Exploring the Connections between the Nonlinear Spectroscopy and the Potential Energy Landscapes of Liquids

Brown University, Providence RI

Investigators

Abstract

Richard M. Stratt of Brown University is supported by the Chemical Theory, Models, and Computational Methods program in the Chemistry division for his research on the relationships between the unusually slow molecular dynamicsseen in liquid mixtures, liquid crystals, and supercooled liquids, and the global features of the potential-energy landscapes of these systems. While slow dynamics is often understandable as a consequence of the need to rearrange individual chemical bonds or surmount other local bottlenecks on the potential energy surface, in many of the most interesting cases, the issue is more the difficulty in locating and traversing even the most efficient global pathways through the landscape. This work investigates how computer simulations can be used to characterize these most efficient ("geodesic") pathways and attempts to forge quantitative connections between the information garnered from these pathways and that available from novel nonlinear spectroscopies. The spectroscopic half of the research is focused on solute pump/solvent-probe spectra, in particular, because those spectra have the potential to reveal how broad swaths of solvent structure can rearrange in the presence of a newly electronically excited solute. By computing the spectra expected from preferentially solvating mixtures, dye-doped liquid crystals, and fragile-glass-forming liquids, and comparing the outcomes with the results of a geodesic landscape analysis, the researchers hope to learn how spectroscopy can be used to discover what is so unique about the potential energy landscapes of these especially slow systems. Professor Stratt and his research group are developing computational methods to explore the relationship between the slow molecular dynamics seen in liquids mixtures, liquid crystals and supercooled liquids and certain features of the potential energy landscapes of these systems. A potential energy landscape is the interaction energy of the atoms in the system as a function of the distances between the atoms. Stratt's research focuses on how to find the most efficient pathways through these landscapes; such pathways determine the actual motions of the atoms. Stratt also develops theoretical methods to predict and analyze spectra that help us to understand how solvents react to molecules that are in an excited electronic state. The eventual payoff for this research may be useful insight into two enormously important practical problems involving slow molecular rearrangements: determining the properties of glassy and amorphous materials, and understanding how and when proteins fold incorrectly. The toughness and ease of processing of amorphous substances often makes them ideal commercial materials, but there is surprisingly little consensus about many of the scientific fundamentals of glassiness. Similarly, proteins that resist folding into well-ordered structures are essential to living organisms, but some of those that fold in an amorphous fashion are markers of critical human health problems, including Alzheimer's disease. The research involves undergraduates as research coworkers, thereby enriching their scientific education.