Ionic liquids from Car-Parrinello simulations. 2. Structural diffusion leading to large anions in chloraluminate ionic liquids.

We present Car-Parrinello molecular dynamics simulations of the liquid imidazolium chloride/AlCl3 by inserting one pair of [C2C1im]Cl into liquid AlCl3, forming an acidic mixture. Two different starting conditions lead to two trajectories from which we harvest structural data. For both simulations, we find large anions within the equilibrium phase: In both trajectories at longer simulation time, the anion size converges to four monomer units, i.e., to Al4Cl13-. The cluster size fluctuations indicate that Grotthus diffusion must play a role. We discuss one possible mechanism of such a reaction changing the anionic species. This process involves many steps of chlorine rattling, bond breaking, and bond forming. With the aid of the electron localization function, a probable rationale for the formation of larger anions is determined: Large anionic species are formed simply to account for the "lack of electrons" present in the acidic melt.