Typical 2 H spectra recorded at 233 K are shown in Fig. , exhibiting a Pake patterns and a small narrow line simultaneously. The 2 H NMR patterns of the high temperature solid phase showed that part of the molecules are still rigidly bound in ground state I (also denoted as static state), while others are found in a highly mobile state II (also denoted as mobile state). From the analysis of the 2 H NMR spectra line shape we can deduce thermodynamic properties characterizing the transition from the rigid to the mobile state of the PIL. The relative populations of the rigid [TEA] + cations pstatic and the mobile fraction pmobile are proportional to the relative intensities of anisotropic and isotropic components of Thus, the present PILs can be regarded as plastic crystals exhibiting states of coupling intermediate between static and mobile units, with a definite three-dimensional lattice, but with rotational motion of the same fraction of the species possible. They have long-range order but short-range disorder, which typically originates from rotational motions of the molecular ions, already present in the solid phases. It has been shown earlier that mainly the cation is determining plasticity of an ionic liquid. The mobile/static equilibrium is described by the equilibrium constant Keq=pmobile/pstatic as a function of temperature. Here pmobile and pstatic represent the corresponding populations and are shown in Fig. . Note that temperature region associated with the transition is centered around 236 K for [TEA-d1][OMs] and about 316 K for [TEA-d16][OMs], which represents an astonishing 80 K shift in temperature due to the deuteration of the alkane chains. As mentioned earlier, a visual inspection of both transitions in Fig. is also suggesting that the transition for [TEA-d16][OMs] is extending over a broader temperature range than for [TEA-d1][OMs]. The molar standard Gibbs free energy difference D𝐺 ! (𝑇) = -𝑅𝑇 ln 𝐾 "# associated with the coexistence of both states obviously has to be temperature dependent for describing such a transition. In the simplest scenario (denoted as the "van 't Hoff scenario") the temperature dependence of D𝐺 $ (𝑇) could be expressed by temperature independent molar standard transition enthalpies and entropies with D𝐺 ! (𝑇) = D𝐻 ! -𝑇D𝑆 ! . In such a case, the data in the van 't Hoff plot shown in Fig. should follow a strictly linear behaviour. Since this condition is not perfectly fulfilled, we also consider a second order expansion of the molar standard Gibbs free energy difference according to D𝐺