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Dynamics In Concentrated LiCl And HCl Solutions

Tuesday, November 30, 2021 - 12:45 to 14:00
Dr. Michael D. Fayer | McDowell Lecture
Department of Chemistry, Stanford University
Event Category: 
LMC - Lectures in Modern Chemistry
Dr. Keng-Chang Chou
Chemistry B250


­­­­­­­­­­­Aqueous salt solutions occur widely in systems ranging from industrial processes to biological materials. Prominent examples include batteries and desalinization. The properties of aqueous electrolyte solutions involve the dynamics of water and the dynamics of ions.  A closely relate problem is proton transfer in acid solutions. Proton transfer in water is ubiquitous and a critical elementary event which, via proton hopping between water molecules, enables protons to diffuse much faster than other ions. While there have been a vast number of experiments and molecular dynamics simulations investigating proton hopping in water, a direct experimental observation of proton hopping has remained elusive due to its ultrafast nature and the lack of direct experimental observables. The dynamics of the formation and dissociation of complexes of Li+ and water with methylthiocyanate (MeSCN) in very concentration LiCl solutions are explicated using two dimensional infrared (2D IR) Chemical Exchange Spectroscopy.  The CN stretch is used as the vibrational probe.  2D IR spectral diffusion measurement show that MeSCN accurately reports on the hydrogen bond dynamics in pure water, making it an excellent probe of dynamics in aqueous systems. Water forms a hydrogen bond and Li+ associates with the nitrogen lone pair of the CN moiety of MeSCN.  These two complexes display distinct CN peaks in the FT-IR spectrum.  2D IR is used to directly measure the chemical exchange of water and Li+ with the nitrogen lone pair of the CN moiety.  2D IR is also used to measure the spectral diffusion, which provides information on the dynamic of the concentrated salt solutions.  In pure water, the spectral diffusion gives rise to a biexponential decay of the 2D IR data.  In the salt solutions, triexponentials are observe.  The slowest component is assigned to the time for ion clusters to randomize. 2D IR Chemical Exchange Spectroscopy was also used to extract the chemical exchange rates between hydronium and water in HCl solutions using MeSCN.  Ab initio molecular dynamics simulations demonstrate that the chemical exchange is dominated by proton hopping. The observed experimental and simulated acid concentration dependences as well as a number of factors obtained from the simulations and spectral diffusion measurements make it possible to extrapolate the measured single step proton hopping time in concentrated HCl to the dilute limit. Within error the 2D IR measure hopping time yields the same value as inferred from measurements of the proton diffusion constant.  It is found that the dilute limit, the proton hopping time is the same as the time for concerted H-bond rearrangement of the extended H-bond network in pure water.  The results indicate that the H-bond rearrangement of the water network in which hydronium ions are embedded triggers proton hopping.