Effects of Methylation at Position 2 of Cation Ring on Rotational Dynamics of Imidazolium-Based Ionic Liquids Investigated by NMR Spectroscopy: [C4mim]Br vs [C4C1mim]Br

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-04, Vol.115 (14), p.2999-3005
• Main Authors: Endo, Takatsugu, Imanari, Mamoru, Seki, Hiroko, Nishikawa, Keiko
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Cations - chemistry; Imidazoles - chemistry; Ionic Liquids - chemistry; Magnetic Resonance Spectroscopy; Methylation; Rotation; Thermodynamics
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp200635h

We investigated the rotational dynamics of two imidazolium-based ionic liquids, 1-butyl-3-methylimidazolium bromide ([C4mim]Br) and 1-butyl-2,3-dimethylimidazolium bromide ([C4C1mim]Br), to reveal the effects of methylation at position 2 of the imidazolium ring (C(2) methylation). The rotational correlation time (τlocal) for each carbon in the cations is derived from the spin−lattice relaxation time of 13C nuclear magnetic resonance. The τlocal results obtained here provide three principle insights into the rotational dynamics of ionic liquids. First, all τlocal values for [C4C1mim]Br are greater than those for [C4mim]Br owing to a viscosity increase due to C(2) methylation. Second, the rate of change in τlocal on C(2) methylation differs among the carbons in the cation, which indicates that each carbon has a different microviscosity. Third, the τlocal increase in the 13C at the root of the butyl group on C(2) methylation is very small compared to both intuitive prediction and the results from quantum chemical calculations. This indicates that the motion of the butyl group root in [C4C1mim]Br is not significantly inhibited by the methyl group at the position 2 of the imidazolium ring. The finding provides conclusive information on the origin of the increases in the melting point on C(2) methylation. Hunt previously found through calculation that decreases in entropy are caused by two factors, namely, reductions in the rotational mobility of the butyl group and in the number of stable anion interaction sites with C(2) methylation, resulting in an increase in melting point and viscosity. Our finding experimentally illustrates that the origin of the increases in melting point is not the inhibition of butyl group motion and that the reduction in stable anion interaction sites plays a major role in the increases. Additionally, it is suggested that the viscosity increase on C(2) methylation can be interpreted in the same manner.