Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach Sergey A. Katsyuba,* ,† Tatiana P. Griaznova, † Ana Vidisˇ, ‡ and Paul J. Dyson ‡ A.E.ArbuzoV Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, ArbuzoV Str. 8, 420088 Kazan, Russia, and Institut des Sciences et Inge´nierie Chimiques, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ReceiVed: September 19, 2008; ReVised Manuscript ReceiVed: February 24, 2009 DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the molecular structure of the ion pairs of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF 4 ], in dichloromethane. According to the computations of the chemical shifts, experimental NMR spectra can be rationalized by an equilibrium between ca. 70-80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20-30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the experimental NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concentration/polarity of their dichloromethane solutions. Introduction Room-temperature ionic liquids (ILs), typically composed of organic cations and inorganic anions and liquid at or near ambient temperature, are currently attracting considerable at- tention. 1 Over the past few years they have started to find diverse applications, such as in separation processes, in nanowire growth, in reaction media for stoichiometric organic synthesis and catalysis, as electrolytes in solar cells, and in analytical processes. 2–5 All the intrinsically useful characteristics of ILs are tunable, and through different combinations of cations and anions there is a tremendous variety of “designer” solvents. 5,6 However, to a large extent the process of design remains a random event, as the features that control the physical properties of ILs remain poorly understood. To rationalize interrelations between structure and properties of ILs, reliable data on the structure and bonding in ionic liquids are needed. Structural investigation of ILs has largely focused on the air and moisture stable imidazolium based systems such as 1-ethyl- 3-methyl-imidazolium tetrafluoroborate ([EMIM][BF 4 ]) (Chart 1). We have demonstrated in recent publications 7–9 that density functional theory (DFT) methods in combination with vibra- tional spectroscopy are highly efficient in elucidating the molecular structure of the ion pairs of various imidazolium halides and perfluorometallates. According to computations, multiple stable structures are possible with the anion positioned (A) near to the C2 atom of the imidazolium ring, (B) between N1 and C5, (C) between N3 and C4, and (D) between C4 and C5. The anion of each ion pair forms a H-bond with the (C2)H proton (structure A) or with the (C4)H or (C5)H protons (structures B-D; see Chart 1). The manifestation of these hydrogen bonds is a red shift of the νC2-H, νC4-H, and νC5-H bands in the IR spectra of the [EMIM]-halides and of the νC2-H band in the IR and Raman spectra of the tetrafluo- roborate and hexafluorophosphate species and their solutions in dichloromethane. There is not significant vibrational spec- troscopic evidence to suggest H-bonding between the perfluo- rometallate anions and the (C4)H or (C5)H protons. The latter observation could be explained by a low concentration of structures B-D in the liquid state and in solution. This explanation agrees well with the computationally predicted higher energetic stability of structure A in comparison with structures B-D. 8,10,11 Nevertheless, during low-temperature crystallization of [EMIM][BF 4 ], the form A, dominant in the liquid state, is not observed; instead the compound crystallizes with structure C. 12,13 The presence of forms different from A in the [EMIM][BF 4 ] liquid is further confirmed by 2D NMR spectroscopy; 14 HOESY reveals that the anion does not occupy a specific position in this salt. Moreover, in CH 2 Cl 2 , where the PGSE data show * E-mail: skatsyuba@yahoo.com. Fax: +7 8432 7322 53. Telephone: +7 8432 7318 92. † Kazan Scientific Centre of the Russian Academy of Sciences. ‡ Ecole Polytechnique Fe´de´rale de Lausanne (EPFL). CHART 1: Numbering Scheme for the [EMIM] + Cation and Possible Positions of the BF 4 - Anion in the [EMIM][BF 4 ] Ion Pair J. Phys. Chem. B 2009, 113, 5046–5051 5046 10.1021/jp8083327 CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009