L-butyl-3-methylimidazolium BMIM

Ionic liquids, having per definition a melting point below 100 °C, and especially room temperature ionic liquids (RTIL) have attracted much interest in recent years as novel solvents for reactions and electrochemical processes [164], Some of these liquids are considered to be green solvents [165]. The scope of ionic liquids based on various combinations of cations and anions has dramatically increased, and continuously new salts [166-168] and solvent mixtures [169] are discovered. The most commonly used liquids are based on imidazolium cations like l-butyl-3-methylimidazolium [bmim] with an appropriate counter anion like hexafluorophos-phate [PFg]. Salts with the latter anion are moisture stable and are sometimes called third generation ionic liquids. 

Certain quaternary salts of imidazoles have become important as ionic liquids, in particular salts of l-butyl-3-methylimidazolium (bmim). Preparation of imidazolium-containing ionic liquids, in the desired product ionic liquid as a solvent, is a virtually solvent-free, one-pot process proceeding in good to excellent yields <2003S2626>. There are also microwave-assisted, solvent-free processes for the preparation of imidazolium-based ionic liquids <2001CC643, 2003GC181>. 

Figure 2.17 Quantitative synthesis of l-butyl-3-methylimidazolium [Bmim] ionic liquids via solvent free anion metathesis.

Hunt et al. have used ab initio methods to study ion pairs in l-butyl-3-methylimidazolium (Bmim) ILs. The anions were Cl, BF4 , and NTf2". The authors established relationships between ion-pair association energy and a derived parameter called the connectivity index . Overall, the results suggest that Bmim-Cl forms a strongly connected and quite highly structured network, which leads to the rather high viscosity observed experimentally. In contrast, Bmim-NTf2 only forms a rather weak network, where the connectivity and the viscosity are much lower [106],... 

The catalytic efficiency of the RTILs derived from l-butyl-3-methylimidazolium (bmim) cation is influenced by the structure of the imidazolium moiety and the counteranion following the order [bmim][MeSO ]>[bmim][HSO ] [bmim]... 

An ionic liquid (IL) is literally an ionic compound (a salt) that is a liquid. Of most current interest are salts that are liquids at room temperature (RTILs), or at least below 100 °C. There is a range of compounds that form room temperature ionic liquids dating back to ethanolammonium nitrate, (EtNH3)+(N03) (m.p. 14 °C), synthesised by Walden in 1914. Perhaps the most popular and well-studied are those based on the l-butyl-3-methylimidazolium (bmim) cation, such as brnim PFg (13.20) and bmim+BF4 which melts at ca. -80 °C. The imidazolium ionic liquids were initially used as their halogenoaluminate salts but they have a major drawback in that they are highly moisture sensitive. 

Scheme 1 Proposed metabolic pathways of the l-butyl-3-methylimidazolium ([BMIM]) cation.

Table 11.4 Melting Point and Decomposition Temperature of Four l-Butyl-3-Methylimidazolium (bmim" ) Salts ...

Fig. 3. IRNMR shift of l-butyl-3-methylimidazolium (bmim) cation with various halide anions.

Fig. 1. Optimized structures of trans-trans (tt) and gauche-trans (gt) conformers of l-butyl-3-methylimidazolium ([bmim]) cation by B3LYP/6-311G+(d) level.

Next, we compare l-butyl-3-methylimidazolium [BMIm] based ILs with a series of hexafluoropnictogenate anions ([XFa]" X is P, As, or Sb) (Shirota et al., 2009). As discussed above, the substitution of C by Si in the cation shifts the first moment of the low-frequency spectrum of an IL to a lower value. However, a change in the dipole moment of an ion caused by atom substitution gives a difference in the spectra. Because the hexafluoropnictogenate anion is octahedral, the substitution of a center atom affects the mass and volume (mean polarizability) of the ion, but does not affect the shape (dipole moment and polarizability anisotropy). Thus, it is more ideal to simply examine the atom substitution effects caused by the mass and volume on the interionic vibration in the ILs. 

FIGURE 5.1 (See color insert.) Electrostatic potential field around various cations as calculated at the PW91/DNP level, (a) l-Ethyl-3-methylmiidazolium (EMIM), (b) l-butyl-3-methylimidazolium (BMIM), and (c) l-hexyl-3-methyltniidazolium (QMIM). 

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