Synthesis, NMR, IR, Raman spectra and DFT calculations of 1‐octyl‐1,4‐diazabicyclo [2.2.2] octan‐1‐ium bis(trifluoromethylsulfonyl)imide

Abstract

In this study, a novel [C8DABCO+] [(CF3SO2)2N−] ionic liquid (IL) was prepared by an alkylation reaction and then characterized it by experimental 1H-, 13C-, and 19F-NMR, IR and Raman spectroscopies together with B3LYP/6-311G** calculations in gas phase and aqueous solution to confirm its structure. DSC and TGA measurements show that it can be classified as a thermally stable ionic liquid. Three different structures (I), (II) and (III) were proposed for the IL with Cis and Trans positions of CF3 groups of anion evidencing better correlations among experimental and predicted spectra for structure (III). Strong S-O···H interactions were predicted in the gas phase while in solution the C-F···H distance is shorter (2.654 Å) than the S-O···H ones (2.827 Å). The analysis of MK charges reveals the presence of positive charges on quaternary N23 atoms both in vaccum and in solution; conversely positive Mulliken charges reside on O49 and O50 in the gas phase, while negative ones are found in solution. Molecular Electrostatic potential (MEP) surfaces support the electrophilic sites on quaternary N23 while the nucleophilic sites are observed on O, N and F atoms of the anion. NBO and AIM calculations reveal the characteristics of cation-anion interactions in the two media, as supported by the different f(νN-C), f(νSO2), f(δSO2) and f(δCH2) force constants values. Gap values suggest that [C8DABCO+] [(CF3SO2)2N−] is less reactive than [C8DABCO+] [Br−] while the characteristics of frontier orbitals justify the cation-anion interactions and support the important role of 1,4-diazabicyclo [2.2.2] octane (DABCO) in the properties of [C8DABCO+] [(CF3SO2)2N−] IL in both media. The complete assignments of 174 vibration modes expected for [C8DABCO+] [(CF3SO2)2N−] IL are reported together with the scaled force constants.