Study of the super acidic properties of imidazolium based ionic liquids by molecular UV-Visible and Raman spectroscopies

Abstract

ILs have attracted attention in the last four decades due to their attractive properties, such as non-volatility, specific solvation abilities and super-acidic properties. These unique features explain the use of ILs in a wide range of applications, including as matrices in Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS), among others. However, this application is highly dependent on the properties of the ILs, and especially on their acidic properties. The main purpose of this Master’s Thesis is to determine the acidity level accessible in several second-generation ILs and to correlate the acidity with their efficiency in ionising analytes in MALDI-MS. Therefore, the synthesis of three imidazolium-based ILs (1-ethyl, 1-butyl or 1-octyl-3-methylimidazolium cations) coupled to three anion types (bis(trifluoromethane)sulfonimide, NTf2; 2,5-dihydroxybenzoic acid, DHB and α-cyano-4-hydroxycinnamic acid, CHCA) was conducted. Thereafter, the acidity level of the ILs was determined by UV-Visible and, for the first time, Raman spectroscopies. The solvation number of the proton was also determined by Raman spectroscopy. Finally, the capability of ionisation and detection of polyethylene oxide (PEO) and rhodamine B (RhB) in MALDI-MS using some ILs were studied and compared to two usual solid matrices (CHCA and DHB). The acidity levels determined by UV-Visible spectroscopy showed a trend between the NTf2-based ILs, but only one acidity value was obtained for the same ILs by Raman spectroscopy. The NTf2-based ILs were inactive in MALDI-MS, as expected from the fact that they do not absorb the excitation UV laser used for MALDI measurements. ILs based on DHB and CHCA anions were not as effective as expected, which is assumed to originate from the presence of bromide alkaline impurities, despite an extensive purification of the ILs.