A new QM/MM method oriented to the study of ionic liquids

The interest on room temperature ionic liquids has grown in the last decades because of their use as all-purpose solvent and their low environmental impact. In the present work, a new theoretical procedure is developed to study pure ionic liquids within the framework of the quantum mechanics/molecular mechanics method. Each type of ion (cation or anion) is considered as an independent entity quantum mechanically described that follows a differentiated path in the liquid. The method permits, through an iterative procedure, the full coupling between the polarized charge distribution of the ions and the liquid structure around them. The procedure has been tested with 1-ethyl-3-methylimidazolium tetrafluoroborate. It was found that, similar to non-polar liquids and as a consequence of the low value of the reaction field, the cation and anion charge distributions are hardly polarized by the rest of molecules in the liquid. Their structure is characterized by an alternance between anion and cation shells as evidenced by the coincidence of the first maximum of the anion-anion and cation-cation radial distribution functions with the first minimum of the anion-cation. Some degree of stacking between the cations is also found.