New evidences about conformational equilibrium in ammonium-TFSI ionic liquids: the X-Ray scattering patterns of TMHA- and TMPA-TFSI interpreted with molecular dynamics simulations

The X-Ray scattering patterns of the two ionic liquids, N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) and N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI), sharing a common anion and differing in the length of the alkyl chain of the cation, were measured at room temperature. Molecular dynamics calculations supported the interpretation of the data. The two force field models, GAFF and DLPOLY 4, were adopted to simulate the scattering patterns. Both of them are able to reproduce the main peaks of the experimental data; however, the DPOLY model seems to better reproduce the finer details. Moreover, from the simulations, the concentration of the two conformers of TFSI are derived. The comparison with previously reported infrared spectroscopy measurements suggests that also under this aspect the DLPOLY model has a better agreement with the experiments.