Ionic Charge Reduction and Atomic Partial Charges from First-Principles Calculations of 1,3-Dimethylimidazolium Chloride

We present a detailed calculation of partial charges for the 1,3-dimethylimidazolium chloride ionic liquid. We first analyze MP2 electronic structure calculations and DFT results on isolated ion pairs with various methods of assigning partial charges to the atomic centers. In a second run we analyze the trajectory of a 25 ps long Car-Parrinello MD run of 30 ion pairs under bulk conditions using a charge fitting procedure due to Blöchl. Both, the single ion pair and the bulk system, provide us with a similar total ionic charge considerably less than unity. Especially the liquid state DFT results give convincing evidence for a reduced ionic charge on the ions. The similarity of both results suggest that the delocalization of the Cl charge is due only to local interactions. The relevance of our results is 2-fold; on the one hand they shed light on the basic property of the liquid and its reduced ionic character, and on the other hand, the ab initio derived partial charges provide a fundamental theoretical basis for the recent attempts to use the total ionic charge as an adjustable parameter. Furthermore, all our partial charges are subject to large fluctuations, hinting to the importance of polarization effects.