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.

IR and NMR Properties of Ionic Liquids: Do They Tell Us the Same Thing?

“…Thus, NMR properties, such as proton and carbon chemical shifts, 13 C relaxation times, and diffusion coefficients, can be measured. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] IR spectroscopy is applied to study the structure of ILs, in particular the strength of the interactions between anions and cations. [19][20][21] However, there is no available systematic study of the structural and dynamical properties by using NMR and IR spectroscopy.…”

Multiple points of view of heteronuclear NOE: Long range vs short range contacts in pyrrolidinium based ionic liquids in the presence of Li salts

“…The 7 Li spectra were acquired on a Bruker DRX 500 spectrometer equipped with a broadband probe. The IL samples (0.5-1.0 g each) were transferred in a 5 mm NMR tube inside the dry-room to avoid any water contamination.…”

“…The first application of NOE to ILs dated back 1995 by the group of Osteyoung [5]. Since then, many applications of NOE for ILs have been published, either homonuclear (typically { 1 H-1 H}NOE) [6][7][8][9] or heteronuclear (most of the cases { 1 H- 19 F}NOE) [10][11][12]. The main strength of the NOE approach is that it is originated by the mutual relaxation (cross-relaxation) of NMR interacting nuclei close in space, thus providing precious information on the three dimensional structure of molecules and/or aggregate containing the interacting nuclei.…”

“…Different models have been developed in order to account for these factors [13][14], all leading to the conclusion that the intermolecular NOE extends beyond the 5 Å threshold, thus assuming a long-range, rather than short-range, character. In the context of ILs, these findings have a quite important fall-out, as many investigations on the local structure of ILs are based on the intermolecular homo- [6][7][8][9] and heteronuclear NOE [10,12]. As remarked, many ILs are composed of cations containing H atoms in an organic frame (e.g.…”

Supramolecular Structures in the Presence of Ionic Liquids

“…As the length of the alkyl chain increases, the nonpolar domains become larger and more connected, and cause swelling of the ionic network, in a manner analogous to microphase separation (Lopes & Padua, 2006). Recently, the results of X-ray diffraction (XRD) experiment (Triolo et al, 2007), fluorescent spectra (Hu & Margulis, 2006), Raman spectra (Iwata et al, 2007;Shigeto & Hamaguchi, 2006), NMR (Mele et al, 2006), femtosecond solvation dynamics study (Adhikari et al, 2007) and optical Kerr Effect spectra (Xiao et al, 2007) of 1-alkyl-3-methylimidazolium ILs also suggested the presence of heterogeneous structures. Furthermore, the result of the XRD experiment also reflected that the size of the inhomogeneous structures is proportional to the length of the alkyl chain (Triolo et al, 2007).…”