Surface of room temperature ionic liquid [bmim][PF_6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy  

作　　者：Ganghua Deng Youqi Guo Xia Li Zhen Zhang Shilin Liu Zhou Lu Yuan Guo 

机构地区：[1]Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China [2]Nanjing University of Aeronautics and Astronautics [3]Beijing National Laboratory for Molecular Sciences [4]State Key Laboratory of Molecular Reaction Dynamics [5]Institute of Chemistry, Chinese Academy of Sciences

出　　处：《Science China Chemistry》2015年第3期439-447,共9页中国科学（化学英文版）

基　　金：supported by the National Natural Science Foundation of China(21073199,91027042,21227802);the National Basic Research Program of China(2013CB834504)

摘　　要：Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids(RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy(SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the(N)–CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up(N)–CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids（RTIL） systems. In this article, the orientation of 1-butyl-3-methylimidazolium（[bmin]） cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate（[bmim][PF6]）, was investigated by the sum frequency generation vibrational spectroscopy（SFG-VS）. Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the（N）–CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up（N）–CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.

关 键 词：ionic liquid sum frequency generation vibrational spectroscopy interface ORIENTATION 

分 类 号：TQ413.2[化学工程] O657.3[理学—分析化学]