Theoretical Research Of1-n-alkyl-3-methylimidazolium Ionic Liquids’ Structures And Properties

In the past decade, ionic liquids (ILs) have been attracting more and more attention. As a new field, room-temperature ionic liquids (RTILs) broaden our horizons in chemistry, particularly in green chemistry. They possess desirable chemical and physical properties, such as good solubility and negligible vapor pressure. Because of the excellent characteristics, RTILs account for the increasing interest in application. For example, they can be used as solvents for organisms, lubricants, and liquid crystals. The theoretical study of ionic liquids contribute to a deeper understanding of ionic liquid from microscopic review. It provides important supports for the interpretation of the relationship between the nature and structure of ionic liquids as well as the excellent performance of the ionic liquid.In this paper, we respectively use quantum method and molecular simulation method to research the structure of the ionic liquid’s multiple ionic pairs and the network properties of hydrogen bonds. The valuable conclusions are as follows,1. The optimized structures of the[1-n-alkyl-3-methylimidazolium][BF4] ion pairs were obtained from calculations of the Gaussian03program. These ion pairs included single ion pair, multiple ion pairs and also included different number of cations and anions. After attaining the stable structures, we analyzed the relative positions of cations and anions and the influences of anions’ existence to the cations. The result shows that the anions prefer to locate close to the imidazolium rings and the methyl, especially close to the imidazolium hydrogen and the methyl hydrogen. The strongest hydrogen bond appears close to hydrogen of the C3. Multiple ion pairs optimization results show that the anions does affect the conformation of the cations. The anions cause the bending of the cations’ alkyl chain. But the Bending amplitude is not the same for different initial configurations. If the ions were tightly arranged in the initial configurations, the bending of the cations’ alkyl chain was apparent. Conversely, If the ions were loosely arranged in the initial configurations, the bending of the cations’alkyl chain was small. 2. Molecular simulations were carried out by the Gromacs4.0.5program to the six [1-n-alkyl-3-methylimidazolium][BF4] ionic liquids. Each simulation contains216ion pairs. The force field parameters were taken from the work of Lopes and Padua which were based on the OPLS-AA/AMBER. The last snapshot of the balanced configuration was taken and all the hydrogen bonds were then printed out by HBAT package. We used the Cytoscape program to analyze the hydrogen bond network and a two dimensional view of the hydrogen bond network was attained. We manually screened a stereo view of cations and anions with zero degree using VMD program. The connection pattern shows that the average length of line shape connection is2.44to2.77for six1-alkyl-3-methylimidazolium ionic liquids, and the connection patterns are different for short and long alkyl side chain length. These properties are closed related to the physical properties of ionic liquids. The degree of each ion was calculated and analyzed. Most of nodes have a degree of1-3, which indicates that the connection of network forms line and T shape and forms a branched tree. The nodes with zero degree were adopted to detect the boundary of the clusters in the ionic liquids, which have no hydrogen bond connecting with neighbor ions. This work indicates that the network analysis method is useful for understanding and predicting the structure and function of RTILs.