Structure of Ionic Liquids The Structure of a Room-Temperature Ionic LiquidwithandwithoutTraceAmountsofWater: The Role of C À H···O and C À H···F Interactions in 1-n-Butyl-3-Methylimidazolium Tetrafluoroborate** Andrea Mele,* Chieu D. Tran, and Silvia H. De Paoli Lacerda Room temperature ionic liquids (RTILs) have been consid- ered as a “green”, recyclable alternative to the traditional volatile organic solvents because of their chemical and physicalproperties,suchasbeingliquidatroomtemperature, air and moisture stability, high solubility power, and virtually novaporpressure. [1,2] Beinghygroscopic,RTILscanabsorba significant amount of water. Their properties (e.g., solubility, polarity,viscosity,andconductivity)arenotonlychangedby, but also are dependent on the amount of absorbed water. [1–3] Rates of chemical reactions and efficiencies of various processes in RTILs are, therefore, dependent on absorbed water. [1,2] As a consequence, information on the structures of RTILs and their interactions with water are important not only fundamentally but also for various industrial applica- tions. Various studies have been made using either exper- imental techniques, such as FT-IR, Near-IR, fluorescence, viscosimetry, conductivity, and pulsed-gradient spin-echo NMR diffusion coefficient measurements, [3,4] or theoretical calculations. [5] Unfortunately, these techniques cannot pro- vide direct information on the molecular level of structure of RTILs and their interactions with water. Herein we present direct experimental evidence of cation–cation, cation–water, and cation–anion interactions by NMR spectroscopy through intermolecular nuclear Overhauser enhancements (NOEs) on the model compound 1-n-butyl-3-methylimidazolium tetrafluoborate ([BMIm] + [BF 4 ] À (1), Figure1). Cation– cation interactions were investigated by homonuclear NOEs intherotatingframe(ROEs).TheROEspatternofthepure liquid (1a) was compared with those of samples containing knownamountsofwater(1b–g,Table1).Waterwasaddedto change the structure of the pure ionic liquid by introducing water–cation interactions. As a complementary picture, intermolecular water–cation ROEs were also observed and evaluated to provide details of 1) the type of water–cation interactionsand2)thesiteofinteraction.Eventually,therole of the anion was investigated by heteronuclear steady-state 1 H{ 19 F} NOE difference spectra. TheanalysisofintermolecularROEsarisingfromcation– cation interactions was carried out by volume integration of the cross peaks attributed to intermolecular interactions. [6] Thehistogramoftherelativeintensities(Figure2)showstwo different trends: 1) ROEs intensity decreases with increasing water content for those interactions involving imidazolium ring protons H2, H4, H5; 2) the opposite (ROEs intensity increases with increasing water content) for interactions between H10 and the protons of the n-butyl group, H6, H7, Figure 1. Chemical structure and atom numbering for 1. Table 1: Composition of samples of compound 1. 1a [a] 1b 1c 1d 1e 1f 1g Water:1 mole ratio 0 0.10 0.20 0.37 0.56 0.81 1.09 Water mole fraction 0 0.09 0.17 0.27 0.36 0.45 0.52 [a] Pure liquid, reference sample. Figure 2. Relative intensity of intermolecular ROESY cross peaks in samples 1b–g. The data reported were obtained as follows: relative intensity (%) I rel = [(VÀV 0 )/V 0 ]100, where V 0 = integrated volume of a given cross peak in 1a (pure ionic liquid) and V = integrated volume of the same cross peak in the sample containing known amounts of water (1b–g, Table 1). All volumes are internally normalized to the H2–H10 cross-peak volume set = 1.00 (arbitrary units). A negative bar for a given cross peak means a decreased relative intensity with respect to the same cross peak in the pure liquid. [*] Dr. A. Mele Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” Politecnico di Milano Via L. Mancinelli 7, 20131 Milano (Italy) Fax: (+ 39)02-2399-3080 E-mail: andrea.mele@polimi.it Prof. Dr. C. D. Tran, Dr. S. H. De Paoli Lacerda Department of Chemistry Marquette University P.O. Box 1881, Milwaukee WI 53201 (USA) [**] We wish to thank Mr. Walter Panzeri (CNR-ICRM) for technical assistance, Prof. Enzio Ragg (University of Milano) and Dr. Giovanni Fronza (CNR-ICRM) for stimulating discussions. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications 4364 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200351783 Angew. Chem. Int. Ed. 2003, 42, 4364 –4366