Structure, interactions, and dynamics of 1-ethyl-3-methylimidazolium methyl-phosphonate studied by pulsed NMR spectroscopy and density functional theory

This article presents a study of the structure, interactions, and dynamics of the ionic liquid (IL) 1-ethyl-3-methylimidazolium methyl-phosphonate ([C 2 C 1 im][MPP]) using pulsed nuclear magnetic resonance (NMR) spectroscopy. Density-functional theory (DFT) calculations on [C 2 C 1 im][MPP] in the gas phase were also performed to determine the structure and ion-pair binding energies. The 2D NMR ion-ion correlations indicate that [C 2 C 1 im][MPP] exists as a 3-D hydrogen-bonded network which is consistent with DFT calculations of the ion-pair binding energies. Diffusion coefficients of the ions were determined using Pulse-Gradient Stimulated Spin Echo (PGSTE) NMR. The translational dynamics of the ions in [C 2 C 1 im][MPP] follow fractional Stokes-Einstein (FSE) behavior governed by hydrodynamic sub-slip boundary conditions. Spin-lattice ( T 1 ) and spin-spin ( T 2 ) relaxation measurements were performed with inversion recovery and Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences to determine the rotational correlation times of the ions. Because T 1 and T 2 are not equal, a T 1 / T 2 ratio technique based on the Bloembergen-Purcell-Pound (BPP) equations for dipolar relaxation was used to obtain unique rotational correlation times for the cation and anion. Analysis of the correlation times indicate that the rotational dynamics are well described by the modified-Stokes-Einstein-Debye (M-SED) equation with slip boundary conditions for the cation and anion. Although the DFT calculations and 2D NMR spectra suggest strong ion-pair interactions, the self-diffusion coefficients and rotational correlation times are consistent with the motion of separated ions and not ion-pairs or higher aggregates.