Professor Michael Fayer
Professor Michael Fayer
Department of Chemistry
Stanford University
Host: Professor Aaron Massari
Dynamics in concentrated LiCl and HCl solutions
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.
Professor Fayer
Professor Fayer earned his bachelor and master's degrees from the University of California, Berkeley. He was a professor of physics at the University of Grenoble, before joining the faculty at Stanford University. He is the David Mulvane Ehrsam and Edward Curtis Franklin professor of chemistry at Stanford University.
Researchers in Professor Fayer's lab are using ultrafast 2D IR vibrational echo spectroscopy and other multi-dimensional IR methods, which they have pioneered, to study dynamics of molecular complexes, water confined on nm lengths scales with a variety of topographies, molecules bound to surfaces, ionic liquids, and materials such as metal organic frameworks and porous silica. They are also studying dynamics in complex liquids, in particular room temperature ionic liquids, liquid crystals, supercooled liquids as well as in influence of small quantities of water on liquid dynamics. In addition, Professor Fayer is studying photo-induced proton transfer in nanoscopic water environments such as polyelectrolyte fuel cell membranes, using ultrafast UV/Vis fluorescence and multidimensional IR measurements to understand the proton transfer and other processes and how they are influenced by nanoscopic confinement.
Bryce L. Crawford Jr.
Bryce L. Crawford Jr. was a renowned Department of Chemistry professor and scientist. He died in September 2011, at the age of 96. He joined the department in 1940, and became a full professor of physical chemistry in 1946. He was chair of the department from 1955 to 1960, and was dean of the graduate school from 1960 to 1972. He retired in 1985. He loved studying molecular vibrations and force constants, and the experimental side of molecular spectroscopy and molecular structure. During World War II, Crawford worked in research on rocket propellants, making significant contributions to rocketry, and the development of solid propellants for the much larger rockets that evolved after the war. Crawford received many honors during his career, including the prestigious American Chemical Society Priestley Medal; and being named a Fellow of the Society for Applied Spectroscopy, a Guggenheim Fellow at the California Institute of Technology, and a Fulbright Fellow at Oxford University. He held the distinction of membership in three honorary science academies, and was actively involved in many professional associations.
