Roger Loring of Cornell University is supported by an award from the Theoretical and Computational Chemistry program for research to develop methods for classical and semi-classical simulations of spectroscopic systems. The PI is developing and testing semiclassical approximations to quantum time-propagation that are specifically designed for the calculation of nonlinear response functions. In a second project, he and his group are extending methods that are currently applicable on larger scales to atomistic models of specific proteins. A third project involves collaboration with scanning probe microscopists who are investigating molecular structure and dynamics of disordered polymer films on longer length and time scales in order to better understand friction in these systems.

A detailed knowledge of molecular motions is essential to understanding, predicting, and even controlling chemical, biological, and transport processes. A theme in this project is the development of new techniques for measuring molecular dynamics over an increasing range of length and time scales. The work is expected to have a broad impact on our understanding of fundamental molecular processes in biology and materials science.

Project Report

Modern experimental physical chemistry is characterized by the quest to interrogate molecules and chemical processes with an unprecedented level of detail, at ever finer length scales and shorter time scales. The development of each new measurement raises basic issues of interpretation whose analysis requires techniques of theoretical and computational chemistry. What is the precise information content of the measurement? How can a maximally detailed picture of molecular properties be extracted from experimental data? This grant has supported three projects in theoretical and computational chemistry that address these questions for three recently developed experimental techniques in physical chemistry. The grant has supported collaborations with three different experimental research groups. This grant has also trained in techniques of theoretical and computational chemistry one undergraduate (A.B. 2010, currently pursuing a Ph. D. elsewhere) and four Ph.D. students, one of whom earned the Ph. D. in 2010 and one of whom will do so in July, 2012. Of these 5 students, one is female and three are US citizens. Conventional one-dimensional infrared spectroscopy has been used for decades as an analytical technique to identify molecules by the frequencies of their vibrations. Modern two-dimensional infrared spectroscopy provides much richer structural and dynamical information arising from the flow of energy among molecular vibrations. This technique has gained prominence as a probe of both dynamics and structure in biomolecules. This grant has supported a collaboration with an experimental group at Stanford University in which we model two-dimensional infrared spectra of a protein, genetically engineered to act as an enzyme, to explore the connections between structural changes and molecular motions. This grant has also supported the development of new computational techniques for calculating two-dimensional spectra of large molecules. A conventional kinetic measurement on a chemical reaction determines reaction rates averaged over enormous numbers of individual events. This grant has supported a collaboration with an experimentalist colleague at Cornell who has succeeded in studying a reaction catalyzed by gold nanoparticles, one reaction event at a time. The catalysis takes place at specific adsorption sites on the metal particles, which are heterogeneous and poorly characterized. The reaction transforms a nonfluorescent molecule to a fluorescent one, and the measurement produces a record in time of a single nanoparticle making transitions between dark and fluorescing states. This grant has enabled the development of a new statistical method for analyzing such time records to address whether the adsorption sites on the nanoparticle change their structure independently or in a correlated fashion. Our findings support a picture in which the surface of the metal particle undergoes structural changes in response to the presence of adsorbed molecules. Scanned probe techniques such as atomic force microscopy provide an atomic view of a solid surface. If the atomic force microscope probe is electrically charged, this electric force microscope can sense weak electrical forces generated by the motions of atoms and molecules in the solid material and can provide an image of the surface based on these forces. This grant has supported a collaboration with an experimental group at Cornell that images the surfaces of organic materials with electric force microscopy. Theoretical models designed with grant support allow the quantitative interpretation of the measurements in terms of the equilibrium motions of polymer molecules and the dynamics of charge carriers in organic semiconductors. The combination of measurements and theory will allow these techniques to provide a new view of charge carrier dynamics in devices such as organic transistors and solar cells. The projects supported by this grant share two common themes: understanding the motion of molecules, and the close and necessary cooperation between theory and measurement.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Evelyn M. Goldfield
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Cornell University
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