NMR Relaxation Studies of the Rotational Dynamics of 1-Ethyl-3-Methylimidazolium Methyl Phosphonate and its Binary Mixtures with Methylimidazole

Edward QUITEVIS¹, Sophia SAGALA¹, Jagdeep KAUR¹, Noah WILLIAMS¹
¹Texas Tech University, Lubbock, United States

The rotational dynamics of both the cation and anion in the ionic liquid (IL) 1-ethyl-3-methylimidazolium methyl phosphonate ([C₂C₁im][(OMe)(H)PO₂]) and its mixtures with methylimidazole (MIM) were studied using NMR relaxation techniques. The spin-lattice relaxation rate 1/T₁ and the spin-spin relaxation rate 1/T₂ of the C2-hydrogen on the cation and the hydrogen on the phosphorous atom of the anion, as well as the viscosity $\eta$ of the neat IL and that of the IL/MIM mixtures were measured as a function of temperature T. The NMR relaxation rates do not vary linearly with the ratio $\small \eta/T$ , indicating NMR relaxation is not in the extreme line-narrowing regime. The rotational correlation time $\small \tau _{c}$ was obtained by fitting the ratio T₁/T₂ versus $\small \eta/T$ by an equation with the relaxation rates given by the Solomon equations for dipolar relaxation. Good fits were obtained when $\small \tau _{c}$ is assumed to follow a power law $\small \small (\eta /T)^{\alpha }$ for both the cation and anion with the exponent $\small \alpha$ less than one in the neat IL and approaching one with increasing dilution in the IL/MIM mixtures. This suggests that the rotational motion of the ions is governed by dynamic heterogeneity in the neat IL which then approaches Debye-Stokes-Einstein behavior with increasing dilution in the IL/MIM mixtures. To obtain a molecular level understanding of the increase of $\small \alpha$ with increasing dilution, ion-ion and ion-MIM interactions were studied by performing 2D NOE measurements (HOESY/NOESY) on the IL and IL/MIM mixtures.