The effect of some amines and alcohols on the organized structure of [bmim][BF4] investigated by 1H NMR spectroscopy

Abstract

The effect exerted by some amines and alcohols on the 1 H NMR spectra of 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] has been studied. This ionic liquid, which is one of the most widely used, is characterized by a high structural order degree, as a consequence of the symmetry and the coordination ability of the anion. In order to have information about the dependence of the detected effects on the alcohol or amine structure, some different primary, secondary and tertiary amines and alcohols have been considered. Furthermore, in the case of amines, their basicity has been also taken into account. Both amines and alcohols induce variation in chemical shifts values and signal multiplicity of imidazolium protons. Collected data show that, among imidazolium protons, the aromatic ones are the most affected by the presence of organic molecule. Cross-correlations among chemical shift values relevant to these protons allow us to explain how 1 H NMR spectrum variations depend on probe structural properties.

Probing structural patterns of ion association and solvation in mixtures of imidazolium ionic liquids with acetonitrile by means of relative1H and13C NMR chemical shifts

“…14,16,27,38,69 However, from the point of view of interpretation, the most relevant reference concentration is that of the corresponding neat compound, and it is the most frequently used approach. 26,32,40,41,48,[70][71][72][73][74] We combined the two ideas outlined above, e.g., the relative chemical shift and the reference concentration in the concept of 'relative chemical shift variation'. It implies that the initial referencing should be done to one of the studied molecule's signal, terminal CH 3 for Rmim + cation and the most shielded group for a solvent (e.g., CH 3 in AN, MeOH).…”

“…Within the proposed approach of looking at the chemical shift changes we have summarized the literature observations on 'imidazolium IL-molecular solvent' systems which can be analyzed in this way (i.e., where the observed absolute chemical shifts of the signals of interest, like those of the imidazolium ring hydrogen atoms, are published along with the terminal methyl group chemical shifts). The relative chemical shift variations DDd i (x) of the imidazolium ring hydrogen atoms were found to be negative in most of the studies which concern the following systems: RmimHal-H 2 O, 63,69,[74][75][76] 36,77,78 HexmimBr-Ac-d 6 , 38 EmimTfO-Ac-d 6 , 25 BmimBF 4 -MeOH, 32 BmimOctSO 4 -ethylene glycol, 79 BmimBF 4 -AN, 48 EmimHal-AN, 24,54 RmimBF 4 /PF 6 -thiophene, 26 Dodecmim-TFSI/NO 3 -C 6 H 6 . 31,56 Nevertheless, there are as well reports where the imidazolium ring hydrogen atoms exhibit more significant deshielding than the terminal methyl ones, i.e., positive relative chemical shift variations are observed: EmimTFSI-Ac-d 6 , 25 RmimPF 6 /BF 4 -Ac-d 6 , 27 EmimTFSI-MeOH/DMSO/H 2 O.…”

“…Formation of such cation-solvent hydrogen bonds upon dilution of IL would contribute to deshielding of the imidazolium ring and of the adjacent alkyl hydrogen atoms. 28,32,42 Apparently, the third and second effects are opposing each other and they are localized at the imidazolium ring and the adjacent hydrogen atoms. Hence, when referencing to the terminal methyl hydrogen atoms which are only prone to the first, non-specific effect, the net observation on the relative chemical shift variation of the hydrogen atoms of the imidazolium ring should reflect the balance between the second and third effects.…”

“…This is the most frequently used approach for IL-molecular solvent mixtures. 24,26,32,[46][47][48] In this technique, a coaxial capillary containing the reference compound and a deuterated lock-solvent is inserted into the NMR tube containing the sample itself. Just as in internal referencing, the deuterated solvent can serve as both lock-compound and chemical shift reference (due to residual protonated solvent molecules for 1 H-NMR).…”

“…Most of the published to date studies dealing with concentration dependence of chemical shifts in mixtures 'imidazolium IL-polar molecular solvent' did not cover a wide range of systems in a systematic manner. Among the most remarkable investigations on the influence of IL (imidazolium cation alkyl chain length and/or nature of the anion) or that of the solvent on the concentration trends of the chemical shifts of cations over the whole range of compositions it is worthy to mention studies on RmimBF 4 /PF 6 in thiophene, 26 RmimPF 6 in Ac-d 6 , 27 RmimTFSI in AN, MeOH, DMSO, and aromatic solvents, [28][29][30][31] BmimBF 4 with alcohols and amines, 32 BmimPF 6 in a set of polar aprotic solvents. 14 As a result, there is still no clear and comprehensive understanding of the influence of the nature of constituents of IL-solvent mixtures on the concentration trends in chemical shifts and, subsequently, on the underlying intermolecular interactions.…”