Comprehensive Raman Spectroscopic Study of 1-(4-Cyanobenzyl)-3-Methylimidazolium Ionic Liquids and Their Melting Processes

• Main Authors: Chen, Hong-Kai, 陳弘凱
• Other Authors: Shinsuke Shigeto
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/58068252488196421420

碩士 === 國立交通大學 === 應用化學系碩博士班 === 102 === Nitrile-functionalized ionic liquids (ILs) are an interesting family of ILs that show better solvent and thermal properties. In this work, Raman spectroscopy was used to understand the structural aspects of nitrile-functionalized ILs. The laboratory-built Raman microspectrometer used in the present study covers a broad range of vibrational spectra from ~3000 cm−1 (CH stretch), to 500–2200 cm−1 (CN stretch and mainly cation modes), to ~10–200 cm−1 (intermolecular and lattice vibrations), facilitating comprehensive vibrational analysis. We studied five newly synthesized ILs, [cbmim]X, where the cation is 1-(4-cyanobenzyl)-3-methylimidazolium (abbreviated as cbmim) and the anion X is Br−, NTf2−, N(CN)2−, BF4−, and PF6−. According to the Raman spectral profile in the 10–1000 cm−1 range, the five ILs can be classified into three groups. [cbmim]PF6 and [cbmim]BF4 show a very similar profile and they both have a melting point higher than 100 °C. Another group, [cbmim]N(CN)2 and [cbmim]Br, also shows quite a similar spectral pattern. In contrast, the Raman spectrum of [cbmim]NTf2, which has the bulkiest anion among the five ILs studied, substantially differs from those of the other two groups. Detailed analysis of the CN stretch band at around 2230 cm−1 and the CH stretch bands provides qualitative structural information on the ILs. Such information is in particular useful for [cbmim]N(CN)2, for which X-ray crystallographic data are yet to be obtained owing to the technical difficulty in growing single crystals of large size and high quality. We also find that [cbmim]Br and [cbmim]N(CN)2 show a small blue shift of the CN stretch band during melting and that stronger hydrogen bond formation revealed by the low-frequency and CH stretch Raman spectra results in low melting points of those compounds. An opposite trend is found in the other groups having high melting points.