The state of the proton in room temperature ionic liquids

M. G. DEL PÓPOLO , J. KOHANOFF AND R. M. LYNDEN-BELL

Schwabisch Gmund, Germany

Congreso; Psi-k 2005 Conference "Towards Atomistic Materials Design"; 2005

Room temperature molten salts, or Ionic Liquids (ILs), have recently come into focus as an alternative to the traditional solvents used for chemical synthesis. Amongst the properties of ILs, the most relevant is their low melting point concomitant with a negligible volatility. Fluidity at room temperature allows, in contrast with classical molten salts, to perform chemical reactions in them without major technical limitations. Ionic Liquids are formed by large and asymmetric cations paired with bulky anions. Their structure results from a balance between molecular packing and screening which in turn depends sensibly on the intermolecular interactions. In the past years the structure of pure ILs has been studied both experimentally and using computer simulations. In particular, we have studied the structure of the ionic liquid dimethylimidazolium chloride ([DMIM][Cl]) by first principles molecular dynamics. Electrostatic properties and charge mobility may play a distinctive role in chemical dynamics in molten salts, as compared with neutral solvents. In this work we study, using first principle molecular dynamics,  proton transfer reactions in [DMIM][Cl]. Hydrogen chloride (HCl) and bromide (HBr) are considered as model reactants.In both cases it is found that complex anions (HCl2- and HClBr-) are formed, indicating that protons remain strongly associated in the molten salt. The solvation structure, energetic and dynamics of these anions is described. Furthermore proton transfer is analysed by calculating the free energy profile for the proton transition between neighbouring anions. This last process may have impact on the electrical conductivity of the fluid and can be analysed in terms of structural diffusion mechanisms known from hydrogen-bonded liquids.