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.

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.