Simulating chemical reactions in room temperature molten salts

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

Lednice

Congreso; 7th Liblice Conference on the Statistical Mechanics of Liquids; 2006

Room temperature ionic liquids (ILs) are fluids composed solely of ions that melt at temperatures below 100 °C [1,2]. The possibility of combining different cations and anions results on a wide variety of ionic liquids which are of interest for physico-chemical studies and industrial applications. The fluidity of ILs at ambient temperature permits their use as solvents for chemical reactions, allowing to explore the action of purely ionic media on chemical reactivity [1,2]. Ions can affect the course of a chemical reaction on a variety of ways, ranging from the solvation of the reaction complex, to the efficiency of the ionic dynamical response to changes in the charge distribution of the reactants. A further possibility involves the active participation of the solvent in the chemical reaction. On this sense, hydrogen and proton transfer reactions can be sensitive to an ionic environment, due to the hydrogen-bond acceptor ability of most anions. As a prototype of an acid-base reaction in an ionic liquid, I will explore the results of adding the Bronsted acid HC1 to dymethyl-imadazolium chloride ([dmim][Cl]). First-principle molecular dynamics simulations, based on Density Functional Theory (DFT), show the immediate formation of hydrogen dichloride, HC12-, as an stable specie in the IL. I will describe the structure of the former anion and the way it is embedded into the structure of the solvent. Afterward, by choosing an appropriate reaction coordinate, one of the H-Cl bonds in HC12- is forced to break, ending with the exchange of one of the chlorine atom with a neighboring CI- belonging to the solvent. Such process results is an effective motion, or hoping, of the hydrogen atom. Two possible reaction mechanisms are discussed, and an estimation of the associated free energy barrier is provided. As a second model reaction, I will present preliminary results on the influence of the ionic liquid in the free energy profile of the classical bimolecular nucleophilic substitution CI- + C1CH3 -> C1CH3 + C1-.[1] Rogers, R. D.; Seddon, K. R.; Volkov, S.. "Green Industrial Applicationsof Ionic Liquids"; NATO Science Series, 2002.[2] Eds. P. Wasserscheid and T. Welton. Ionic Liquids in Synthesis'. Wiley-VCH, 2003.