Raman Spectroscopy study of imidazolic ionic liquids: interionic interactions, structural organization and micro-environment effects

Author(s)

Fabio Rodrigues

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ)
• Defense date: 2010-09-02

Examining board members

Paulo Sérgio Santos; Koiti Araki; Fernanda Ferraz Camilo; Antonio Martins Figueiredo Neto; Roberto Manuel Torresi

• Advisor: Paulo Sérgio Santos

Abstract

The present work aims to be a systematic study, by Raman spectroscopy, of imidazolic ionic liquids, which means low temperature molten salts derived from imidazole ring. The study involved the cations 1-alkil-3-methylimidazolium and 1-alkyl-2,3-dimethylimidazolim, being alkyl chain composed of 2, 4, 6, 8 or 10 carbon atoms and with the anions bromide, hexafluorophosphate and bis(trifluoromethanesulfonyl)imide (TFSI-), in a grand total of 30 species with distinct properties. The study has been separated in three steps. The first one aimed to study both neutral and cationic molecules derived from imidazolic ring to understand the difference between cations that can form ILs and their precursors. Raman spectroscopy gave information about the changes in the vibrational modes with the substitutions. The results show that the intensity of modes from the ring decreases with the substitutions, while the contribution of -CH modes increases. In accordance with this are the results obtained from theoretical calculation of Mulliken charge, used as an auxiliary technique. The second step consisted in the study of pure ILs, analyzing both cation and anion effects. The carbonic chain plays a major role in Raman spectra, exhibiting bands attributed to different conformers. These bands become more abundant and less intense when the chain length increases. The anions have been studied in both ILs and inorganic salts, being observed very similar spectra for all the ILs but different ones for the inorganic salts. These results have been confirmed by XANES (X-ray absorption near edge structure) spectroscopy, which makes possible to probe the electronic structure of different atoms. After the analysis of both cation and anion, it was possible to better understand the ion pair formation in ILs, focusing in hydrogen bonds in the carbonic chain, since the Coulombic interaction is too weak. For ILs with hydrogen in carbon 2, it could be observed that bromide anion forms a stronger ion pair when compared to PF6- and TFSI-, less coordinating anions. The addition of methyl group in carbon 2 modifies this behavior, and even bromide does not form the strong ion pair. At last, the third step focused in the better understanding of binary systems containing ILs and a molecular solvent, dimethylformamide (DMFA), used also as probe since the shift of the band attributed to C=O group was used in this study. In a first stage it has been studied equimolar solutions of ILs and DMFA, in which it has been observed the organization of ILs in the mixture, and the contribution of both cation and anion. The second stage was dedicated to the understanding of these mixtures with different molar fraction of ILs and DMFA, for which some ILs have been selected. For low concentration of ILs, their behavior was similar to molecular solvents, but when the concentration of IL was increased, an anomalous behavior in the shift of C=O band has been observed, which can be attributed to the ionic character of these systems. (AU)