Structure and Dynamics of 1-Ethyl-3-methylimidazolium Acetate via Molecular Dynamics and Neutron Diffraction

The liquid state structure of the ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), an excellent
nonderivitizing solvent for cellulosic biomass, has been investigated at 323 K by molecular dynamics (MD)
simulation and by neutron diffraction using the SANDALS diffractometer at ISIS to provide experimental
differential neutron scattering cross sections from H/D isotopically substituted materials. Ion-ion radial
distribution functions both calculated from MD and derived from the empirical potential structure refinement
(EPSR) model to the experimental data show the alternating shell structure of anions around the cation, as
anticipated. Spatial probability distributions reveal the main anion-to-cation features as in-plane interactions
of anions with the three imidazolium ring hydrogens and cation-cation planar stacking above/below the
imidazolium rings. Interestingly, the presence of the polarized hydrogen-bond acceptor (HBA) anion (acetate)
leads to an increase in anion-anion tail-tail structuring within each anion shell, an indicator of the onset of
hydrophobic regions within the anion regions of the liquid. MD simulations show the importance of scaling
of the effective ionic charges in the basic simulation approach to accurately reproduce both the observed
experimental neutron scattering cross sections and ion self-diffusion coefficients.