Crystal polymorphism. BmimCl

Crystal polymorphism of bmimCl was reported almost simultaneously by two groups [8,9]. We found by chance that two different types of crystals. Crystal (1) and Crystal (2), formed when liquid bmimCl was cooled down to 18°C and was kept for 48 h [8]. Orthorhombic Crystal (2) dominantly formed but monoclinic Crystal (1) also formed occasionally (see Fig. 2). 

Subsequent to the discovery of the crystal polymorphism of bmimCl, the crystal structures of bmimCl and bmimBr were determined. We determined the crystal structures of bmimCl Crystal (1) and bmimBr at room temperature [11, 12]. Independently, Holbrey et al. [9] reported the crystal structures of bmimCl Crystal (1) and Crystal (2), as well as that of bmimBr at — 100°C. The two sets of structures determined at different temperatures agree well with each other except for the lattice constants that vary with temperature. They also show that the molecular structure of the bmim cation in bmimCl Crystal (2) is different from that in (1) but that it is the same as that in bmimBr, as already indicated by the Raman spectra. In the following, we discuss the crystal structures of bmimCl Crystal (1) and bmimBr as the two representative stmctures at room temperature. 

Saha et. al. [4] and Holbrey et. al.[31] that bmimCl crystal shows two crystal polymorphism. Schematic diagram of bmim cation has been presented in Figure 1. In crystal 1 (Figure 2a), the /i-butyl group takes trans-trans (TT) conformation with respect to Cy-Cg and C8-C9 bonds, while in crystal 2 (Figure 2b), n-butyl group takes gauche-trans (GT) conformation with respect to Cz-Cg and Cg-C9 bonds similar to that of bmimBr conformation. [4, 25] Powdered x-ray diffraction patterns of the two polymorphs of bmimCl and bmimBr was recorded and found that the pattern for crystal 2 and bmimBr somewhat resembles each other, while that of bmimCl crystal 1 is distinct from the other two. [4,21]... 

The Raman spectra of bmimCl Crystals (1) and (2), and that of bmimBr, are compared in Fig. 4 [8]. The polymorphs of bmimCl give two distinct Raman... 

---