Effect of the alkyl chain length in 1-alkyl-3-methylimidazolium ionic liquids on inter-molecular interactions and rotational dynamics A combined vibrational and NMR spectroscopic study

We have investigated the effect of increasing the alkyl chain length, n, in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (CnC1-ImTFSI) ionic liquids on both inter-molecular interactions and rotational dynamics. This has been addressed by combining 1D H-1 and T-1 relaxation NMR spectroscopy with vibrational spectroscopy, including both Raman and infrared. We find that the vibrational modes sensitive to inter-molecular interactions change only slightly with an increased alkyl chain length, with negligible changes for chains longer than the hexyl (n > 6). This is nicely corroborated by 1H chemical shift changes, in particular of the aromatic protons. The C-13 correlation times, tau(c), of the individual carbon atoms reveal that while the overall rotational mobility decreases with n, different dynamical properties are observed with the polar domains becoming the most dynamic and the imidazolium ring and its closest chain segment the most rigid. We conclude that the formation of segregated nano-domains in C(n)C(1)ImTFSI is accompanied by the formation of nano-dynamical heterogeneities, which can explain why classical theories fail to describe the dependence on n of macroscopically observed properties. (C) 2015 Elsevier B.V. All rights reserved.