Chapter 1
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Investigation of the Structure of 1-Butyl-3methylimidazolium Tetrafluoroborate and Its Interaction with Water by Homo- and Heteronuclear Overhauser Enhancement Andrea Mele Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta" Politecnico di Milano, Via L. Mancinelli, 7 I-20131 Milano, Italy
The application of some N M R techniques based on homonuclear 1H{1H}- and heteronuclear 1H{19F}-NOE are illustrated for the room temperature ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate [bmim] [BF ]- in the presence of known amount of water. The experimental results on this model system provide information on cation-cation, cation-anion, water-cation and water-anion interactions. The results are discussed in terms of non-covalent C -H...Ο and C— H...F and O - H...F interactions. The results are compared to experimental and theoretical data taken from the most recent literature on similar systems. +
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© 2005 American Chemical Society
In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 3, 2015 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch001
3 The use of room temperature ionic liquids (RTILs) as non-volatile reaction media for synthesis and catalysis is constantly gaining popularity in both academia and industry (1-4). Nevertheless, their unique physical and physicochemical properties, as well as the mechanism of interaction with molecular solutes, are far from being fully understood. This, in turn, can severely hamper one of the most fascinating features of RTILs, that is the possibility of "designing" specifically targeted new solvents due to the large number of combinations of anions and cations potentially able to afford new RTILs (J, 6). The rational design of new RTILs, and the choice of optimum RTIL for a given synthetic or catalytic process, require die knowledge of the structure of the liquid at atomic level in terms of type and intensity of intermolecular interactions within the liquid phase, the interaction sites and the way the intermolecular interactions affect or determine the behavior of the RTIL in the presence of a given solute. In a schematic way, a detailed study of the structure of RTILs should provide data concerning cation-cation, cationanion, solute-cation and solute-anion interaction. Special interest is aroused by water as solute, litis is basically due to the fact that many RTILs, either hydrophilic or hydrophobic, are known to absorb relevant amount of water from the environment. It is proven that important physicochemical properties of RTILs are dependent on the amount of the absorbed water. A detailed description of the influence of dissolved water on viscosity, conductivity, polarity, rate and selectivity of chemical reactions carried out in ionic liquids is beyond die purpose of this chapter. For the sake of clarity suffice it to mention the variation of RTILs' polarity (7) and the change of CO2 solubility in RTILs (8) in the presence of water. The latter parameter is of interest for die application of carbon dioxide for the separation of RTILs from organic mixtures (9,10). Hie purpose of this chapter is to illustrate die application of some NMR methods based on the detection of homonuclear and heteronuclear Overhauser enhancement (or effect, NOE) to the structural problems mentioned above. To this end, some applications of H{*H}- and ^ { ^ F J - N O E experiments in one and two dimensions for the investigation of 1 -butyl-3-methylimidazolium tetrafluoroborate [bmim] [BF ]~ (compound 1, Figure 1) and its interaction water (11) are exposed in detail as a case study. The discussion is largely focused on the aspects concerning the repertoire of intermolecular interactions consistent with the experimentally observed NOEs, such as "weak" hydrogen bonds (12% as possible source of local structure of the liquid and their role in the mechanism of interaction with water. The experimental results are discussed and compared to other experimental and theoretical data of the most recent literature on the same systems or on structurally related RTILs (Figure 1). !
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In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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The Structure of RTILs by NMR spectroscopy: Applications of Nuclear Overhauser Enhancement The physical principles of the nuclear Overhauser effect, the applications of NMR experiments based on NOE and the details of the pulse sequences used for NOE experiments are extensively reported in fundamental textbooks {1315) and will not be repeated here. In the first part of this section some examples of application of two-dimensional homonuclear NOE correlation spectroscopy in the laboratory frame (NOESY) and in the rotating frame (ROESY) are proposed. The latter is often used in order to avoid the region of vanishing NOE corresponding to the condition
