*3.1. Ionic Liquids*

Over the past two decades, ionic liquids have become very versatile and "green" solvents in which hydrogen bonding is also important. This has spurred research into the properties, but also led to new types of ionic liquids.

Moyna et al. [12] studied the deuterium isotope effects at 19F chemical shifts of the counter ions PF6 − and BF4 − caused by deuterium of deuterated side-chains of 1-n-butyl-3 methylimidazolium (Figure 2).

**Figure 2.** 1-n-butyl-3-methylimidazolium PF6 − on top and the BF4 − below. The numbers are the deuterium isotope effects from that particular set of deuterium seen at the fluorine signals of the counter ions. In the case of BF4 −, the effects caused by D-2 and D-3 are actually also caused by D-5. Data for the aliphatic deuterations from Ref. [12] and those caused by deuteration at C-2, C-3 and C-5 from Ref. [13].

The effects caused by deuteration at the aliphatic chain are largest closest to the ring. The effects correlate with the change in polarization of the C-H bonds. The effects are largest for BF4 − as the electron density is largest at the fluorines of this counter ion.

In a similar vein, deuterium isotope effects have been observed at the Cl− chemical shifts when Cl− is the counter ion [14] (Figure 3).

**Figure 3.** Numbers are deuterium isotope effects observed at the Cl resonance caused by the deuterium. Uncertainties ~0.1 ppm. Data from Ref. [14].

Combined *o*-hydroxy Schiff bases and ionic liquids based on amino acids (Figure 4) showed based on <sup>2</sup>ΔC-2(D) isotope effects that the COO<sup>−</sup> group stabilizes the NH-form [15].

**Figure 4.** Ionic liquids based on amino acids.

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In the reinvestigation of the diisopropylethylammonium formate, it was found that the 1:1 complex claimed by Anouti et al. [16] could not be reproduced. A more complex scheme was suggested (Figure 5) [17].

**Figure 5.** Suggested reaction scheme for the formation of the "ionic liquid". Reprinted with permission from Ref. [17]. Copyright 2016 American Chemical Society.

Primary deuterium isotope effects at the OH and NH<sup>+</sup> resonances at 243 K were measured as 2.24 and 0.34 ppm, respectively. These values dropped to 1.08 and 0.30 ppm at 193 K. The mere fact that the OH resonance could be observed and with that, the primary isotope effect, proves that this is not a 1:1 complex. A study of the acetic acid dimer gave a primary deuterium isotope effect of 0.3 ppm and that of the acetic acid-acetate dimer 0.6 ppm [18]. It is obvious, based on an analogy with that result, that the OH(D) primary isotope effects are partly equilibrium isotope effects (Section 4). On the other hand, the NH+ is hydrogen bonded to the monomeric or to the dimeric acetate ion in the same fashion.
