear overhauser effect (NOE) is a powerful investigative tool, as it originates from dipolar interaction between pairs of nuclei and thus provides information on the molecular sites involved in the interactions. A pioneering study by Osteryoung and co-workers [3] demonstrated the existence of intermolecular NOE interactions between ring protons in 1-ethyl-2-methylimidazolium [EMIM]Cl·AlCl 3 . The cation-cation contacts suggested a local or short-range structure of the liquid. The concept of a local structure in liquid methylimidazolium salts that resembles those found in the solid state has been stressed by some authors on the basis of X-ray diffraction, [4] neutron scattering, [5] and NMR spectroscopy.[6]Herein, we present the first attempt to provide cationcation distances in neat liquid 1-butyl-3-methylimidazolium tetrafluoroborate ( À (2 g), which bear a bulky and noncoordinating anion, possibly because of the effect of increased intercation distance rather than unfavorable correlation times. The methodology for the assessment of distances in the liquid relies upon the distance dependence of the crossrelaxation rate s obtained by intermolecular NOE build-up rates. [7] The cations 1 a and 2 a can be depicted as a polar head (the charged imidazolium ring) and an apolar tail (the n-butyl chain). NOESY spectra showed both head-to-head and headto-tail cation-cation contacts. Herein, we assume that a) the tumbling of the system is isotropic and can be described by a single correlation time and b) the contact time of the cationcation association is long enough to contribute to dipolar relaxation. These are reasonable hypotheses for head-to-head contacts and could be verified if individual correlation times À (2). Curved arrows and Greek letters refer to the torsion angles of the butyl chain. b) Effective correlation times t eff (ns) at 305 K for 1 a and 315 K for 2 a. Tf= triflate.

Magnetic resonance spectroscopy in ionic liquids

“…More recently, NOESY measurements at mixing times smaller than 50 ms on the BF À 4 and Tf 2 N À salts of [bmim] and [bmmim] cations were conducted by Mele et al [99]. While, interestingly, no intermolecular contacts were detected for neat bistriflic amide salts, data from BF À 4 salts could be used to craft a model of preferred relative orientations of the cations in the neat liquid.…”

Fabrication of gold nano- and microstructures in ionic liquids—A remarkable anion effect

“…Recently, it was reported that imidazolium-based ILs exhibit a high degree of self-organization due to the hydrogen bond network between the cations and anions [45,46]. Moreover, they are also composed of polar and nonpolar microdomains [47][48][49].…”

Pyrazolium- versus Imidazolium-Based Ionic Liquids: Structure, Dynamics and Physicochemical Properties

“…This approach has been successfully applied several times in the study of the imidazolium series of ILs. [23][24][25][26][27] The separation of the intra-and intermolecular dipolar contacts can be done a posteriori by applying the distance threshold of 4 Å for vanishing ROEs. The average distances between the H atoms of interest in cations 1 and 3 were calculated at DFT level (see the SI) and used to assign intra-and intermolecular ROEs, e.g.…”