Probing Low Frequency Vibrational Modes in Molecular Crystals and Developing Terahertz Spectroscopic Polarimetry

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With this award, funded by the Chemical Structure, Dynamics and Mechanisms (CSDM-A) Program of the Division of Chemistry, Professor Charles Schmuttenmaer of Yale University and his postdoctoral, graduate and undergraduate student colleagues are studying a technique called terahertz vibrational circular dichroism. Vibrational circular dichroism (VCD) in the mid-infra-red region is a very useful technique that provides three-dimensional structural information about molecules. It has applications for both small and complex systems. Species such as muscle proteins, DNA, amino acids, and proteins have been studied. This project is extending VCD in the terahertz region of the spectrum (the middle ground between microwave and infra-red radiation). This is a new frontier that allows the exploration of motions that occur over an entire molecule. Applications are envisaged from amino acid crystals and carbohydrates, to pharmaceuticals, to explosives, and others. There is a strong outreach component to socioeconomically disadvantaged middle- and high-school students. In addition, undergraduate research opportunities at Yale will be strengthened.There has been a great deal of recent work using THz time-domain spectroscopy to characterize the low frequency vibrational modes in organic molecular crystals (OMCs). While THz spectra of many OMCs have been reported, assigning them has been exceedingly difficult. This project is using new approaches to understand the complex motions underlying these low frequency modes. Insight is gained through isotopic substitution as well as dependence on external parameters such as pressure and temperature. The spectra of several chemically related OMCs are calculated and measured in order to identify and describe which motions are common to them, and which are unique. The THz VCD spectra are measured and provide experimental data that constrain possible peak assignments. One objective is to measure the THz VCD spectrum of an OMC for the first time.