Decoding the Infrared Spectra of High Frequency Molecular Vibrations

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Molecules can be detected and identified by how they vibrate. Their vibrations typically occur at frequencies corresponding to infrared light. Thus, by observing how molecules absorb different wavelengths of infrared light (the laboratory technique is called vibrational spectroscopy), we can tell something about their structure and properties. However, as molecules become large (in other words “polyatomic”, meaning many atoms), or if a small molecule is surrounded by other molecules (for example, an alcohol molecule dissolved in water), their vibrations become very complicated and difficult to interpret. In this project, funded by the Chemical Structure, Dynamics and Mechanisms A of the Chemistry Division, Professor Edwin Sibert of the University of Wisconsin-Madison is extending current theoretical models in order to enhance our ability to extract structural information from the vibrations of polyatomic molecules and molecules dissolved in liquids. This research is expected to enhance our ability to interpret get a detailed picture of a molecule’s structure and its interaction with other molecules in its environment. The models developed in this research also have implications for understanding complex biochemical processes as well as the practical design of molecules for a variety of applications, including pharmaceuticals and fuels. Finally, the Sibert research group is also developing electronic learning resources for the undergraduate chemistry program at UW Madison. Professor Silbert is involved in the design of electronic resources for students. These resources extend from helping students understand the forces at play in solvation phenomena to educating them about the significant contributions of chemistry to our society.Liquid phase infrared spectra of the CH, NH and OH stretch regions often lack the spectral resolution required to interrogate how the motions of nuclei evolve upon molecular excitation. The analogous molecular motions sampled in cold clusters are governed by similar forces, yet, due to the limited cluster size and cold temperatures, the experimental spectra of these high frequency vibrations have sufficiently narrow line shapes that spectral features are revealed. The aim of this project is to provide the theoretical tools that are required for interpreting these features in order to both reveal the underlying dynamics and extract structural and environmental information. The limited size of these clusters also allows for the rigorous testing of the theoretical models. This testing includes quantifying the quality of the techniques used to determine the underlying force fields. Designing drugs and predicting outcomes of combustion reactions depend on chemists’ ability to determine such force fields in order to calculate the relative stabilities of various chemical species and understand how environment affects those stabilities. The broader impact of this project includes generating open source computer software to enable scientists to measure and understand these stabilities as well as to provide a training ground for students in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.