Peculiarities of NMR relaxation in ionic liquids: Difficulties in interpretation and novel possibilities

An applicability of ¹³C and ¹H relaxation rates (1/T ₁) to direct calculations of the characteristic reorientation time (τ_c) in imidazolium-based ionic liquids has been analyzed. It has shown that ¹³C NMR relaxation technique can be applied to ionic liquids as successfully as it took place for other liquid systems and allows one to get τ_c values for each carbon directly. The corresponding ¹H data are affected by dipole-dipole interaction only at “higher temperature” range while at lower temperatures spin-diffusion process controls the proton relaxation. Both carbon and hydrogen 1/T ₁ dependences are suitable for calculation of τ_c and reveal equal numerical values for a number of functional groups of [emim]Ac at the proper temperature range. On the other hand, ¹H relaxation curves of some functional groups allow one to detect motions, unobservable in the carbon relaxation and, thereby, to extract more information concerning details of the dynamics of the [amim]⁺ cations.