Proton transfer reactions in room temperature molten salts

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

Cambridge, UK

Congreso; Proton conduction in diverse media; 2005

Room temperature molten salts, or ionic liquids (ILs), have recently come into focus as a green alternative to traditional organic solvents. Amongst the various 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.Modern ionic liquids are formed by highly asymmetric organic cations paired with bulky inorganic 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 some research has been performed on the structure of pure ILs both experimentally and using computer simulations. In particular, we have previously studied the structure of the ionic liquid dimethylimidazolium clhoride ([DMIM][Cl]) by means of first principle molecular dynamics.Electrostatic properties and charge dynamics are expected to play a distinctive role in chemical dynamics in molten salts, as compared with neutral solvents. Describing the behaviour of protons in ILs is relevant to understand their acid-base behavior and offers a way to asses the features of chemical reactivity in fluids composed solely of ions.   In the present work we study proton transfer reactions in [DMIM][Cl] using first principle molecular dynamics simulation. Hydrogen chloride (HCl) and bromide (HBr) are considered as our model reactants. In both cases it is found that protons remain strongly associated in the IL as complex anions (HCl2- and HClBr-) are formed. The solvation structure, energetic and dynamics of the proton is described. Furthermoreproton transfer dynamics is analysed by calculating  the free energy profile for the proton transition between neighbouring anions. This last process can have a direct impact on the electrical conductivity of the liquid and possible connections with other structural diffusion  mechanism in hydrogen-bonded liquids are discussed.